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context: tag/antimatter/ question: Can one bit of light bounce off another bit of light?

Yes, one bit of light can bounce off another bit of light, but not directly, and the effect is very rare. Light is made out of small quantum objects called photons. When you turn on a lamp, the light bulb begins creating and emitting trillions upon trillions of photons. Photons are in a class of quantum particles known as bosons. Bosons are special because many bosons can occupy the exact same quantum state at the same time. Light being made of bosons is what makes a laser beam possible. A laser beam is a collection of many photons all in the same quantum state. In contrast, particles that are not bosons cannot occupy the same state at the same time. This is one of the effects that keeps the atoms in an object from collapsing to a single point. The principle that dictates that non-bosons cannot be in the same state is called the Pauli Exclusion Principle. Non-bosons are also called fermions. The fact that bosons such as light can occupy the same state means that they don't get in each other's way. Also, light dominantly interacts with objects that have electric charge. Since light itself does not have electric charge, one photon cannot directly interact with another photon. Instead, they just pass right through each other without being affected. Because they are bosons and because they carry no electric charge, one photon cannot directly bounce off another photon. If you point one jet of water towards another jet of water, then at the point where they cross you will get a mess of water spraying all over due to the collisions. In contrast, if you shine one light beam such that it crosses another light beam, they will just pass through each other unaffected. However, two photons heading towards each other can indeed collide indirectly. The process goes like this. A photon can spontaneously degenerate into a particle with mass and its antiparticle in a process known as pair production. In this process, the energy of the photon is completely transformed into the mass of the two particles. For example, a photon can turn into an electron and an anti-electron. If two photons head towards each other and they both turn into electron/anti-electron pairs at about the same time, then these particles can interact. The anti-electron from one photon will collide with an electron from the other photon, and turn back to light. The same thing happens to the other anti-electron and electron. The overall effect is that you get two photons going into the interaction and you get two photons coming out of the interaction, so it looks like the photons simply bounced off each other. In a sense, the one bit of light did indeed bounce off the other bit of light, but only indirectly by transforming into other particles. This interaction is shown in the diagram. The red squiggles represent photons, the e+ blue lines are anti-electrons (positrons), and the e- blue lines are electrons. A photon comes from the left of the diagram and decays into an electron and an anti-electron. At the same time, another photon comes from the right and turns into an electron and an anti-electron. Each anti-electron collides with an electron, they mutually annihilate and turn back into a new photon. Photon-photon scattering is therefore possible through an indirect mechanism, but it is rare. There are two reasons that it is rare. First, light can only turn into other particles if it has enough energy to create the mass needed for the new particles, according to E = mc2. Because c is such a huge number, it takes a large amount of energy to make a little bit of mass. In order to turn into an electron and an anti-electron, the photon must have at least as much energy as the equivalent energy of their combined masses. Only gamma rays (one step higher than X-rays) have enough energy to do this. Secondly, the photons have to transform at just the right moment in order for the new particles from both photons to collide. For both of these reasons, light-light scattering is very rare. In fact, light-light scattering has never been conclusively observed. All the steps in light-light scattering have been observed (pair production and pair annihilation), so that we know that it is possible. The whole effect is just so rare that it has never been observed. However, the Large Hadron Collider (LHC) has both the ability to create high-energy light, and the ability to create a lot of it, making the rarity of light-light scattering more manageable. It is only a matter of time before the LHC observes light bouncing off light.

context: tag/antimatter/ question: Does the negative mass of antimatter mean that it's gravitationally repelled from stars?

Antimatter does not have negative mass. In our universe, there is no such thing as negative mass. Mass only comes in positive form. In contrast, electric charge can be positive or negative. The nonexistence of negative mass has profound consequences. First, gravity is always attractive and never repulsive. Presumably, if negative mass existed, it would be repelled by objects with positive mass such as the earth or sun. A cup made out of negative mass would fall up and not down when you let it go. As interesting as this sounds, negative mass does not exist, so heavier objects always fall down. Second, the lack of negative mass means that gravitational fields can never be shielded, blocked, or canceled. In contrast, electric charge comes in both positive and negative varieties. An electric field created by a positively charged object can therefore be blocked by a wall of negative charges. This is the principle used in metal shielding, which keeps the microwaves in a microwave oven from cooking everything in the kitchen. But there is no negative mass to cancel out gravity. If negative mass did exist, you could use it as a form of anti-gravity. If you built a floor on earth out of negative mass of sufficient size, then people above that floor would feel no gravity and would float around. Again, negative mass does not exist, so genuine anti-gravity is not possible. Gravity permeates every cell of every creature, and into the deepest dungeon with the thickest walls. Antimatter is a physical entity with positive mass that is identical to regular matter in every way except that the charge and some other properties are flipped. Every bit of matter in the universe has a potential antimatter counterpart. Every fundamental particle of regular matter has an antimatter version. For instance, the antimatter version of the electron is the positron. Electrons and positron have the exact same mass, the same spin, and the same charge magnitude. The only difference is that electrons are negatively charged and positrons are positively charged (positrons should not be confused with protons which are completely different particles) and a few other properties are flipped. When an electron meets a positron, they annihilate each other and their combined mass is converted completely into energy in the form of gamma rays. This effect is used routinely in medical PET scans. In general, antimatter annihilates its regular matter version when they meet. Antimatter can be thought of as regular matter traveling backwards in time. In this picture, a particle-antiparticle annihilation event can be thought of as a forward-time-traveling particle being knocked by gamma rays so that it becomes the same particle, but just traveling backwards in time. Note that this time travel concept applies only to specific antimatter events that obey the conservation of energy and does not open up the possibility for humans to travel back in time, which would violate the conservation of energy. Although antimatter comes in very small quantities in our universe, it is not as exotic or unnatural as once thought. Every minute of every day, high energy cosmic rays from distant supernovas are slamming into earth's atmosphere and creating a very small amount of antimatter. Also, the natural radioactive decay that takes place constantly in earth's rocks creates small amounts of antimatter. But this antimatter does not stick around for long because it quickly bumps up against regular matter and destroys itself in the process.

context: tag/antimatter/ question: How can you tell a black hole made out of antimatter from a black hole made out of matter?

According to our current understanding, there is no way to distinguish an antimatter black hole from a regular-matter black hole. In fact, there is no difference between an antimatter black hole and a regular-matter black hole if they have the same mass, charge, and angular-momentum. First of all, antimatter is just like regular matter except that its charge and some other properties are flipped. Antimatter has positive mass just like regular matter and experiences gravity the same way. Antimatter is exotic in the sense of being very rare in our universe, but it is not exotic in how it obeys the laws of physics. An antimatter cookie would look just like a regular-matter cookie. Therefore, adding the concept of antimatter to the discussion does not really lead to anything new or exotic. We could just as easily ask, "what is the difference between a black hole made of hydrogen and a black hole made of helium?" The answer is that there is no difference (as long as the total mass, charge, and angular-momentum are the same). According to the No-Hair Theorem, a black hole has the interesting property that all information and structure that falls into a black hole becomes trapped from the rest of the universe, and perhaps even destroyed, except for its effect on the total mass, charge, and angular momentum of the black hole. The overall mass of a black hole is what determines the strength of its gravity. When scientists talk about large or small black holes, they are actually talking about the mass of the black hole. Large black holes have more mass, more gravity, and therefore more effect on their surroundings. When matter falls into a black hole, it increases the overall mass of the black hole. The overall electric charge of a black hole determines the strength of the electric field that it creates. When matter with electric charge of the same polarity as the black hole falls in, it increases the charge of the black hole. The overall angular momentum of a black hole describes how fast it is spinning. When matter falls into a black hole with a swirling motion (as opposed to falling straight in), it can increase the black hole's total angular momentum if the matter swirls in the same direction, or decrease the black hole's total angular momentum if it swirls in the opposite direction. In the book The Nature of Space and Time by Stephen Hawking and Roger Penrose, Hawking states: The no-hair theorem, proved by the combined work of Israel, Carter, Robinson, and myself, shows that the only stationary black holes in the absence of matter fields are the Kerr solutions. These are characterized by two parameters, the mass M and the angular momentum J. The no-hair theorem was extended by Robinson to the case where there was an electromagnetic field. This added a third parameter Q, the electric charge... What the no-hair theorems show is that a large amount of information is lost when a body collapses to form a black hole. The collapsing body is described by a very large number of parameters. These are the types of matter and the multipole moments of the mass distribution. Yet the black hole that forms is completely independent of the type of matter and rapidly loses all the multipole moments except the first two: the monopole moment, which is the mass, and the dipole moment, which is the angular momentum. We don't know exactly what goes on in a black hole. The matter inside a black hole could be condensed down to an indistinguishable blob. Or the matter could retain some structure but remain trapped in the black hole by the black hole's intense gravity. The problem is that a black hole's center is so small that the theory of General Relativity, which describes gravitational effects, becomes inaccurate. We need quantum theory to accurately describe physics on the very small scale. But we have not yet developed a correct theory of quantum gravity. Therefore, we won't have a good idea of what goes on inside a black hole until we have an accurate theory of quantum gravity. The fact that the inside of black holes is shielded from all experimental observations makes the task even more difficult.

context: tag/antimatter/ question: Is there any difference between antimatter, dark matter, dark energy, and degenerate matter?

Yes. Although the names sound vague and almost fictional, the types of matter called antimatter, dark matter, dark energy, and degenerate matter are all different, specific entities that really exist in our universe. Antimatter is just regular matter with a few properties flipped, such as the electric charge. For example, the antimatter version of an electron is a positron. They both have the same mass, but have opposite electric charge. Antimatter is not as exotic as science fiction makes it out to be. For starters, antimatter has regular mass and accelerates in response to forces just like regular matter. Also, antimatter is gravitationally attracted to other forms of matter just like regular matter. For every particle that exists, there is an antimatter counterpart (some particles such as photons are their own anti-particles). What makes antimatter unique is that when antimatter comes in contact with its regular matter counterpart, they mutually destroy each other and all of their mass is converted to energy. This matter-antimatter mutual annihilation has been observed many times and is a well-established principle. In fact, medical PET scans routinely use annihilation events in order to form images of patients. Antimatter is therefore only distinct from regular matter in that it annihilates when meeting regular matter. For instance, a proton and a positron are somewhat similar. They both have regular mass. They both have a positive electric charge of the same strength. They both have a quantum spin of one half. But when a proton meets an electron, it forms a stable hydrogen atom. When a positron meets an electron, they destroy each other. The key difference is that a positron is antimatter and a proton is not. Antimatter is very rare in our universe compared to regular matter, but there are small amounts of antimatter all over the place in the natural world, including inside your body. Antimatter is created by many types of radioactive decay, such as by the decay of potassium-40. When you eat a banana, you are eating trace amounts of antimatter-producing atoms. The amount is so small, that it does not really affect your health. But it is still there. Why doesn't antimatter build up in your body? The key is that our universe is mostly made of regular matter, so antimatter cannot stick around for very long. Very soon after antimatter is created, it bumps into regular matter and gets destroyed again. Antimatter is also produced by lightning and cosmic rays. It is well understood by physicists, and is predicted by standard particle physics theories. Dark matter is matter that does not interact electromagnetically, and therefore cannot be seen using light. At the same time, dark matter does interact gravitationally and can therefore be "seen" through its gravitational effect on other matter. It is common throughout the universe and helps shape galaxies. In fact, recent estimates put dark matter as five times more common than regular matter in our universe. But because dark matter does not interact electromagnetically, we can't touch it, see it, or manipulate it using conventional means. You could, in principle, manipulate dark matter using gravitational forces. The problem is that the gravitational force is so weak that you need planet-sized masses in order to gravitationally manipulate human-sized objects. There remains much unknown about dark matter since it is so hard to detect and manipulate. Dark matter is not predicted or explained by standard particle physics theories but is a crucial part of the Big Bang model. Dark energy is an energy on the universal scale that is pushing apart galaxies and causing the universe to expand at an increasing rate. Like dark matter, dark energy is poorly understood and is not directly detectable using conventional means. Several lines of evidence make it clear that our universe is expanding. Not only that, our universe is expanding at an increasing rate. Dark energy is the name of the poorly understood mechanism that drives this accelerating expansion. While dark matter tends to bring matter together, dark energy tends to push matter apart. Dark energy is weak and mostly operates only on the intergalactic scale where gravitational attraction of dark matter and regular matter is negligible. Dark energy is thought to be spread thinly but evenly throughout the entire universe. Dark energy is also not predicted or explained by standard particle physics theories but is included in modern versions of the Big Bang model. Dark energy may have a connection with the vacuum energy predicted by particle physics, but the connection is currently unclear. Degenerate matter is regular matter that has been compressed until the atoms break down and the particles lock into a giant mass. Degenerate matter acts somewhat like a gas in that the particles are not bound to each other, and somewhat like a solid in that the particles are packed so closely that they cannot move much. A white dwarf star is mostly composed of electrons compressed into a state of degenerate matter. A neutron star is mostly composed of degenerate neutrons. Further compression of a neutron star may transform it to a quark star, which is a star composed of quarks in a degenerate state. But not enough is known about quarks to determine at present whether quark stars really exist or are even possible. These concepts are summarized in the list below. Regular Matter Antimatter Dark Matter Dark Energy Degenerate Matter

context: tag/atom/ question: Are there nuclear reactions going on in our bodies?

Yes, there are nuclear reactions constantly occurring in our bodies, but there are very few of them compared to the chemical reactions, and they do not affect our bodies much. All physical objects are made of molecules. A molecule is a series of atoms linked together by chemical (electromagnetic) bonds. Inside each atom is a nucleus which is a collection of protons and neutrons linked together by nuclear bonds. Chemical reactions are the making, breaking, and rearranging of bonds between atoms in molecules. Chemical reactions do not change the nuclear structure of any atoms. In contrast, nuclear reactions involve the transformation of atomic nuclei. Most of the processes surrounding us in our daily life are chemical reactions and not nuclear reactions. All of the physical processes that take place to keep a human body running (blood capturing oxygen, sugars being burned, DNA being constructed,etc.) are chemical processes and not nuclear processes. Nuclear reactions do indeed occur in the human body, but the body does not use them. Nuclear reactions can lead to chemical damage, which the body may notice and try to fix. There are three main types of nuclear reactions: Note that nuclear fission and radioactive decay overlap a little bit. Some types of radioactive decay involve the spitting out of nuclear fragments and could therefore be seen as a type of fission. For the purposes of this article, "fission" refers to large-scale nucleus fragmentation events that can clearly not be classified as radioactive decay. Nuclear fusion requires high energy in order to be ignited. For this reason, nuclear fusion only occurs in stars, in supernovas, in nuclear fusion bombs, in nuclear fusion experimental reactors, in cosmic ray impacts, and in particle accelerators. Similarly, nuclear fission requires high energy or a large mass of heavy, radioactive elements. For this reason, significant nuclear fission only occurs in supernovas, in nuclear fission bombs, in nuclear fission reactors, in cosmic ray impacts, in particle accelerators, and in a few natural ore deposits. In contrast, radioactive decay happens automatically to unstable nuclei and is therefore much more common. Every atom has either a stable nucleus or an unstable nucleus, depending on how big it is and on the ratio of protons to neutrons. Nuclei with too many neutrons, too few neutrons, or that are simply too big are unstable. They eventually transform to a stable form through radioactive decay. Wherever there are atoms with unstable nuclei (radioactive atoms), there are nuclear reactions occurring naturally. The interesting thing is that there are small amounts of radioactive atoms everywhere: in your chair, in the ground, in the food you eat, and yes, in your body. Radioactive decay produces high-energy radiation that can damage your body. Fortunately, our bodies have mechanisms to clean up the damage caused by radioactivity and high-energy radiation before they become serious. For the average person living a normal life, the amount of radioactivity in his body is so small that the body has no difficulty repairing all the damage. The problem is when the radioactivity levels (the amount of nuclear reactions in and around the body) rise too high and the body cannot keep up with the repairs. In such cases, the victim experiences burns, sickness, cancer, and even death. Exposure to dangerously high levels of radioactivity is rare and is typically avoided through government regulation, training, and education. Common causes of human exposure to high radioactivity include: Note that if you have a single medical scan performed that requires drinking or being injected with a radioactive tracer, you do indeed end up with more nuclear reactions in your body than normal, but the level is still low enough to not be dangerous, and therefore was not included on this list. Low levels of radioactive atoms are constantly accumulating in every person. The ways we end up with radioactive atoms in our bodies include: eating food that naturally contains small amounts of radioactive isotopes, breathing air that naturally contains small amounts of radioactive isotopes, and being bombarded with cosmic rays that create radioactive atoms in our bodies. The most common natural radioactive isotopes in humans are carbon-14 and potassium-40. Chemically, these isotopes behave exactly like stable carbon and potassium. For this reason, the body uses carbon-14 and potassium-40 just like it does normal carbon and potassium; building them into the different parts of the cells, without knowing that they are radioactive. In time, carbon-14 atoms decay to stable nitrogen atoms and potassium-40 atoms decay to stable calcium atoms. Chemicals in the body that relied on having a carbon-14 atom or potassium-40 atom in a certain spot will suddenly have a nitrogen or calcium atom. Such a change damages the chemical. Normally, such change are so rare, that the body can repair the damage or filter away the damaged chemicals. The textbook Chemistry: The Practical Science by Paul B. Kelter, Michael D. Mosher and Andrew Scott states: Whereas potassium-39 and potassium-41 possess stable nuclei, potassium-40 is radioactive. This means that when we consume a banana, we get a measurable amount of radioactive potassium-40. How much? The natural abundance of potassium-40 is only 0.012%, or approximately 1 atom in 10,000. A typical banana has approximately 300 mg of potassium. Therefore, with each banana that we eat, we ingest approximately 0.036 mg of radioactive potassium-40. The natural occurrence of carbon-14 decay in the body is the core principle behind carbon dating. As long as a person is alive and still eating, every carbon-14 atom that decays into a nitrogen atom is replaced on average with a new carbon-14 atom. But once a person dies, he stops replacing the decaying carbon-14 atoms. Slowly the carbon-14 atoms decay to nitrogen without being replaced, so that there is less and less carbon-14 in a dead body. The rate at which carbon-14 decays is constant and well-known, so by measuring the relative amount of carbon-14 in a bone, archeologists can calculate when the person died. All living organisms consume carbon, so carbon dating can be used to date any living organism, and any object made from a living organism. Bones, wood, leather, and even paper can be accurately dated, as long as they first existed within the last 60,000 years. This is all because of the fact that nuclear reactions naturally occur in living organisms.

context: tag/atom/ question: Are two atoms of the same element identical?

No. Two atoms of the same chemical element are typically not identical. First of all, there is a range of possible states that the electrons of an atom can occupy. Two atoms of the same element can be different if their electrons are in different states. If one copper atom has an electron in an excited state and another copper atom has all of its electrons in the ground state, then the two atoms are different. The excited copper atom will emit a bit of light when the electron relaxes back down to the ground state, and the copper atom already in the ground state will not. Since the states of the electrons in an atom are what determine the nature of the chemical bonding that the atom experiences, two atoms of the same element can react differently if they are in different states. For instance, a neutral sodium atom (say, from a chunk of sodium metal) reacts with water much more violently than an ionized sodium atom (say, from a bit of salt). Chemists know this very well. It's not enough to say what atoms are involved if you want to fully describe and predict a reaction. You have to also specify the ionization/excitation states of the electrons in the atoms. Even if left alone, an atom often does not come with an equal number of protons and electrons. But what if two atoms of the same element both have their electrons in the same states. Then are they identical? No, they are still not identical. Two atoms of the same element and in the same electronic state could be traveling or rotating at different speeds, which affects their ability to chemically bond. Slower moving atoms (such as the atoms in solid iron) have time to form stable bonds, while faster moving atoms (such as the atoms in liquid iron) cannot form such stable bonds. A slow moving tin atom acts differently from a rapidly moving tin atom. But what if two atoms of the same element both have their electrons in the same states, and the atoms are both traveling and rotating at the same speed. Then are they identical? No. Although two such atoms are essentially chemically identical (they will chemically react in the same way), they are not completely identical. There's more to the atom than the electrons. There's also the nucleus. The nucleus of an atom contains neutrons and protons bonded tightly together. The same chemical element can have a different number of neutrons and still be the same element. We refer to the atoms of the same element with different numbers of neutrons as "isotopes". While the particular isotope involved does not affect how an atom will react chemically, it does determine how the atom will behave in nuclear reactions. The most common nuclear reaction on earth is radioactive decay. Some isotopes decay very quickly into other elements and emit radiation, while other isotopes do not. If you are doing carbon dating, the fact that a carbon-12 atom is not identical to a carbon-14 atom is essential to the dating process. Simply counting the number of carbon atoms in a sample will not give you any information about the age of a sample. You will have to count the number of different isotopes of carbon instead. But what if two atoms are the same element, have electrons in the same state, are traveling and rotating at the same speed, and have the same number of neutrons; then are they identical? No. Just like the electrons, the neutrons and protons in the nucleus can be in various excited states. In addition, the nucleus as a whole can rotate and vibrate at various speeds. Therefore, even if all else is identical, two gold atoms can have their nuclei in different excited states and behave differently in nuclear reactions. To state the case succinctly, it is very hard to have two atoms of the same element be exactly identical. In fact, succeeding in coaxing a group of atoms to be very close to identical was worthy of a Nobel Prize. With that said, don't think that atoms have individual identities beyond what has been mentioned here. If two carbon atoms are in the exact same molecular, atomic, electronic and nuclear states, then those two carbon atoms are identical, no matter where they came from or what has happened to them in the past.

context: tag/atom/ question: Can sound waves generate heat?

Yes, sound waves can generate heat. In fact, sound waves almost always generate a little bit of heat as they travel and almost always end up as heat when they are absorbed. Sound and heat are both macroscopic descriptions of the movement of atoms and molecules. Sound is the ordered movement of atoms and molecules in rapid waving patterns. Heat is the disordered, random, movement of atoms and molecules. Therefore, all you have to do in order to turn sound into heat is transform some of the ordered movement of the atoms and molecules into disordered movement. This effect always happens to some extent. This effect happens a lot whenever the sound wave encounters irregularities as it travels. For instance, dust particles in air are irregularities that randomly interfere with the vibrating motion of some of the air molecules that make up the sound wave. The dust particles mess up some of the ordered motion, and therefore convert some of the sound to heat. As another example, the rough surface of an object constitutes a collection of irregularities that the sound wave encounters. Therefore, when a sound wave hits a rough surface, the motion of the air molecules gets scrambled up a bit. Note that the air molecules already have a motion that is somewhat disordered. In other words, air through which sound is traveling already contains some amount of heat. When some of the sound wave is converted to heat, the motion of the air molecules becomes more disordered and the amount of heat increases. The ordered movement of atoms is also made more disorderly when sound travels through acoustically absorbent materials. Materials can be made absorbent by embedding an array of little irregularities directly into the material, such as air bubbles. For this reason, materials that are soft and porous, like cloth, are good at converting sound to heat. The sound is said to be "absorbed" or "lost" when it is converted to heat inside a material. Even without irregularities, a material can be highly absorbent if the atoms and molecules that make up the material cannot slide past each other smoothly. In this case, an atom or molecule that is trying to participate in the ordered vibrational motion of the sound wave roughly slides past the neighboring atoms or molecules that are off to the side, such that motion gets diverted in sideways directions rather than continuing in the forward direction as part of the sound wave. The ordered motion therefore becomes disordered. You can think of it as a kind of internal friction that all materials posses to some extent. In this way, some of the sound energy is converted to thermal energy. All materials, even air, have some amount of resistance to smooth atomic/molecular sliding and therefore are somewhat absorbent to sound. In summary, sound waves always generate a little heat as they travel and they ultimately almost always end up completely as heat when they are absorbed by materials. However, the amount of energy carried by sound waves is very small, so that the amount of heat they generate is typically insignificant. In short, cranking up the volume on your speakers is a terrible way to try to heat up your room. Yelling at your soup does indeed warm it up, but the amount is far too small to be noticeable.

context: tag/atom/ question: Can the decay half-life of a radioactive material be changed?

Yes, the decay half-life of a radioactive material can be changed. Radioactive decay happens when an unstable atomic nucleus spontaneously changes to a lower-energy state and spits out a bit of radiation. This process changes the atom to a different element or a different isotope. Since radioactive decay is a spontaneous event, you may think that the half-life of the decay process is completely fixed and cannot be altered by outside influences. However, this statement is not completely true. First of all, it is worth pointing out that the time when an individual radioactive atom decays is completely random. It is impossible to predict when an individual radioactive atom will decay. The half-life of a certain type of atom does not describe the exact amount of time that every single atom experiences before decaying. Rather, the half-life describes the average amount of time it takes for a large group of atoms to reach the point where half of the atoms have decayed. The half-life of a radioactive material can be changed using time dilation effects. According to relativity, time itself can be slowed down. Everything that experiences time can therefore be given a longer effective lifetime if time is dilated. This can be done in two ways. Traveling at a speed close to the speed of light causes time to slow down significantly, relative to the stationary observer. For instance, a number of radioactive atoms shot through a tube at high speed in the lab will have their half-life lengthened relative to the lab because of time dilation. This effect has been verified many times using particle accelerators. Time can also be dilated by applying a very strong gravitational field. For instance, placing a bunch of radioactive atoms near a black hole will also extend their half-life relative to the distant observer because of time dilation. The half-life of radioactive decay can also be altered by changing the state of the electrons surrounding the nucleus. In a type of radioactive decay called "electron capture", the nucleus absorbs one of the atom's electrons and combines it with a proton to make a neutron and a neutrino. The more the wavefunctions of the atom's electrons overlap with the nucleus, the more able the nucleus is to capture an electron. Therefore, the half-life of an electron-capture radioactive decay mode depends slightly on what state the atom's electrons are in. By exciting or deforming the atom's electrons into states that overlap less with the nucleus, the half-life can be increased. Since the chemical bonding between atoms involves the deformation of atomic electron wavefunctions, the radioactive half-life of an atom can depend on how it is bonded to other atoms. Simply by changing the neighboring atoms that are bonded to a radioactive isotope, we can change its half-life. However, the change in half-life accomplished in this way is typically small. For instance, a study performed by B. Wang et al and published in the European Physical Journal A was able to measure that the electron capture half-life of beryllium-7 was made 0.9% longer by surrounding the beryllium atoms with palladium atoms. In addition to altering the chemical bonds, the half-life can be altered by simply removing electrons from the atom. In the extreme limit of this approach, all of the electrons can be ripped off of a radioactive atom. For such an ion, there are no longer any electrons available to capture, and therefore the half-life of the electron capture radioactive decay mode becomes infinite. Certain radioactive isotopes that can only decay via the electron capture mode (such as rubidium-83) can be made to never decay by ripping off all the electrons. Other types of radioactive decay besides electron capture have also been found to have the decay half-life depend on the state of the surrounding electrons, but the effects are smaller. The change in half-life due to changing the electron environment is generally very small, typically much less than 1%. Lastly, the half-life of a radioactive material can be changed by bombarding it with high-energy radiation. This should not come as a surprise since radioactive decay is a nuclear reaction, and inducing other nuclear reactions at the same time as the decay can interfere with it. However, at this point, you don't really have stand-alone radioactive decay. Rather, you have nuclear reaction soup, so this approach may not really count as "changing the half-life". When reference books list values for the half-life of various materials, they are really listing the half-life for the material when its atoms are at rest, in the ground state, and in a particular chemical bonding configuration. Note that most changes to the half-life of radioactive materials are very small. Furthermore, large changes to a half-life require elaborate, expensive, high-energy equipment (e.g. particle accelerators, nuclear reactors, ion traps). Therefore, outside of specialized labs, we can say that as a good approximation radioactive decay half-lives don't change. For instance, carbon dating and geological radiometric dating are so accurate because decay half-lives in nature are so close to constant.

context: tag/atom/ question: Do atoms ever actually touch each other?

The answer depends on what you mean by "touch". There are three possible meanings of touch at the atomic level: 1) two objects influence each other, 2) two objects influence each other significantly, or 3) two objects reside in the exact same location. Note that the everday concept of touch (i.e the hard boundaries of two objects exist at the same location) makes no sense at the atomic level because atoms don't have hard boundaries. Atoms are not really solid spheres. They are fuzzy quantum probability clouds filled with electrons spread out into waving cloud-like shapes called "orbitals". Like a cloud in the sky, an atom can have a shape and a location without having a hard boundary. This is possible because the atom has regions of high density and regions of low density. When we say that an atom is sitting at point A, what we really mean is that the high-density portion of the atom's probability cloud is located at point A. If you put an electron in a box (as is done in quantum dot lasers), that electron is only mostly in the box. Part of the electron's wavefunction leaks through the walls of the box and out to infinity. This makes possible the effect of quantum tunneling, which is used in scanning tunneling microscopes. With the non-solid nature of atoms in mind, let us look at each of the possible meanings of touching. 1. If "touching" is taken to mean that two atoms influence each other, then atoms are always touching. Two atoms that are held a mile apart still have their wavefunctions overlapping. The amplitude of one atom's wavefunction at the point where it overlaps with the other atom's center will be ridiculously small if they are a mile apart, but it will not be zero. In principle, two atoms influence each other no matter where they are in the universe because they extend out in all directions. In practice, if two atoms are more than a few nanometers apart, their influence on each other typically becomes so small that it is overshadowed by the influence of closer atoms. Therefore, although two atoms a mile apart may technically be touching (if we define touching as the overlap of atomic wavefunctions), this touching is typically so insignificant that it can be ignored. What is this "touching"? In the physical world, there are only four fundamental ways for objects to influence each other: through the electromagnetic force, through the strong nuclear force, through the weak nuclear force, and through the force of gravity. Neutrons and protons that make up the nucleus of an atom are bound to each other and undergo reactions via the two nuclear forces. The electrons that make up the rest of the atom are bound to the nucleus by the electromagnetic force. Atoms are bound into molecules, and molecules are bound into everyday objects by the electromagnetic force. Finally, planets (as well as other large astronomical objects) and macroscopic objects on the planet's surface are bound together by gravity. If two atoms are held a meter apart, they are touching each other through all four fundamental forces. However, for typical atoms, the electromagnetic force tends to dominate over the other forces. What does this touching lead to? If two atoms are too far apart, their interaction is too weak compared to other surrounding bodies to amount to anything. When the two atoms get close enough, this interaction can lead to many things. The entire field of chemistry can be summed up as the study of all the interesting things that happen when atoms get close enough to influence each other electromagnetically. If two atoms are non-reactive and don't form covalent, ionic, or hydrogen bonds, then their electromagnetic interaction typically takes the form of the Van der Walls force. In the Van der Walls effect, two atoms brought close to each other induce electric dipole moments in each other, and these dipoles then attract each other weakly through electrostatic attraction. While the statement that "all atoms on the planet are always touching all other atoms on the planet" is strictly true according to this definition of touching, it is not very helpful. Instead, we can arbitrarily define an effective perimeter that contains most of the atom, and then say that any part of the atom that takes extends beyond that perimeter is not worth noticing. This takes us to our next definition of touching. 2. If "touching" is taken to mean that two atoms influence each other significantly, then atoms do indeed touch, but only when they get close enough. The problem is that what constitutes "significant" is open to interpretation. For instance, we can define the outer perimeter of an atom as the mathematical surface that contains 95% of the atom's electron mass. As should be obvious at this point, a perimeter that contains 100% of the atom would be larger than the earth. With 95% of the atom's electron probability density contained in this mathematical surface, we could say that atoms do not touch until their 95% regions begin to overlap. Another way to assign an effective edge to an atom is to say it exists halfway between two atoms that are covalently bonded. For instance, two hydrogen atoms that are covalently bonded to each other to form an H2 molecule have their centers separated by 50 picometers. They can be thought of as "touching" at this separation. In this approach, atoms touch whenever they are close enough to potentially form a chemical bond. 3. If "touching" is taken to mean that two atoms reside in the exact same location, then two atoms never touch at room temperature because of the Pauli exclusion principle. The Pauli exclusion principle is what keeps all the atoms in our body from collapsing into one point. Interestingly, at very low temperatures, certain atoms can be coaxed into the exact same location. The result is known as a Bose-Einstein condensate. Again, atoms never touch in the everyday sense of the word for the simple reason that they don't have hard boundaries. But in every other sense of the word "touch" that has meaning at the atomic level, atoms certainly touch.

context: tag/atom/ question: Does an atom have a color?

The answer really depends on how you define "having a color". The term "color" refers to visible light with a certain frequency, or a mixture of visible light frequencies. Therefore, the word "color" describes the frequency content of any type of visible light. Anytime visible light is present, we can describe it as having a certain color. With this in mind, there are many different ways an object can reflect or emit visible light. Thus, there are many ways an object can "have a color". While a single, isolated, atom can reflect or emit visible light in several of these ways, it does not participate in all the ways. If you define "having a color" very narrowly such that it only includes certain mechanisms, then atoms do not have color. If you define "having a color" more broadly, then atoms do have a color. Let us look at the different ways an object can reflect or emit visible light and apply each one to an atom. 1. Bulk reflection, refraction, and absorptionThe most common, everyday manner in which objects can send visible light to our eyes is through bulk reflection, refraction, and absorption. These three effects are all part of the same physical mechanism: the interaction of an external beam of light with many atoms at the same time. When white light, which contains all colors, hits the surface of a red apple, the light waves that are orange, yellow, green and blue get absorbed by the atoms in the apple's skin and converted to heat, while the red waves are mostly reflected back to our eyes. Some of the light is also transmitted through the apple skin and bent slightly as it goes through. We call this bent transmission of light "refraction". Some materials such as glass transmit a lot of the light while other materials such as apples transmit very little. The key point here is that traditional reflection, refraction, and absorption constitute a bulk phenomenon where each ray of light interacts with dozens to millions of atoms at the same time. This makes sense when you consider that visible light has a wavelength that is about a thousand times bigger than atoms. Visible light waves have a wavelength from 400 nanometers to 700 nanometers, depending on the color. In contrast, atoms have a width of about 0.2 nanometers. This discrepancy is why you can't see individual atoms using an optical microscope. The atoms are far smaller than the light you are trying to use to see them. The color of an object that results from traditional bulk reflection, refraction, and absorption is therefore a result of how several atoms are bound together and arranged, and not a result of the actual color of individual atoms. For example, take carbon atoms and bind them into a diamond lattice, and you get a clear diamonds. In contrast, take carbon atoms and bind them into hexagonal planes and you get gray graphite. The nature of the bonds between many atoms is what determines the traditional color of a material and not the type of atoms themselves. If you have no bonds at all between any atoms, you get a monoatomic gas, which is invisible (at least according to traditional reflection, refraction, and absorption). The color of most of the everyday objects around us, from apples to pencils to chairs, arises from traditional bulk reflection, refraction, and absorption. This mechanism of light delivery is so common and intuitive that we could define "having a color" narrowly to only include this mechanism. With this narrow definition in mind, therefore, a single atom is too small to have a color. 2. Thermal radiationHeat up a bar of iron enough and it glows red. You could therefore say that the color of a hot iron bar is glowing red. The red color of the iron bar in this case, however, is due to thermal radiation, which is a mechanism that is very different from bulk reflection, refraction, and absorption. In the mechanism of thermal radiation, the atoms of an object knock into each other so violently that they emit light. More accurately, the collisions cause the electrons and atoms to be excited to higher energy states, and then the electrons and atoms emit light when they transition back down to lower energy states. Since the collisions due to thermal motion are random, they lead to a wide range of energy excitations. As a result, the thermal radiation emitted contains many colors that span a broad band of frequencies. The interesting thing about thermal radiation is that its color is more a result of the temperature of the object and less a result of the material of the object. Every solid material glows red if you can get it to the right temperature without it evaporating or chemically reacting. The key to thermal radiation is that it is an emergent property of the interaction of many atoms. As such, a single atom cannot emit thermal radiation. So even if we expand the definition of "having a color" to include thermal radiation, individual atoms still have no color. 3. Rayleigh scatteringMore informatively called "long-wavelength scattering", Rayleigh scattering is when light does bounce off of single atoms and molecules. But because the light is so much bigger than the atoms, Rayleigh scattering is not really the "bouncing" of a light wave off of a small particle such as an atom, but is more a case of immersing the particle in the electric field of the light wave. The electric field induces an oscillating electric dipole in the particle which then radiates. Because the mechanism is so different, Rayleigh scattering of white light off of small particles always creates the same broad range of colors, with blue and violet being the strongest. The color of Rayleigh scattering is always the same (assuming the incident light is white) and is mostly independent of the material of the scattering object. Therefore, a single atom does have a color in the sense that it participates in Rayleigh scattering. For example, earth's atmosphere is composed mostly of small oxygen molecules (O2) and nitrogen molecules (N2). These molecules are far enough apart that they act like single, isolated molecules. When the daytime white sunlight hits isolated air molecules, it scatters according to Rayleigh scattering, turning the sky whitish-bluish-violet. The fact that we can see the daytime sky attests to the fact that small, individual molecules can exhibit some form of color. While we are talking about small molecules when it comes to the sky, the same principle applies to single atoms. Properly understood, the color in Rayleigh scattering belongs more to the interaction itself than to the actual types of atoms involved. Just because the sky is blue does not necessarily mean that nitrogen atoms are blue. Raman scattering is much rarer than Rayleigh scattering, but is nearly identical in the context of this discussion. Raman scattering is different in that some of the energy of the incident light is lost internally to the particle so that the scattered light is shifted lower in frequency. 4. Gas DischargeGas discharge (e.g. a Neon light) is perhaps the mechanism that would best fit the notion of an individual atom "having a color". Gas discharge is what happens when you take pure atoms, isolate them from each other in a low-density gas state and then excite them using an electric current. When the atoms de-excite, they emit visible light. The key here is that a particular atom can only being excited, de-excited, and emit light in certain ways. This leads to the color of an atom during gas discharge being very strongly tied to the type of atom involved. The frequency spectrum of an atom during gas discharge is considered the color "fingerprint" of that particular type of atom. For instance, true neon signs are always red because neon atoms themselves are red under gas discharge. Argon atoms are lavender under gas discharge, while sodium atoms are yellow and mercury atoms are blue. Many of the colors generated by "Neon" lights are attained by mixing different gases together. The "flame test" used in chemistry to detect certain atoms is essentially a less-controlled, less-pure version of a gas discharge lamp. Note that florescence (such as in a florescent light bulb), phosphorescence, and gas laser emission are all similar to gas discharge in that they involve exciting electrons in single atoms or simple molecules. As opposed to gas discharge, which forces an atom to emit all of its characteristic colors; florescence, phosphorescence, and laser emission all involve exploiting certain transitions so that only certain atomic colors are emitted. They can be considered special cases of gas discharge, as far as atomic color characterization is concerned. There are many other ways an object or material can emit or reflect visible light; such as through semiconductor electron-hole recombination (in LED's), Cherenkov radiation, chemical reactions, synchrotron radiation, or sonoluminescence; but all of these involve the interaction of many atoms or no atoms at all, and so are not pertinent to the current discussion. In summary: in the sense of traditional reflection, refraction, absorption, and thermal radiation, individual atoms are invisible. In the sense of Rayleigh scattering and gas discharge atoms do have a color.

context: tag/atom/ question: Does an electron in an atom move at all?

First of all, I assume you meant to ask the question, "Does an electron in a stable (non-transitioning) atomic state experience any movement?" Obviously, an electron that is transitioning between states is moving from one state to the other. But for an electron that is just staying in one stable state in an atom, the question is more interesting. Does it move? The answer could be yes or no depending on how we define motion and what form of the electron we consider to be truly real. The problem is that an electron is not a solid little ball that we can watch zip around. An electron is a quantum object. As such, an electron is partially particle-like and partially wave-like, but is really something more complex that is neither a simple wave nor a simple particle. The electron is described by a probabilistic quantum wavefunction, which spreads out through space and vibrates, but in such a way that it still has certain discrete properties such as mass. When bound in a stable state in an atom, the electron wavefunction spreads out into a certain shape called an "orbital". The orbital does not contain the electron or describe the average location of a little hard electron orbiting around. Rather, the orbital is the electron. When bound in a stable state in an atom, an electron behaves mostly like an oscillating three-dimensional wave, i.e. the orbital vibrates. It's a bit like a vibrating guitar string. When you pluck a guitar string, you get the string shaking, which is what creates the sound. Scientifically, we would say that you have excited a standing wave in the string. The guitar string is not moving in the sense of shooting off to the other side of the room. In this sense, the guitar string is not moving at all, but remains clamped to the guitar. But the guitar string is moving in the sense that it is vibrating when you pluck it. If you pick one spot on the plucked string and look at it closely, it is definitely moving from one location in space to another, back and forth repeatedly. By pulling the string, you transferred chemical energy in your arm to elastic energy in the stretched string. When you let go, the elastic energy was converted to motional energy (kinetic energy) as the string snapped back and started vibrating. The total kinetic energy of the entire string averaged over time is zero, since the overall string is not going anywhere with respect to the guitar. But the kinetic energy of any small part of the string at a given moment is not zero. In this way, a plucked guitar string experiences local motion but not overall motion. An electron in an atomic orbital state acts somewhat like a plucked guitar string. It is spread out in a three-dimensional cloud-like wavefunction that vibrates. Whereas a guitar string vibrates up and down, an atomic electron wavefunction simply vibrates strong and weak. The frequency at which the electron wavefunction vibrates is directly proportional to the total energy of the electron. Electrons in higher-energy atomic states vibrate more quickly. Because an electron is a quantum object with wave-like properties, it must always be vibrating at some frequency. In order for an electron to stop vibrating and therefore have a frequency of zero, it must be destroyed. In an atom, this happens when an electron is sucked into the nucleus and takes part in a nuclear reaction known as electron capture. With all of this in mind, an electron in a stable atomic state does not move in the sense of a solid little ball zipping around in circles like how the planets orbit the sun, since the electron is spread out in a wave. Furthermore, an electron in a stable atomic state does not move in the sense of waving through space. The orbital electron does move in the sense of vibrating in time. But the truth is more complicated than this simple picture depicts. There are two things that describe the electron in quantum theory: the electron's quantum wavefunction, and the magnitude squared of the electron's quantum wavefunction. (The "magnitude squared" operation just means that you drop phase factors such as negative signs and then take the square. For instance, the magnitude squared of negative three is nine.) Interestingly, experiments can only directly measure the magnitude squared of the electron wavefunction, and yet we need the original wavefunction in order to predict the outcome of many experiments. For this reason, some people say that the magnitude squared of the wavefunction is the only real entity, whereas the original wavefunction itself is just a mathematical crutch that is needed because our theory is inelegant. Is the magnitude squared of the electron wavefuntion the real physical entity or is the original wavefunction the real physical entity? This question is really a philosophical one and not a physical one, so I will not pursue the question here. To scientists, the question, "What is actually real?" is unimportant. We are more concerned with making the equations match the experiments. So what does all this have to do with an electron in an atom? The point is that an atomic electron's raw wavefunction does vibrate, but the magnitude squared of the wavefunction does not vibrate. In fact, physicists call stable atomic electron states "stationary states" because the magnitude squared of the wavefunction is constant in time. If you consider the raw wavefunction to be the truly physical entity, then you have to say that an electron in an atom experiences motion in the form of a vibration. If you consider the magnitude squared of the wavefunction to be the truly physical entity, then you have to say that an electron in an atom experiences no vibration, and therefore no motion. I consider the first choice to make more sense. You can mathematically show that certain atomic electron states contain angular momentum (i.e. rotational momentum). It's hard to make sense of the claim that an atomic electron contains angular momentum and at the same claim that the electron is completely motionless in every sense of the word. For this reason, I prefer to view the raw wavefunction as the truly physical entity, and therefore an electron in an atom experiences motion in the form of vibrations. But, again, the question, "What is actually real?" is a philosophical one and is unimportant in science. The bottom line is that the raw wavefunction of an electron in a stable atomic state experiences vibrational motion. Whether you consider this motion real or not is up to you.

context: tag/atom/ question: Does the human body contain minerals?

For the most part, the human body does not contain minerals. Scientifically speaking, a mineral is a naturally-occurring inorganic crystalline solid with a single chemical formula. Rocks are aggregates of minerals and organic materials. Except for in bones and teeth, the atoms and molecules making up a healthy body are not crystalline and are not solid. In this way, most of the molecules making up a human body fail to meet the definition of a mineral. Confusion often arises because many health professionals, nutritionists, and biologists misuse the word "mineral". When they say "mineral" in the context of human nutrition, they really mean "dietary element". Scientifically, the phrase "trace element" should really be used instead of "trace mineral" when talking about rare atoms required by the human body. The words "element" and "mineral" do not mean the same thing. A chemical "element" is a material containing only one kind of atom. In some cases, elements can form minerals, but they don't have to. For example, hydrogen is an element, but it is not a mineral because it is neither crystalline nor a solid. In contrast, quartz is indeed a mineral, but it is not an element because it contains more than one kind of atom. A gold nugget found in the ground is both an element (because it contains only gold atoms) and a mineral (because it has a natural crystalline solid structure). The small subset of materials in the world that contain only one kind of atom and have the atoms naturally bonded into a solid crystalline lattice are called "native element minerals". 1. Materials that are elements, but not minerals 2. Materials that are minerals, but not elements 3. Materials that are both minerals and elements (native element minerals): For example, table salt contains sodium atoms and chlorine atoms bound into a solid, ionic, cubic crystalline lattice. Naturally occurring salt is therefore a mineral. But as soon as you sprinkle salt on your tongue and begin to eat it, the salt dissolves in the water on your tongue. This means that the sodium and chlorine atoms break apart and float around in the water. You no longer have a mineral. You have elemental ions in solution. Your body then uses the dissolved elemental sodium ions to regulate fluid pressure levels and to send electrical signals along your nerves. In this way, you can eat minerals, but once you eat them, they aren't minerals anymore. Furthermore, you can get dietary elements from non-mineral sources. For example, you can get dietary sodium from milk, which is not a mineral. In fact, we get most of our dietary elements from non-mineral sources. The only mineral we really eat on a regular basis is table salt. The one exception in a healthy human is bone mineral, such as in bones and teeth. Bone mineral is indeed an inorganic, crystalline, solid with a single chemical formula and therefore qualifies as a genuine mineral. The mineral in your bones is called hydroxyapatite and has the chemical formula Ca5(PO4)3(OH). Our bodies build bone mineral on the spot, so we don't have to swallow hydroxyapatite crystals. But we do have to eat food with enough of the right kinds of atoms to build bone mineral. Looking at the chemical formula, we see that our bodies can't build bone mineral unless we supply it with enough calcium, phosphorus, oxygen, and hydrogen. A typical person has almost unlimited access to hydrogen and oxygen atoms through the water he drinks and the air he breaths. In contrast, a person can only get enough calcium and phosphorus to build healthy bones if he eats and drinks foods containing these elements. Minerals can also form in the human body as part of disease states such as in kidney stones.

context: tag/atom/ question: How can an electron leap between atomic levels without passing through all the space in between?

An electron that is transitioning between two atomic states does not skip any intervening space. The idea of a quantum leap is highly misleading and commonly misunderstood. First of all, an electron is a quantum object. As such, it acts both as a wave and as a particle at the same time. When bound as part of an atom, an electron mostly acts like a wave. An atomic electron spreads out into cloud-like wave shapes called "orbitals". If you look closely at the various orbitals of an atom (for instance, the hydrogen atom), you see that they all overlap in space. Therefore, when an electron transitions from one atomic energy level to another energy level, it does not really go anywhere. It just changes shape. The orbital shapes with more fluctuations (with more highs, lows, and bends to its shape) contain more energy. In other words, when an electron transitions to a lower atomic energy level, its wave shape changes to have less kinks in it. But the electron does not "leap" anywhere. The wave behavior of an electron in an atom is very similar to the behavior of classical waves on a guitar string. When you pluck a guitar string, you excite standing waves in the string, which are what make the sound. A certain string can only experience certain types of standing waves because the string is clamped down on both ends. The types of waves allowed on a particular string are called its "harmonics". The harmonics of a string depend on the string's length, tension, and mass density. A particular guitar string (of a particular length, tension, and mass) can therefore only play a certain type of sound, which is a combination of its harmonics. If you are very careful about how you pluck the string, you can create a wave on the string which is mostly the lower, fundamental harmonic (which has very few kinks), or you can create a wave on the string which is mostly a higher harmonic (which has many kinks). It takes more energy and is therefore harder to strongly excite the higher harmonic in a guitar string. Furthermore, if you pluck the string properly so as to strongly excite a higher harmonic wave in the string, you can even coax the string to transition down to the lower-energy harmonic. The wave on the guitar string does not go anywhere when the string transitions from a higher-energy state to a lower-energy state. The wave just changes shape. In a similar way, the discrete set of electron orbitals possible in a certain atom are effectively the harmonics of the atom. The electron can transition to a higher harmonic wave shape by absorbing energy and kinking more, or transition to a lower harmonic wave shape by emitting energy and kinking less (relaxing). It should be clear at this point that an electron that transitions in an atom does not make any kind of leap from one location in space to another location in space. But you may still be worried that the electron makes a leap from one energy level to another, and therefore bypasses all the in-between energy states. Although we are talking about a leap on the energy scale, and not a leap in space, such a leap may still strike you as unnatural, as it should. The fact is that an electron transitioning in an atom does not actually discontinuously leap from one energy level to another energy level, but makes a smooth transition. You may wonder, "Doesn't quantum theory tell us that an electron in an atom can only exist at certain, discrete energy levels?" Actually, no. Quantum theory tells us that an electron with a stationary energy can only exist at certain, discrete energy levels. This distinction is very important. By "stationary energy" we mean that the electron's energy stays constant for a fairly long period of time. The orbitals of a particular atom are not the only allowed states that an electron can take on in the atom. They are the only stable states of the atom, meaning that when an electron settles down to a particular state in an atom, it must be in one of the orbital states. When an electron is in the process of transitioning between stable states, it is not itself stable and therefore has less restrictions on its energy. In fact, an electron that transitions does not even have a well-defined energy. Innate quantum uncertainty arises in the electron's energy because of its transition. The quicker an electron transitions, the more uncertain its energy. This "innate quantum uncertainty" is not some metaphysical mystery, but is better understood as the wave spreading out over many values. Just as the electron can spread out into a wave that extends over a region of space, it can also spread out into a wave that extends over a region along the energy scale. If you calculate the average energy (the "expectation value") of this transitioning electron's spread of energies, you find that the electron's average energy does not instantaneously jump from one energy level to another. Rather, it smoothly transitions on average from the one energy level to the other energy level over a period of time. There is really no "instantaneous quantum leap" at all. The electron does not leap in space, and it does not leap up the energy scale. In fact, the term "quantum leap" is almost universally shunned by scientists as it is highly misleading. If you want a better mental image, you can think of the electron as quickly, but smoothly sliding along the energy scale from one stable state to the next. Because a typical atomic electron transition is so fast (often on the order of nanoseconds), it can seem to be nearly instantaneous to the slow human senses, but fundamentally it is not.

context: tag/atom/ question: How can radioactive decay just happen with nothing triggering it?

Although a radioactive decay event seems spontaneous and is unpredictable, it is indeed triggered by a physical agent. That physical agent is a vacuum fluctuation. Due to the quantum nature of the universe, a vacuum always contains vacuum fluctuations. Vacuum fluctuations are also called vacuum energy and zero-point energy. You can think of vacuum fluctuations as a sea of particles and antiparticles briefly popping into and out of existence. These particles originate from the vacuum itself due to intrinsic quantum uncertainty. Vacuum fluctuations are very short-lived (short-lived enough that they do not violate any conservation laws, within the level of quantum uncertainty). However, vacuum fluctuations are physically real and cause real effects. For instance, vacuum fluctuations tend to weaken, or screen, electromagnetic fields. Vacuum fluctuations also give rise to the Casimir effect as well as the Lamb shift in hydrogen energy levels. Every spontaneous quantum transition is actually triggered by a vacuum fluctuation. Lasers crucially depend on vacuum fluctuations. The light emitted from a laser is generated by a chain reaction of coherent photon emissions. This chain reaction is triggered at the beginning by a vacuum fluctuation. When an electron is put in an excited atomic state and left alone, it will eventually, naturally transition back down to its original state. When exactly this happens seems spontaneous, but it is actually triggered by a vacuum fluctuation. Similarly, vacuum fluctuations are what trigger a radioactive decay event. As part of the background quantum fluctuations that are intrinsic to the vacuum, a particle pops into existence just long enough to collide with the nucleus and trigger radioactive decay. The exact moment that radioactive decay happens is random and unpredictable because vacuum fluctuations are random and unpredictable. All of this just leads to the question, what triggers vacuum fluctuations? The answer is that nothing triggers vacuum fluctuations. They are constantly happening due to the quantum nature of the universe. Vacuum fluctuations are a well-established principle of mainstream physics. Note that there are a lot of incorrect, unscientific notions online about vacuum energy. Even though vacuum energy is real, it cannot be harnessed as a free source of energy. Quantum physics is strange, but it still must obey the physical conservation laws of the universe, including the law of conservation of energy. A vacuum fluctuation cannot permanently give its energy to another object. That would violate the law of conservation of energy. In fact, conservation of energy is what prevents a vacuum fluctuation from continuing to exist beyond its short life. However, if conditions are right, a vacuum fluctuation can be given enough energy from another object for it to be promoted to a stable particle that continues to exist. For instance, in an effect called spontaneous emission, an electron transitions spontaneously from an excited quantum state to a lower quantum state and emits a bit of light called a photon in the process. It looks like the photon is just created out of nowhere. However, the more accurate description is that a photon vacuum fluctuation collides with the electron, triggering the electron to transition. In the process, the electron gives some of its energy to the vacuum fluctuation, thereby promoting it to a regular photon that can continue to exist forever.

context: tag/atom/ question: How does dissolving a salt molecule in water make its atoms ionize?

Dissolving a salt molecule in water does not make its atoms ionize. The atoms in solid salts are already ionized long before touching water. Electrons in an atom can only take on specific wave states, and only one electron can occupy one wave state at a time. As a result, electrons in an atom take different states, starting from the lowest energy state and going upwards in energy until the electrons have all found distinct states. For various reasons that are not worth mentioning here, electron states in atoms tend to form various groups, with the states in the same group having very similar energies and states. Chemists call these groups of electron states "shells", even though they have nothing to do with literal shells. The interesting thing is that an atom with completely filled shells is very stable (all the available states in each group are occupied by electrons). On the other hand, an atom with its outermost shell only partially filled has a strong tendency to steal, lose, or share electrons from other atoms in order to fill its outermost shell and become stable. Such atoms are therefore chemically reactive. A well-known salt is sodium chloride (table salt), so let's use it as an example. A single neutral sodium atom has eleven electrons. Ten of these electrons fill states such that they form complete shells. The eleventh electron of sodium, however, is alone in the outermost, partially filled shell. Electrons are bound in atoms because their negative electric charge experiences electric attraction to the positive charge of the atom's nucleus. But for sodium, the negatively-charged electrons in the inner, completed shells do a good job of blocking, or screening, the attractive force of the nucleus on the eleventh electron. As a result, the eleventh electron of sodium is loosely bound to the atom and is ripe for being stolen by a more powerful atom. In contrast, chlorine (17 electrons) has all of its shells filled with electrons except for its outermost shell which is one electron short of being complete. There is a very strong attraction by the chlorine atom on an outside electron which is needed to complete its shell. Sodium and chlorine are therefore a perfect match. Sodium has one electron it is not holding onto very strongly, and chlorine is looking for one more electron to steal to fill its shell. As a result, a pure sample of sodium reacts strongly with a pure sample of chlorine and the end product is table salt. Each chlorine atom steals an electron from the sodium atom. Each sodium atom now has 11 positive protons and 10 negative electrons, for a net charge of +1. Each chlorine atom now has 17 positive protons and 18 negative electrons for a net charge of -1. The atoms have therefore been ionized by the reaction that forms solid table salt, all without the presence of water. Both the sodium and the chlorine ions now have completely filled shells and are therefore stable. This is a good example of an atom that naturally has an unequal number of electron and protons. The net positive sodium ion is now attracted to the net negative chlorine ion and this attraction forms what we call an "ionic bond". But, in reality, we don't have just one sodium ion sticking to ion chlorine ion. Instead, a lattice of many sodium ions ionically bonds to a lattice of chlorine ions, and we end up with a crystalline solid. Each sodium ion in the crystalline lattice of table salt is bound to the 6 nearest chlorine ions, and the same goes for each chlorine ion. The atoms in table salt are therefore already in the ionized state. Adding water does not ionize the atoms in salt, because they are already ionized. Instead, the water molecules stick to the already formed ions in the salt. The textbook titled Cell and Molecular Biology: Concepts and Experiments by Gerald Karp states, "A crystal of table salt is held together by an electrostatic attraction between positively charged Na+ and negatively charged Cl– ions. This type of attraction between fully charged components is called an ionic bond (or a salt bridge). Ionic bonds within a salt crystal may be quite strong. However, if a crystal of salt is dissolved in water, each of the individual ions becomes surrounded by water molecules, which inhibit oppositely charged ions from approaching one another closely enough to form ionic bonds." Each water molecule has a permanent dipole, meaning that one end is always slightly positively charged and the other end is always slightly negatively charged. The charged ends of the water molecules are so strongly attracted to the charged ions in the salt crystal that the water destroys the solid lattice structure of the salt and each sodium and chlorine ion becomes surrounded by a layer of sticky water molecules. In chemistry, we say the salt has been dissolved by the water. It's like a rock band exiting the limousine into a crowd of fans and becoming separated as each band member gets surrounded by his own circle of fans. If the atoms in solid salt were not ionized to begin with, the water would not do such a good job dissolving the salt.

context: tag/atom/ question: If I hammered and flattened a penny enough, could I cover the entire earth with it?

No. If you spread out the atoms from a single penny over the entire surface of the earth, you would no longer have a single piece of solid material since the atoms would be too far apart to bond to each other. Let's do some careful calculations to show this result. A modern United States penny has a mass of 2.500 grams according to the US Mint. Since a penny is composed of 97.50% zinc and 2.50% copper, it therefore contains 2.4375 grams of zinc and 0.0625 grams of copper. At a molar mass of 65.380 grams per mole for zinc and 63.546 grams per mole for copper, a penny therefore contains 0.037282 moles of zinc and 0.00098354 moles of copper. Since a mole of atoms contains 6.0221 x 1023 atoms, there are 2.2452 x 1022 zinc atoms and 5.9230 x 1020 copper atoms in a penny, for a total of 2.3044 x 1022 atoms in a penny. The earth has a surface area of 510,072,000 square kilometers, or 5.10072 x 1032 square nanometers. The surface area of the earth really depends on what you include in your definition of surface. For instance, if we wish to cover the area of every leaf on every tree and shrub with atoms from the penny, then this will change our answer. Surprisingly, it will not change our answer very much. Most of the earth is covered in relativity flat oceans, sandy deserts, snow fields, barren rocks, and meadows. Trees, shrubs, buildings, and other irregularly-shaped objects only cover a very small percentage of the earth (trees and buildings seem common to most of us humans because most of us live near crowded concentrations of trees and/or buildings). At any rate, we must pick some definition of earth's surface area to make any calculations. The number cited above does not include the surface area of tree leaves and other small irregularities. In the context of trying to cover the earth with a flattened penny, you can think of this definition of surface area as us lowering a sheet of zinc so that it drapes along the tops of the trees, but does not wrap around any of the leaves or branches of the trees. The thinnest we could ever hammer a sheet of material is one atom thick. We therefore assume that we are creating a one-atom-thick planar sheet of material. Using the above value for earth's surface area, we divide it by the number of atoms in a penny to find how much area each atom will occupy when the atoms are spread evenly across earth's surface. We get the value of 2.21347 x 1010 square nanometers per atom, or 0.0000343 square inches per atom. This may seem like a small area, but it is huge compared to the types of areas spanned by simple molecules. From here on out we will assume that all of the atoms in the penny are zinc atoms. This is a good assumption because almost all of the atoms in the penny are zinc atoms (97.5%). Also, in terms of atomic size and bonding distance, zinc and copper are nearly identical. When allowed to bond into a solid piece of material, zinc atoms arrange themselves into stacks of planar hexagonal grids. Therefore, in creating our one-atom thick sheet of zinc, we will arrange our zinc atoms along a planar hexagonal grid. If the penny's atoms are spread out uniformly on a hexagonal grid covering earth's surface, then each atom will have to be 159,870 nanometers away from its next nearest atoms in order to cover the entire earth (for a hexagonal grid of objects, the distance between nearest-neighbor objects is 1.0746 times the square root of the area that each object has to itself). In other words, taking the atoms of a single penny and spreading them out over the entire earth in a hexagonal arrangement will cause each atom to be about 0.16 millimeters away from its neighboring atoms. In order to form a solid chunk of material, atoms have to be close enough to form stable bonds. In regular pieces of zinc metal, stable chemical bonds are formed when each zinc atom is a distance of 0.26649 nanometers away from its next nearest zinc atoms. Therefore, the atoms of our smashed penny will be almost exactly 600,000 times too far apart to maintain stable bonds and constitute a solid piece of metal. With this kind of separation, we don't have a solid penny at all. We have a very dilute zinc gas spread over the earth. These widely separated atoms would blow around, dissolve into the ocean, mix with the clouds, and react with other atoms, so that we no longer have any type of distinct object that we would say is covering the earth. For this reason, you cannot hammer and flatten a penny until it covers the entire. The earth is simply too big and a penny has simply too few atoms to accomplish this task. Even if we break each zinc atom into 30 hydrogen atoms (ignoring all the messy details of the nuclear reactions involved), the atoms are still about a hundred thousand times too far apart to form stable chemical bonds. Besides, hydrogen atoms don't bond to form a solid chunk of material under normal conditions. These thoughts lead to another question: How much area can a smashed penny cover and still remain a solid chunk of material? To calculate this, we again realize that the thinnest a material can get is one atom thick. Also, the distance that zinc atoms need to be from each other and still stay bonded as a solid is 0.26649 nanometers, as already mentioned. This means that as part of a hexagonal planar arrangement of atoms, each zinc atom needs to occupy 0.0615 square nanometers of area. Multiply this number by the 2.3044 x 1022 atoms in a penny and we get a total area of 1417 square meters, which is about a quarter of the size of an American football field. In other words, if you had a special machine that carefully hammered a penny until it was everywhere just an atom thick, it would only cover a quarter of a football field. Keep in mind that at this thickness, you would be hard pressed to even see the penny and you would rip the penny when walking on it without feeling any resistance (think of walking on aluminum foil, but much, much thinner). To cover the entire earth's surface with one-atom-thick smashed pennies, you would need at least 360 billion pennies.

context: tag/atom/ question: What is the shape of an electron?

Depending on how you define "shape", an electron either has no shape, or an electron can take on various wave shapes. The shape of an electron is never statically round like an orange. The reason for this is that an electron is not a solid little ball, despite being so often portrayed this way in the popular media and in elementary-level science texts. Rather, electrons are quantum objects. Along with all other quantum objects, an electron is partly a wave and partly a particle. To be more accurate, an electron is neither literally a traditional wave nor a traditional particle, but is instead a quantized fluctuating probability wavefunction. This wavefunction looks in certain ways like a wave and in other ways like a particle. An electron looks like a particle when it interacts with other objects in certain ways (such as in high-speed collisions). When an electron looks more like a particle it has no shape, according to the Standard Model. In this context, physicists call an electron a "point particle," meaning that it interacts as if it is entirely located at a single point in space and does not spread out to fill a three-dimensional volume. If you find the concept of a fixed amount of mass being contained in the infinitely small volume of a single point illogical, then you should. But you have to realize that the electron is not literally a solid ball. This means that the electron's mass is not literally squeezed into an infinitely small volume. Rather, in certain cases where the electron looks somewhat like a particle, it interacts as if it were completely located at a single point. Therefore, in the sense of particle-like interactions, an electron has no shape. Note that an electron is a fundamental particle; it is not made out of anything else (according to our current experiments and theories). All fundamental particles interact as shapeless points when acting like particles. But not all quantum objects are fundamental, and therefore not all quantum objects are point particles. The proton, for instance, is not fundamental, but is instead composed of three quarks. The existence of particles inside a proton means that a proton must spread out to fill a certain space and have a certain shape. A proton is not a point particle, but is in fact a sphere with a radius of 8.8 × 10-16 meters. (Note that as a quantum object, a proton is not a solid sphere with a hard surface, but is really a quantized wave function that interacts in particle-like collisions as if it were a cloud-like sphere.) If the electron was composed of other particles, it could indeed have a shape when interacting like a particle. But it doesn't. The electron is a point particle. When an electron is behaving more like a wave, it can have all sorts of shapes, as long as its shape obeys the electron wave equation. An electron's wave equation, and therefore its shape, is a function of its energy and the shape of the potential well trapping it. For instance, when an electron is bound in a simple hydrogen atom, an electron can take on the familiar orbitals taught in elementary physics and chemistry classes, such as the shape shown on the right. In fact, the word "orbital" in this context really just means "the shape of an electron when acting as a wave bound in an atom". Each atomic orbital is not some mathematical average of where the electron has been, or some average forecast of where the electron may be. Each orbital is the electron, spread out in the quantum wavefunction state. In the sense of its wave-like state, an electron in a hydrogen atom can have the shape of layered spheres (the "s" states), layered dumbbells (the "p" states), layered four-leaf clovers (the "d" states), and other shapes at higher energies. In other atoms and molecules, an electron can take on even more complex shapes. An electron can also be trapped by other objects besides atoms. For instance, electrons trapped in the potential wells of a quantum cascade laser take on shapes that look more like traditional waves. An example of electron wavefunction shapes in a quantum cascade laser is shown on the right. Note that when scientists or journalists say "the shape of an electron is round," they are not talking about the literal shape. They are talking about the electric field distribution created by a free electron, which is entirely different from the actual shape.

context: tag/atom/ question: What is the strongest magnetic field possible? Is there a limit?

There is no firmly-established fundamental limit on magnetic field strength, although exotic things start to happen at very high magnetic field strengths. A magnetic field exerts a sideways force on a moving electric charge, causing it to turn sideways. As long as the magnetic field is on, this turning continues, causing the electric charge to travel in spirals. Once traveling in spirals, an electric charge acts like a small, oriented, permanent magnet and is therefore repelled from regions of high magnetic field gradient. Therefore, electric charges tend to spiral around magnetic field lines and be pushed away from regions where magnetic field lines bunch up. These two effects cause electric charges to get trapped along magnetic field lines that are strong enough. Examples of this effect include ions trapped in earth's ionosphere, radiation trapped in earth's radiation belts, hot plasma looping over the sun's surface in solar prominences, and plasmas contained in the laboratory using magnetic traps. The stronger the magnetic field gets, the more violently an electric charge is pushed sideways by the magnetic field, the faster and tighter it therefore spirals around in circles, and the stronger it gets pushed away from regions of high magnetic field gradient. Interestingly, all normal objects are made out of atoms, and all atoms are made out of electric charges: electrons and protons. Therefore, strong enough magnetic fields have the ability to deform and even break objects. When a magnetic field gets stronger than about 500,000 Gauss, objects get ripped to pieces by the intense forces. For this reason, scientists cannot build a machine that creates a magnetic field stronger than 500,000 Gauss and survives longer than a fraction of a second. Strong enough magnetic fields therefore destroy objects as we know them. Note that the magnetic fields used in medical MRI scanners are much weaker than 500,000 Gauss and are perfectly safe when used properly. While the destructive nature of strong magnetic fields places a practical limit on how strong of a field earthlings can create, it does not place a fundamental limit. Magnetic fields that surpass about a billion Gauss are so strong that they compress atoms to tiny needles, destroying the ordinary chemical bonds that bind atoms into molecules, and making chemistry as we know it impossible. Each atom is compressed into a needle shape because the electrons that fill most of the atom are forced by the magnetic field to spin in tiny circles. While such extremely strong magnetic fields are not possible on earth, they do exist in highly-magnetized stars called magnetars. A magnetar is a type of neutron star left over from a supernova. The intense magnetic field of a magnetar is created by superconducting currents of protons inside the neutron star, which were established by the manner in which the matter collapsed to form a neutron star. In a review paper presented at the Fifth Huntsville Gamma-Ray Burst Symposium, Robert C. Duncan summarized many of the theoretically-predicted exotic effects of magnetic fields that are even stronger: "In particular, I describe how ultra-strong fields At the most extreme end, a magnetic field that is strong enough could form a black hole. General Relativity tells us that both energy and mass bend spacetime. Therefore, if you get enough energy in one region, then you bend spacetime enough to form a black hole. The black hole does not destroy the magnetic field, it just confines it. Even stronger magnetic fields create larger black holes. It is currently not known whether this is actually possible, as there may be unknown mechanisms that limit a magnetic field from ever getting this strong. Certain unconfirmed extensions of current theories state that there is a fundamental limit to the strength of a magnetic field. For instance, if a magnetic field gets too strong, it may create magnetic monopoles out of the vacuum, which would weaken the magnetic field and prevent it from getting any stronger. However, since there is currently no evidence that magnetic monopoles actually exist, this purported limit is likely not real. We may someday discover a fundamental limit to the magnetic field strength, but there is currently no experimental evidence or well-established theoretical prediction that a limit exists.

context: tag/atom/ question: What makes radioactive atoms get old so quickly and decay?

Atoms don't age. Atoms radioactively decay when a lower-energy nuclear configuration exists to which they can transition. The actual decay event of an individual atom happens randomly and is not the result of the atom getting old or changing through time. The phrases "getting old" or "aging" are rather vague and could refer to a lot of things. For biological organisms and mechanical devices, "aging" usually refers to the progression of complex internal processes. A single atom does not have any internal biological or mechanical systems, and therefore does not age in this way. There is no clock inside an atom telling it that it is now a minute older. For other objects, "aging" refers to the wearing down or corrosion of the object because of repeated use or exposure to the environment. Atoms are too simple to wear down, corrode, or steadily change. No matter what reasonable definition we use for the word "aging", individual atoms don't do it. Note that aging is different from experiencing time. Everything, including atoms, experiences time. An atom can sit at on my desk on Tuesday and then fall off and sit on the carpet on Wednesday, because it experiences time. However, an isolated atom does not deterministically change from one day to the next. (An atom's electrons and nucleons can be excited, but these excited particles quickly relax back down to the ground state. Therefore, excitations do not fundamentally change the atom. Also, an atom's nucleus can change via nuclear reactions, but these changes are random rather than the result of aging.) If atoms don't age, how do radioactive atoms know when to decay? How can we possibly say that a radioactive isotope has a lifetime if it does not age? The answer is that radioactive atoms don't know when to decay. In fact, an individual radioactive atom does not decay at a particular, predictable time. It's not like an atom has an internal clock ticking away telling it when it's time to fall apart. Rather, an atom decays at a random time, completely independent of how long it has been in existence. Radioactive decay is governed by random, statistical effects and not by internal deterministic machinery. A particular radioactive atom can and will decay at any time. The "lifetime" of a radioactive isotope is not a description of how long a single atom will survive before decaying. Rather, it is a description of the average amount of time it takes for a significant portion of a group of radioactive atoms to decay. A characteristic lifetime does not come about by the progression of internal machinery, but by the statistical behavior of a large group of atoms governed by probability. An analogy may be helpful. A standard six-sided die will show a single number between "1" and "6" when rolled. Let us agree that when we roll a "6", we smash the die to pieces and the game is over for that particular die. We begin rolling the die and get a "3", and then a "1" and then a "5". Next we roll a "6" and destroy the die as agreed upon. Since the die was destroyed after four rolls, we say that this particular die had an individual lifetime of four rolls. Now we get a new die and repeat the game. For this die, we roll a "2", then a "1", then "4", "3", "1", "5", and then finally a "6". This die therefore had an individual lifetime of seven rolls. When we repeat this game for many dice, we discover that the individual lifetime of a particular die can be anything from one roll to hundreds of rolls. However, if we average over thousands of individual lifetimes, we find that the dice consistently have an average lifetime of about six rolls. Since an individual die has no internal machinery telling it to show a "6" after a certain number of rolls, the individual lifetime of a die is completely random. However, since the random events are governed by probabilities, we can experimentally find a fixed characteristic average lifetime of a group of dice by averaging over a large ensemble of dice. We can also mathematically find the average lifetime by calculating probabilities. For the die, there are six possible outcomes to a single roll, each with equal probability of occurring. Therefore, the probability of rolling a "6" and destroying the die is 1 out of 6 for every roll. For this reason, we expect it to take six rolls on average to roll a 6 and destroy the die, which is just what we found experimentally. The dice do not have a predictable average lifetime because they age, but because they experience probabilistic events. In the same way, atoms do not age and yet we can identify a meaningful decay lifetime because of the probabilities.

context: tag/atom/ question: Why do atoms always contain the same number of electrons and protons?

Atoms do not always contain the same number of electrons and protons, although this state is common. When an atom has an equal number of electrons and protons, it has an equal number of negative electric charges (the electrons) and positive electric charges (the protons). The total electric charge of the atom is therefore zero and the atom is said to be neutral. In contrast, when an atom loses or gains an electron (or the rarer case of losing or gaining a proton, which requires a nuclear reaction), the total charges add up to something other than zero. The atom is then said to be electrically charged, or "ionized". There is a major difference between the neutral state and the ionized state. In the neutral state, an atom has little electromagnetic attraction to other atoms. Note that the electric field of a neutral atom is weak, but is not exactly zero because the atom is not a point particle. If another atom gets close enough to the atom, they may begin to share electrons. Chemically, we say that the atoms have formed bonds. In contrast to neutral atoms, the field due to an ionized atom is strong, even at larger distances. The strong electric field of ions makes them strongly attracted to other atoms and molecules, to the point of being highly chemically reactive. Ionized atoms can be free radicals, which are atoms with a dangling bond that are highly reactive. In the human body, free radicals can react with DNA, leading to mutations and possibly cancer. Atoms become ionized when light with enough energy knocks off some of their electrons. Only light waves at the frequencies of X-rays and gamma rays have enough energy to ionize atoms and therefore lead to cancer. The cancer-causing power of only certain frequencies is why you can use your cell phone as much as you want, but you can only get an X-ray image taken on rare occasions. Free radicals occur naturally in your body. They only become dangerous when there are more free radicals than your body can handle. But not all ions in the body are bad. Because of the charged nature of ions, the human body makes use of them to pass electric signals through nerves. The body also uses ions to control fluid levels and blood pressure. The most-used ions in the human body are sodium, potassium, calcium, magnesium and chloride. Ions are also created whenever you electrostatically charge an object, such as when you rub a balloon on your hair. For this reason, your clothes dryer machine can be thought of as an ion maker. As clothes rub together in the machine, electrons get knocked from one atom to another. The result is the all-too familiar static cling. Electricity and strong electric fields do a good job of creating ions (think lightning). The neutral state of an atom is typically the most stable configuration (unless molecular bonds and the chemical environment complicates the picture), so ions tend to discharge and return to their neutral state over time. The reason for this is that, as an ion, the atom has a strong electric field that attracts the needed electron or the needed atom to take its extra electron. But once the atom becomes neutral, it has an equal number of electrons and protons, it does not have a very strong field, and therefore has little possibility of changing.

context: tag/atom/ question: Why don't atoms collapse if they are mostly empty space?

Atoms are not mostly empty space because there is no such thing as purely empty space. Rather, space is filled with a wide variety of particles and fields. Sucking all the particles and fields out of a certain volume won't make the space completely empty because new particles will still flash into existence due to vacuum energy. Additionally, the Higgs field can't be removed. Even if we ignore every kind of field and particle except electrons, protons and neutrons, we find that atoms are still not empty. Atoms are filled with electrons. It's true that a large percentage of the atom's mass is concentrated in its tiny nucleus, but that does not imply that the rest of the atom is empty. Rather, it implies that the rest of the atom has relatively low density. The misconception of an empty atom is taught by incorrect elementary-level science books and is based on the false picture of electrons as balls. In this view, the atom consists of electron balls whizzing around the atomic nucleus which is itself a ball. In this picture, the space between the electrons and the nucleus is therefore empty space. While this picture (the Bohr model) is simple to imagine, it was shown to be wrong almost a century ago. Electrons (as well as all particles) are partially particle-like and partially wave-like, depending on the situation. When bound in atoms in an undisturbed state, electrons act like waves. These waves are three-dimensional probability density waves that spread out to fill the entire atom. The electrons do not spread out uniformly, but rather follow specific distribution patterns called "orbitals". The shape of the orbitals underpin all chemical reactions. As an example of some orbitals, the single-electron density distribution is shown on the right for hydrogen in the first few lowest states. The lighter points indicate regions where the electron has a higher density. Note that each image represents a single electron. The different light spots and bands in a single image are all part of a single electron's wave state. Because bound electrons spread out into fuzzy density waves, there is no definite "edge" to an atom. The electron actually spreads out to fill all space, although far away from the atom it is thin enough to be negligible. Interestingly, electrons in the atom even spread out so as to overlap with the nucleus itself. This electron-nucleus overlap makes possible the effect of electron capture, where a proton in the nucleus can react with an electron and turn into a neutron. If atoms were mostly empty space, we could remove this space and shrink atoms. In reality, atoms do not contain any empty space. Rather, they are filled completely with spread-out electrons, making the shrinking of atoms impossible.

context: tag/atom/ question: Why don't electrons in the atom enter the nucleus?

Electrons in the atom do enter the nucleus. In fact, electrons in the s states tend to peak at the nucleus. Electrons are not little balls that can fall into the nucleus under electrostatic attraction. Rather, electrons are quantized wavefunctions that spread out in space and can sometimes act like particles in limited ways. An electron in an atom spreads out according to its energy. The states with more energy are more spread out. All electron states overlap with the nucleus, so the concept of an electron "falling into" or "entering" the nucleus does not really make sense. Electrons are always partially in the nucleus. If the question was supposed to ask, "Why don't electrons in the atom get localized in the nucleus?" then the answer is still "they do". Electrons can get localized in the nucleus, but it takes an interaction to make it happen. The process is known as "electron capture" and it is an important mode of radioactive decay. In electron capture, an atomic electron is absorbed by a proton in the nucleus, turning the proton into a neutron. The electron starts as a regular atomic electron, with its wavefunction spreading through the atom and overlapping with the nucleus. In time, the electron reacts with the proton via its overlapping portion, collapses to a point in the nucleus, and disappears as it becomes part of the new neutron. Because the atom now has one less proton, electron capture is a type of radioactive decay that turns one element into another element. If the question was supposed to ask, "Why is it rare for electrons to get localized in the nucleus?" then the answer is: it takes an interaction in the nucleus to completely localize an electron there, and there is often nothing for the electron to interact with. An electron will only react with a proton in the nucleus via electron capture if there are too many protons in the nucleus. When there are too many protons, some of the outer protons are loosely bound and more free to react with the electron. But most atoms do not have too many protons, so there is nothing for the electron to interact with. As a result, each electron in a stable atom remains in its spread-out wavefunction shape. Each electron continues to flow in, out, and around the nucleus without finding anything in the nucleus to interact with that would collapse it down inside the nucleus. It's a good thing too, because if electron capture was more common, matter would not be stable but would collapse down to a handful of nuclei.

context: tag/atom/ question: Why don't metals burn?

Metals do burn. In fact, most metals release a lot of heat when they burn and are hard to put out. For example, thermite is used to weld train rails together. The fuel in thermite is the metal aluminum. When thermite burns, the aluminum atoms bond with oxygen atoms to form aluminum oxide, releasing a lot of heat and light in the process. As another example, hand-held sparklers use aluminum, magnesium, or iron as the fuel. The flame of a sparkler looks different from the flame of a wood fire because metal tends to burn hotter, quicker, and more completely than wood. This is what gives a lit sparkler its distinctive sparkling flame. In fact, most fireworks contain metal fuels. As another example, old flash tubes used in photography were nothing more than burning bits of magnesium in a glass bulb. Also, the space shuttle's solid rocket boosters used aluminum as the fuel. Some metals, such as sodium, burn so well that we don't make everyday objects out of them. Any boy scout who has started a fire using steel wool can attest to the fact that metals burn. Still, you may wonder why holding up a lit match to aluminum foil does not make it burn. Similarly, placing a metal pan on a kitchen flame does not make the pan burst into flames. In everyday situations, metal objects don't seem to burn so much. How can this be possible if metals actually do burn? There are three main factors involved. First, if you have a solid chunk of metal, it is hard to get oxygen atoms close enough to the majority of the metal atoms to react. In order to burn the metal, each metal atom has to get close enough to an oxygen atom to bond to it. For large chunks of metal; like spoons, pots, and chairs; most of the atoms are simply too deeply buried to have any access to oxygen molecules. Furthermore, metals don't vaporize easily. When you burn a chunk of wood or a wax candle, the fuel particles readily vaporize, meaning that with just a little heat, they shoot out into the air where they have better access to oxygen atoms. In contrast, solid metals tend to have their atoms very tightly bound together, meaning that it is much harder to use heat to vaporize the metal. Also, organic materials like wood or cloth contain a lot of their own oxygen, whereas raw metals don't. This is one reason why it is much harder to burn a metal spoon than a wooden spoon, even though they both consist of large chunks of material. With this fact in mind, all we have to do is manually break apart the metal atoms in order to get them to burn better. In practice, this means grinding the metal down to a fine powder. When used as a fuel in commercial products and industrial processes, metals usually come in the form of a powder. Although, even if you have ground a metal block down to a powder, it still won't burn as efficiently as it could if you just use the oxygen in the ambient air. The problem is that air does not actually contain that much oxygen. Air is mostly nitrogen. The best approach is to mix oxygen directly into the powder. Raw oxygen won't work so well because it is a gas at room temperature and will float away. Instead, solid compounds containing loosely bound oxygen atoms can be mixed into the metal powder. In this way, the oxygen atoms can stably sit right next to the metal atoms, ready to react. This approach is the most efficient way to get metals to burn well. For example, thermite is just aluminum powder (the fuel) mixed in with iron oxide (the oxygen source). The second reason that everyday metal objects don't burn so well is that metals generally have a higher ignition temperature. Because the atoms in a typical metal are so tightly bound to each other, it takes more energy to break them apart and free them up, even if the oxygen atoms are sitting right next to them. Candle flames, match flames, campfires, and kitchen stove flames simply don't get hot enough to ignite most metals, even if the metal is in the ideal powder form. Chemical reactions that produce higher temperatures must be used to ignite most metals. For example, the combustion of magnesium strips can be used to ignite thermite. The last reason that everyday metal objects don't burn so well is that metals tend to be excellent thermal conductors. This means that if a spot on a metal object starts to build up some heat, the heat very quickly flows through the metal to cooler parts of the object. This makes it hard to build up enough heat in one spot to reach the ignition temperature. Even if you have a flame torch running at a high enough temperature, it is difficult to use the torch to ignite a chunk of metal because the heat keeps flowing away through the metal. In summary, because most atoms in a solid chunk of metal don't have access to oxygen atoms, because metals have a high ignition temperature, and because metals are good thermal conductors, they don't burn very well in everyday situations. The ideal way to get a metal to burn is to grind it into a powder, mix in an oxidizer, contain it so heat can't escape, and then apply a high temperature ignition device.

context: tag/black-hole/ question: Are there different types of black holes?

Yes, there are different types of black holes. The most straightforward way to classify black holes is according to their mass. You may think that because a black hole is in essence just a clump of matter that is dense enough to trap light, black holes of all masses should exist. In other words, black holes should exist along a continuous range of masses. However, that is not what we find in practice. The only types of black holes that have been firmly established to exist are stellar-mass black holes and supermassive black holes. Stellar-mass black holes are formed from the gravitational collapse of a single star or from the merger of two neutron stars. Therefore, stellar-mass black holes have masses similar to the masses of stars. More specifically, stellar-mass black holes have masses ranging from about 3 times the mass of our sun to about 50 times the mass of our sun. In contrast, supermassive black holes have a mass greater than about 50,000 times the mass of our sun and are typically millions to billions times the mass of our sun. Supermassive black holes are far too large to have formed from the gravitational collapse of a single star. However, scientists do not currently know how supermassive black holes form. Supermassive black holes are always found at the center of a galaxy and almost all galaxies have a supermassive black hole at its center. This seems to suggest that each supermassive black hole is formed as part of the formation of its galaxy. Interestingly, there seems to be zero or very few black holes with a mass between that of stellar black holes and supermassive black holes. The range between about 50 times the mass of our sun to about 50,000 times the mass of our sun seems like a huge range over which black holes typically do not exist. Any black hole with a mass in this range is called an intermediate black hole. A few decades ago, intermediate black holes were thought to not exist at all. However, recent observations seem to suggest that intermediate black holes may exist but are very rare. There may be many reasons why intermediate black holes are very rare, but one reason is likely the most important. This reason is that there are not any common physical mechanisms in the universe that can collapse matter down to a black hole of intermediate size. Most stars are too small to collapse down to intermediate black holes and whatever galactic mechanism produces supermassive black holes seems to involve masses that are too large to produce intermediate black holes. This is an ongoing area of research. Also interestingly, there seems to be no black holes that have a mass smaller than that of the stars (which spans a huge range from the mass of planets down to masses smaller than that of electrons). Termed mini black holes or micro black holes, the laws of physics as currently understood seem to suggest that it is indeed physically possible for them to exist. However, scientists cannot find any evidence of mini black holes existing. Perhaps mini black holes can exist but there is no natural physical mechanism that can produce them. Or perhaps mini black holes cannot exist for fundamental physical reasons. If mini black holes did exist, it is likely that they would quickly evaporate away to nothing through Hawking radiation. This is also an ongoing area of research. These concepts are summarized in the table below, where the numbers shown are approximate and the values for mass are that value times the mass of our sun. Another way to classify black holes is according to physical structure. The point-of-no-return nature of black holes means that most of the information that enters a black hole is destroyed or permanently locked away from the rest of the universe. For instance, there are no such things as rocky black holes or gaseous black holes like there are rocky planets and gaseous planets. All the rocks, gases, and dust particles that fall into a black hole get crushed down to a featureless speck of mass or ring of mass. Similarly, there are no such things as hot black holes or cold black holes. Also, there is no difference between a black hole formed from regular matter and a black hole formed from antimatter (although I should note here that there are not actually clumps of antimatter in our universe that are large enough to form black holes). The very nature of a black hole leads it to collapse all of its mass and energy down to an indistinguishable clump and to smooth out all irregularities and asymmetries. Because everything becomes indistinguishable within a black hole, the word "mass" in this context actually refers to mass and energy. However, a black hole does indeed retain a few properties that are externally measurable: its overall mass, its overall electric charge, and its overall spinning rate. Note that a few other properties of a black hole such as radius and magnetic moment are externally measurable, at least in principle, but these are not independent parameters. In other words, they are directly dependent, and arise from, the black hole's mass, charge, and spin. These three properties are the only independent, externally-observable black hole properties. If two isolated black holes had the same mass, charge, and spin, they would be indistinguishable. The reason that the total mass, total charge, and total spin of a black hole are measurable from outside of the black hole despite being internal properties is that they obey universal conservation laws. Another way of saying this is that these properties are connected to fundamental symmetries in spacetime and therefore affect spacetime curvature. We can therefore classify black holes according to mass, charge, and spin. I have already described classifying black holes by mass. If we just focus on charge and spin, we can make the following classification categories: black holes that are not spinning and have no net electric charge (Schwarzschild black holes), black holes that are spinning and have no net electric charge (Kerr black holes), black holes that are not spinning and do have a net electric charge (Reissner-Nordstrom black holes), and black holes that are spinning and do have a net electric charge (Kerr-Newman black holes). In our universe, black holes are almost always spinning (because they form from spinning bodies of matter) and almost always have zero net electric charge (because of the tendency of electric charge to attract opposite types of electric charge and self-neutralize). Therefore, Kerr black holes are by far the most common. These concepts are summarized in the table below. Combining all of the concepts in this article, we see that the most common black holes in our universe are spinning, uncharged stellar-mass black holes and spinning, uncharged supermassive black holes. Black Hole TypeNameHow Common non-spinning, unchargedSchwarzschild Black Holerare spinning, uncharged Kerr Black Holecommon non-spinning, chargedReissner-Nordstrom Black Holerare spinning, chargedKerr-Newman Black Holerare Topics: black hole, charge, energy, gravity, mass, spin The reason that the total mass, total charge, and total spin of a black hole are measurable from outside of the black hole despite being internal properties is that they obey universal conservation laws. Another way of saying this is that these properties are connected to fundamental symmetries in spacetime and therefore affect spacetime curvature. We can therefore classify black holes according to mass, charge, and spin. I have already described classifying black holes by mass. If we just focus on charge and spin, we can make the following classification categories: black holes that are not spinning and have no net electric charge (Schwarzschild black holes), black holes that are spinning and have no net electric charge (Kerr black holes), black holes that are not spinning and do have a net electric charge (Reissner-Nordstrom black holes), and black holes that are spinning and do have a net electric charge (Kerr-Newman black holes). In our universe, black holes are almost always spinning (because they form from spinning bodies of matter) and almost always have zero net electric charge (because of the tendency of electric charge to attract opposite types of electric charge and self-neutralize). Therefore, Kerr black holes are by far the most common. These concepts are summarized in the table below. Combining all of the concepts in this article, we see that the most common black holes in our universe are spinning, uncharged stellar-mass black holes and spinning, uncharged supermassive black holes. Black Hole TypeNameHow Common non-spinning, unchargedSchwarzschild Black Holerare spinning, uncharged Kerr Black Holecommon non-spinning, chargedReissner-Nordstrom Black Holerare spinning, chargedKerr-Newman Black Holerare Topics: black hole, charge, energy, gravity, mass, spin The reason that the total mass, total charge, and total spin of a black hole are measurable from outside of the black hole despite being internal properties is that they obey universal conservation laws. Another way of saying this is that these properties are connected to fundamental symmetries in spacetime and therefore affect spacetime curvature. We can therefore classify black holes according to mass, charge, and spin. I have already described classifying black holes by mass. If we just focus on charge and spin, we can make the following classification categories: black holes that are not spinning and have no net electric charge (Schwarzschild black holes), black holes that are spinning and have no net electric charge (Kerr black holes), black holes that are not spinning and do have a net electric charge (Reissner-Nordstrom black holes), and black holes that are spinning and do have a net electric charge (Kerr-Newman black holes). In our universe, black holes are almost always spinning (because they form from spinning bodies of matter) and almost always have zero net electric charge (because of the tendency of electric charge to attract opposite types of electric charge and self-neutralize). Therefore, Kerr black holes are by far the most common. These concepts are summarized in the table below. Combining all of the concepts in this article, we see that the most common black holes in our universe are spinning, uncharged stellar-mass black holes and spinning, uncharged supermassive black holes.

context: tag/black-hole/ question: Can you go fast enough to get enough mass to become a black hole?

Traveling at high speed does not affect your mass, even in Einstein's theory of Special Relativity. For some reason, pre-college teachers, popular science books, and older physics textbooks claim that objects gain mass when they are traveling at higher speeds. This claim is wrong. If you define something called "relativistic mass" that is completely different from regular mass, then this claim could be made to look true. But doing so is very confusing and misleading. Today's physicists no longer treat the motion energy of an object as "relativistic mass" because doing so is misleading. When an object gains speeds, the entity that it gains is called "kinetic energy", even in Special Relativity. The total energy of a moving object is therefore its rest energy plus its kinetic energy. The rest energy of an object is contained in its mass. The relativistic total energy of a moving object is: E = mc2/(1-v2/c2)1/2 In this equation, m is the mass of the object (which does not change no matter how fast the object is moving), c is the speed of light, and v is the speed of the object. If the object is not moving, v = 0, then there is no kinetic energy and the total energy just equals the rest energy. Plugging v = 0 into the equation above, we end up with the famous equation E = mc2. The rest energy of an object is therefore mc2, telling us that the rest energy is contained completely in the form of mass. The kinetic energy EK is therefore the total energy minus the rest energy: EK = E – mc2 EK = mc2(1/(1-v2/c2)1/2 – 1) This equation shows us that as the speed increases, the kinetic energy increases, but the mass never changes. In the limit that the speed of the object v approaches the speed of light in vacuum c, the kinetic energy becomes infinite. The law of conservation of energy tells us that to get an object traveling with infinite kinetic energy, we have to give it infinite energy. This act is clearly impossible as there is only a finite amount of energy in the observable universe. This facts means that objects with mass can never travel exactly at the speed of light in vacuum, as that state would require infinite energy. But objects can get very close to the speed of light. Mass is the property of an object that describes two things: When an object is traveling at a high speed, its resistance to acceleration does not change and its ability to experience gravity does not change. The mass of an object therefore does not change when it travels at high speed. This fact is predicted by Einstein's theories and verified by experiment. An object can never be turned into a black hole, or even be made slightly overweight by speeding it up.

context: tag/black-hole/ question: Does every black hole contain a singularity?

In the real universe, no black holes contain singularities. In general, singularities are the non-physical mathematical result of a flawed physical theory. When scientists talk about black hole singularities, they are talking about the errors that appear in our current theories and not about objects that actually exist. When scientists and non-scientists talk about singularities as if they really exist, they are simply displaying their ignorance. A singularity is a point in space where there is a mass with infinite density. This would lead to a spacetime with an infinite curvature. Singularities are predicted to exist in black holes by Einstein's theory of general relativity, which is a theory that has done remarkably well at matching experimental results. The problem is that infinities never exist in the real world. Whenever an infinity pops out of a theory, it is simply a sign that your theory is too simple to handle extreme cases. For example, consider the simplest physical model that accurately describes how waves travel on a guitar string. If you drive such a string at its resonant frequency, the simplest model predicts that the vibration of the string will increase exponentially with time, even if you are driving it gently. The string actually does this... up to a point. The problem is that the exponential function quickly approaches infinity. The model therefore predicts that a guitar string driven at its resonant frequency will, in time, vibrate passed the moon, passed the stars, out to infinity, and then back. Does the string actually vibrate infinitely just because the model says so? Of course not. The string snaps long before vibrating out to the moon. The appearance of the infinity in the model therefore indicates that the model has reached its limitations. The simple model of waves on a string is correct as long as the vibrations are small. To avoid the infinity in the equations, you need to build a better theory. For vibrating guitar strings, all you have to do is add to the model a description of when guitar strings snap. As another example, consider a thin glass drinking goblet. If a singer sings a note at the right pitch, the goblet begins to shake more and more. The simplest model would predict that, in time, the goblet will be shaking infinitely. In real life, this does not happen. Instead, the singing causes the goblet to shatter to pieces when the shaking becomes too violent. Every scientific theory has its limitations. Within its realm of validity, a good theory matches experimental results very well. But go beyond the limitations of a theory, and it starts giving predictions that are inaccurate or even just nonsense. Physicists hope to one day develop a theory of everything that has no limitations and is accurate in all situations. But we do not have that yet. Currently, the best physics theories are quantum field theory and Einstein's general relativity. Quantum field theory very accurately describes the physics from the size of humans down to the smallest particle. At the same time, quantum field theory fails on the planetary and astronomical scales, and, in fact, says nothing at all about gravity. In contrast, general relativity accurately predicts gravitational effects and other effects on the astronomical scale, but says nothing about atoms, electromagnetism, or anything on the small scale. Using general relativity to predict an electron's orbit around an atomic nucleus will give you embarrassingly bad results, and using quantum field theory to predict earth's orbit around the sun will likewise give you bad results. But as long as scientists and engineers use the right theory in the right setting, they mostly get the right answers in their research, calculations, and predictions. The good thing is that general relativity does not overlap much with quantum field theory. For most astronomical-scale and gravitational calculations, you can get away with using just general relativity and ignoring quantum field theory. Similarly, for small-scale and electromagnetic calculations you can get away with using quantum field theory and ignoring general relativity. For example, you use just quantum field theory to describe what the atoms in the sun are doing, but use just general relativity to describe what the sun is doing as a whole. Many efforts are underway to consistently unite quantum field theory and general relativity into one complete theory, but none of these efforts have been fully solidified or confirmed by experiments. Until a successful theory of everything comes along, physicists can mostly get by with using both general relativity and relativistic quantum theory in a patchwork manner. This approach mostly works because the realms of validity of both theories do not overlap much. But this approach breaks down when you have an astronomical object collapsed down to quantum sizes, which is exactly what a black hole is. A black hole forms when a massive star runs out of the fuel needed to balance out gravity, and collapses under its own gravity to a very small size. General relativity predicts that the star collapses to an infinitely small point with infinite density. But, as should now be clear, such a beast does not really exist in the real world. The appearance of a black hole singularity in general relativity simply indicates that general relativity is inaccurate at very small sizes, which we already knew. You need quantum field theory to describe objects of small sizes. But, quantum field theory does not include gravitational effects, which is the main feature of a black hole. This fact means that we will not known exactly what is going on in a black hole until scientists can successfully create a new theory that accurately describes small sizes and strong gravitational effects at the same time. Whatever the new theory ends up telling us, it will most certainly not say that there are singularities in black holes. If it did, that outcome would simply indicate that the new theory is just as bad as the old theory. In fact, one of the requirements for the future theory of everything is that it not predict singularities in black holes. In this sense, the interiors of black holes are the final frontier for theoretical physics. Just about everything else in the universe can be accurately described (at least in principle) using our current theories.

context: tag/black-hole/ question: How can there be anything left in the universe? Don't black holes suck everything in?

Black holes don't suck everything in. Black holes have gravity in the same ways stars have gravity. For this reason, planets orbit safely around black holes in the same way they orbit around our sun. The only difference between a black hole and a star is that the black hole has a small enough radius that light inside this radius cannot escape. There is a gigantic black hole at the center of our galaxy, and yet we are not in any danger of being sucked into it. Our entire galaxy orbits safely around the black hole at its center, as described in the book "The Galactic Supermassive Black Hole" by Fulvio Melia. It is true that if anything gets too close to a black hole, it will "hit it" and fall in, but this is generally no different from an earth satellite crashing into the ocean after years of slowing down due to friction

context: tag/black-hole/ question: How does a black hole give off light?

A black hole itself does not give off any light. That is why it is called black. However, matter that is near a black hole can give off light in response to the black hole's gravity. A black hole is a region of space where gravity is so strong that nothing can escape, not even light. It might be surprising to you to hear that gravity can affect light even though light has no mass. If gravity obeyed Newton's law of universal gravitation, then gravity would indeed have no effect on light. However, gravity obeys a more modern set of laws known as Einstein's general theory of relativity. According to general relativity, gravity is actually caused by a curving of space and time. Since light travels in a straight line through straight spacetime, the curving of spacetime causes light to follow a curved path. The gravitational curvature of light's path is a weak enough effect that we don't notice it much on earth. However, when gravity is very strong, the bending of light's path becomes significant. A black hole is a region where spacetime is so curved that every possible path which light could take eventually curves and leads back inside the black hole. As a result, once a ray of light enters a black hole, it can never exit. For this reason, a black hole is truly black and never emits light. However, this restriction only applies to points inside the black hole. Light that is near a black hole, but not actually inside it, can certainly escape away to the rest of the universe. This effect is in fact what enables us to indirectly "see" black holes. For instance, there is a supermassive black hole at the center of our galaxy. If you point a high-power telescope exactly at the center of our galaxy and zoom way in, you don't see anything. A black hole by itself is truly black. However, the black hole's gravity is so strong that it causes several nearby stars to orbit the black hole. Since these stars are actually outside of the black hole, the light from these stars can reach earth just fine. When scientists pointed a high-power telescope at the center of our galaxy for several years, what they saw was several bright stars orbiting around the same blank spot. This result indicated that the spot is the location of a supermassive black hole. As another example, a large cloud of gas and dust can fall towards a black hole. In the absence of friction, the black hole's gravity would simply cause the gas particles to orbit the black hole rather than fall in, similar to how stars orbit a black hole (i.e. black holes don't suck). However, the gas particles constantly smash into each other, thereby converting some of their kinetic energy into heat. With the loss of kinetic energy, the gas particles fall closer to the black hole. In this way, friction causes a large gas cloud to swirl toward a black hole and heat up along the way. Eventually, the cloud of gas falls into the black hole and becomes part of it. However, before the gas actually enters the black hole, it heats up enough to begin glowing, just like how a toaster element glows when it heats up. The light that is emitted consists mostly of x-rays but can also include visible light. Since this light is emitted by the gas before the gas enters the black hole, the light can escape away to the rest of the universe. In this way, light can be emitted from a glowing gas cloud just outside of a black hole even though the black hole itself emits no light. Therefore, we can indirectly "see" a black hole by seeing the glowing gas cloud that surrounds it. This gas cloud is called an accretion disk. When atoms of gas become hot enough, the atoms' electrons are ripped off, causing the atoms to become ions. A cloud of gas that is mostly ionized is called a plasma. The situation gets even more interesting. As the plasma cloud gets pulled ever closer to the black hole, the plasma gets moving faster and faster. At the same time, there is less and less room for all of this plasma. As a result of this high speed and this crowding effect, some of the plasma ricochets far away from the black hole. In this way, two giant jets of glowing plasma are formed, which are called astrophysical jets. The jets shoot plasma far away from the black hole, never to return. Again, this is possible because the plasma in the jets was never actually inside the black hole. When such jets are created by supermassive black holes, they can stretch out for hundreds of thousands of light years. For instance, the image below shows a photograph of galaxy M87 captured by the Hubble Space Telescope. The bright yellow spot in the upper left of the image is the central region of the galaxy and the violet line is the glowing astrophysical jet created by the supermassive black hole at the center of the galaxy. In summary, a black hole itself cannot emit light, but it's intense gravity can create accretion disks and astrophysical jets outside the black hole which emit light. Photograph captured by the Hubble Space Telescope. The violet line is light emitted by a giant plasma jet created by a supermassive black hole located at the center of galaxy M87. Public Domain Image, source: NASA. Topics: black hole, light, relativity, spacetime A black hole is a region of space where gravity is so strong that nothing can escape, not even light. It might be surprising to you to hear that gravity can affect light even though light has no mass. If gravity obeyed Newton's law of universal gravitation, then gravity would indeed have no effect on light. However, gravity obeys a more modern set of laws known as Einstein's general theory of relativity. According to general relativity, gravity is actually caused by a curving of space and time. Since light travels in a straight line through straight spacetime, the curving of spacetime causes light to follow a curved path. The gravitational curvature of light's path is a weak enough effect that we don't notice it much on earth. However, when gravity is very strong, the bending of light's path becomes significant. A black hole is a region where spacetime is so curved that every possible path which light could take eventually curves and leads back inside the black hole. As a result, once a ray of light enters a black hole, it can never exit. For this reason, a black hole is truly black and never emits light. However, this restriction only applies to points inside the black hole. Light that is near a black hole, but not actually inside it, can certainly escape away to the rest of the universe. This effect is in fact what enables us to indirectly "see" black holes. For instance, there is a supermassive black hole at the center of our galaxy. If you point a high-power telescope exactly at the center of our galaxy and zoom way in, you don't see anything. A black hole by itself is truly black. However, the black hole's gravity is so strong that it causes several nearby stars to orbit the black hole. Since these stars are actually outside of the black hole, the light from these stars can reach earth just fine. When scientists pointed a high-power telescope at the center of our galaxy for several years, what they saw was several bright stars orbiting around the same blank spot. This result indicated that the spot is the location of a supermassive black hole. As another example, a large cloud of gas and dust can fall towards a black hole. In the absence of friction, the black hole's gravity would simply cause the gas particles to orbit the black hole rather than fall in, similar to how stars orbit a black hole (i.e. black holes don't suck). However, the gas particles constantly smash into each other, thereby converting some of their kinetic energy into heat. With the loss of kinetic energy, the gas particles fall closer to the black hole. In this way, friction causes a large gas cloud to swirl toward a black hole and heat up along the way. Eventually, the cloud of gas falls into the black hole and becomes part of it. However, before the gas actually enters the black hole, it heats up enough to begin glowing, just like how a toaster element glows when it heats up. The light that is emitted consists mostly of x-rays but can also include visible light. Since this light is emitted by the gas before the gas enters the black hole, the light can escape away to the rest of the universe. In this way, light can be emitted from a glowing gas cloud just outside of a black hole even though the black hole itself emits no light. Therefore, we can indirectly "see" a black hole by seeing the glowing gas cloud that surrounds it. This gas cloud is called an accretion disk. When atoms of gas become hot enough, the atoms' electrons are ripped off, causing the atoms to become ions. A cloud of gas that is mostly ionized is called a plasma. The situation gets even more interesting. As the plasma cloud gets pulled ever closer to the black hole, the plasma gets moving faster and faster. At the same time, there is less and less room for all of this plasma. As a result of this high speed and this crowding effect, some of the plasma ricochets far away from the black hole. In this way, two giant jets of glowing plasma are formed, which are called astrophysical jets. The jets shoot plasma far away from the black hole, never to return. Again, this is possible because the plasma in the jets was never actually inside the black hole. When such jets are created by supermassive black holes, they can stretch out for hundreds of thousands of light years. For instance, the image below shows a photograph of galaxy M87 captured by the Hubble Space Telescope. The bright yellow spot in the upper left of the image is the central region of the galaxy and the violet line is the glowing astrophysical jet created by the supermassive black hole at the center of the galaxy. In summary, a black hole itself cannot emit light, but it's intense gravity can create accretion disks and astrophysical jets outside the black hole which emit light.

context: tag/black-hole/ question: How does a supernova completely destroy a star?

A supernova does not completely destroy a star. Supernovae are the most violent explosions in the universe. But they do not explode like a bomb explodes, blowing away every bit of the original bomb. Rather, when a star explodes into a supernova, its core survives. The reason for this is that the explosion is caused by a gravitational rebound effect and not by a chemical reaction, as explained by NASA. It is true that within most stars there are violent hydrogen fusion reactions churning away, but these do not cause the supernova. Stars are so large that the gravitational forces holding them together are strong enough to keep the nuclear reactions from blowing them apart. It is the gravitational rebound that blows apart a star in a supernova. Consider the typical momentum transfer exhibit found in many science museums, as depicted in the animation on the right. Rubber balls of different sizes are held at different heights. The balls are then let go at the same moment. Gravity pulls them all down and they all fall towards the ground. In the next few moments, the bottom ball hits the ground and bounces back, and then the balls start colliding. Momentum equals mass times velocity. This means that a heavy object going slow has as much momentum as a light object going fast. When two objects collide, they transfer some momentum. When a heavy slow object collides with a light object, it can give it a very high velocity because of the conservation of momentum. As this animation shows, by arranging the rubber balls from heaviest on the bottom to lightest on the top, momentum is transferred to ever lighter objects, meaning ever higher speeds. As a result, even though gravity is pulling all the balls downwards, the upper balls rebound at incredible speeds. This is all in keeping with the law of conservation of momentum. The lower balls are too heavy and too slow to fly off. They remain behind as the surviving core of the original system. On the other hand, the upper balls are blown away (in a science museum exhibit, they are captured at the top of the apparatus so that the demonstration can be rerun). This explosion of rubber balls occurs without any significant chemical or nuclear reactions taking place. This explosion is simply due to gravity and momentum transfer, i.e. a gravitational rebound. If you look closely at the animation, you see that the rebound takes the form of an outward shock wave that gains in intensity as it spreads. A supernova is the same kind of explosion as this rubber-balls demonstration. An aging star is composed of denser layers down towards the center, and thinner layers near the surface. The star's nuclear reactions typically balance out the force of gravity. But when the star runs out of fuel, the nuclear reactions slow down. This means that gravity is no longer balanced. Gravity begins collapsing the star. After the core of a collapsing star reaches a critical density, its pressure becomes strong enough to hold back the collapse. But, like the rubber balls, the star has been falling inwards and now bounces back. The outer layers are blown off into space in a giant explosion, spreading fertile dust clouds through-out the universe . But because of the momentum transfer, the star's core survives. The collapsing event has so intensely squeezed the star's core, that it transforms into something exotic. If the star started out with between 5 and 12 times the mass of our sun, the core becomes a big ball of neutrons called a neutron star. If the star started out with more than 12 times the mass of our sun, the core becomes a black hole. You may be tempted to argue that when a star explodes so that all that remains is a black hole, there is nothing left and the star has therefore been completely destroyed. But a black hole is not nothing. Black holes have mass, charge, angular momentum, and exert gravity. A black hole is just a star that is dense enough, and therefore has strong enough gravity, to keep light from escaping. The black hole created by a supernova is the leftover core of the star that exploded. Not all stars experience a supernova. Stars that have less than 5 times the mass of our sun are too light to experience this violent transformation. They simply don't have enough gravity to collapse and rebound so violently. Instead, when lighter stars run out of nuclear fuel, they go through a series of stages and then settle down as long-lived white dwarfs. Whether stars end up as neutron stars, black holes, or white dwarfs, they never go completely away.

context: tag/black-hole/ question: Is a black hole a 2D or a 3D object?

A black hole is actually a four-dimensional object. A black hole extends across all four physical dimensions of the universe. The four dimensions that form the background framework of the universe consist of three spatial dimensions and one time dimension. These four dimensions are inseparably connected into one unified framework called spacetime. While it may sound exotic to say that a black hole is a four-dimensional object, the mundane truth is that all physical objects are four-dimensional objects*. For instance, a desk extends a few feet in the x direction (we call this its width), extends a few feet in the y direction (we call this its length), extends a few feet in the z direction (we call this its height), and extends a few years in the t direction (we call this its lifetime). A desk is therefore spread out across many points in each of the three spatial dimensions and across many points in time. The size of the desk in the time dimension extends from the moment that it was built to the present moment and will continue to extend through time until the moment it is destroyed. In this way, the desk is a physical four-dimensional object that fills a four-dimensional volume. The same is true of chairs, apples, kites, asteroids, stars, and all other physical objects. Calling the time dimension the fourth dimension is more than just a clever use of words. There are profound physical effects that force us to consider time as a dimension attached to the three spatial dimensions. An object that in one reference frame is observed to have a large extent in the length dimension and a small extent in the time dimension will, in another reference frame, be observed to have a small extent in the length dimension and a large extent in the time dimension. In other words, because of relativistic effects such as length contraction and time dilation, how an object fills out its four-dimensional volume depends on the reference frame. Therefore, the time component of spacetime cannot be ignored. While a black hole is similar to a chair or a tree in that it exists extended across the four physical dimensions of the universe, a black hole is uniquely different for another reason. A black hole consists of spacetime that is so warped that nothing can escape, not even light. In fact, the spacetime of a black hole is so warped that, to a distant observer, spacetime itself is observed to cease to exist at the black hole's event horizon (which can roughly be thought of as the surface of the black hole). To a distant observer, a black hole is literally a hole in the spacetime framework; there is no "inside" of a black hole. All of the black hole's mass and trapped light is observed from far away (if it could be observed) to exist at its event horizon. To a distant observer, nothing exists inside a black hole, not even space or time. If this concept does not sit right with you, then you can think of it as an apparent effect arising from being in a reference frame that is observing the black hole from far away. To an observer very close to the event horizon of a black hole or even inside the event horizon, spacetime does not end at the event horizon and there is indeed an inside to a black hole. The views of both observers are correct, without there being a contradiction, because of the relativistic nature of spacetime. If a black hole is a four-dimensional object, then what is its shape? A black hole that is not rotating is spherical in shape (i.e., its event horizon is spherical) and extends linearly through the time dimension. You can roughly think of a black hole as a star that traps all of its light, and therefore it seems natural that a black hole is spherical. Furthermore, a black hole that is rotating is very close to being spherical in shape but is slightly flattened along its rotational axis. This shape is called an oblate spheroid. A rotating black hole also extends linearly though the time dimension. Interestingly, all real black holes are rotating because they form from giant rotating clouds of matter. However, black holes that are rotating very slowly can be approximated to be not rotating. Note that if a black hole is changing (e.g., matter is falling non-uniformly into the black hole), then things get more complicated and its shape is not exactly spherical, but these basic ideas still apply to a good approximation. In summary, a black hole is a four-dimensional object with a spherical or nearly spherical shape that extends linearly through time. *Note that fundamental particles such as electrons act in certain ways like point particles, meaning that they act like they have exactly zero width, zero length, zero height, and extend across many points in time. You could therefore argue that fundamental particles are physical one-dimensional objects. However, fundamental particles are quantum particles and therefore simple concepts such as size and volume do not have direct, literal, physical meaning. An electron (when acting perfectly as a particle) indeed does not have a fixed, definite, non-zero physical radius. In fact, in many experiments, an electron acts externally as if it had a radius of exactly zero. However, an electron also has a finite mass and does not have an infinite mass density, which implies that it cannot have a fixed radius of exactly zero. Furthermore, if all the mass of an electron were truly packed into an infinitely small volume, it would become a black hole, which it does not. This apparent contradiction is resolved by the fact that fundamental quantum particles simply do not have a definite radius at all. Quantum particles are not definite, hard, fixed balls of matter and/or energy. Rather, quantum particles are fluctuating, ambiguous, smeared-out probability clouds of matter and/or energy. Fundamental particles do not have zero radius in a fixed, definite, classical sense and therefore are ultimately four-dimensional objects.

context: tag/black-hole/ question: Where is the center of the universe?

According to all current observations, there is no center to the universe. For a center point to exist, that point would have to somehow be special with respect to the universe as a whole. Let us think about all the different types of effects that could create a center. First, if an object is rotating, you can define a center of rotation. The center of rotation is the one spot on a rotating object that is stationary. For the earth, the center of rotation is the axis connecting the North and South pole. For a basketball player spinning a basketball on his finger, the center of rotation is the point where the ball touches his finger. The center of rotation for a wheel on an axle is the center of the axle. Observations of the universe have not found any rotation at all to the universe as a whole. With no rotation, there is no center of rotation. Next, you can define a center of mass. If an object is finite, the center of mass is just the point that, on average, has an equal amount of mass surrounding it in all directions. The situation gets more complicated for an infinite object. If an object is infinite and uniform, you simply cannot define a center of mass because all points are identical. On the other hand, if an object is infinite but not uniform (for instance it has a single knot of high density at one point), you can define the center of mass of the entire object as the center of mass of the non-uniformity. For instance, consider a cloud in the sky. Certain kinds of clouds don't have a well-defined boundary, but instead just stretch out in all directions, getting thinner and thinner. Even though the cloud stretches out effectively to infinity, the high density region of the cloud exists in a limited volume, so you can find a center of mass through a limiting procedure. Observations currently indicate that the universe is infinite in size. Although planets and stars do represent non-uniformities in the spacetime structure, on the universal scale, such uniformities are randomly dispersed. On average, therefore, the universe is uniform. Being infinite and uniform, there is no way to define a center of mass for the universe. Another possibility is a center of charge. Similar to the center of mass, this would be a point in an object where the amount of electric charge is on average the same in all directions surrounding it. The center of charge for a uniformly charged sphere would just be the center of the sphere. Similar to the mass distribution, the charge distribution of the universe is infinite and uniform on average so that there is no center of charge. Next, there could be a center of curvature. Like a salad bowl, there could be a central point to the universe from which all other points curve away from. But current observations have found the universe to be flat and not curved at all. Yet another possibility is a center of expansion. If you bolt a rubber sheet to the ground and then have people pull on all sides, the place where the sheet is bolted becomes the center of expansion. The center of expansion is the point in space from which all other points are moving away. A wealth of astronomical observations has revealed that the universe is indeed expanding. These observations are the foundation for the concept that a Big Bang started the universe. Because the universe is expanding, if you run time backwards, there had to be a time when the universe was all compacted to one point. Since the universe is expanding, you would think there is a center of expansion. But observations have revealed this not to be the case. The universe is expanding equally in all directions. All points in space are getting uniformly distant from all other points at the same time. This may be hard to visualize, but the key concept is that objects in the universe aren't really flying away from each other on the universal scale. Instead, the objects are relativity fixed in space, and space itself is expanding. You might be tempted to say that the location of the Big Bang is the center of the universe. But because space itself was created by the Big Bang, the location of the Big Bang was everywhere in the universe and not at a single point. The major aftereffect of the Big Bang was a flash of light known as the Cosmic Background Radiation. If the Big Bang happened at one location in space, we would only see this flash of light coming from one spot in the sky (we can see a flash that happened so long ago because light takes time to travel through space and the universal scale is so big). Instead, we see the flash as coming equally from all points in space. Furthermore, once the motion of the earth is accounted for, the flash of light is equally strong in all directions on average. This indicates that there is no center of expansion. Another way to define a center would be to identify some object or feature that exists only at one spot, such as a supermassive black hole or super-large nebula. But observations indicate that all types of objects are randomly peppered through the universe. No matter how we try to define and identify it, the universe simply has no center. The universe is infinite and non-rotating. Averaged over the universal scale, the universe is uniform.

context: tag/black-hole/ question: Why does everything in our galaxy orbit the supermassive black hole at the center?

Strictly speaking, everything in our galaxy does not orbit the supermassive black hole at the center. Everything in the galaxy orbits the center of mass of the galaxy. The supermassive black hole just happens to be at the center. If the black hole at the center were removed, the galactic orbits of almost all objects in the galaxy would not change (except for the few stars that are very close to the black hole). Our galaxy contains a lot of mass, which includes stars, gas, planets, and dark matter. The black hole in the center is only about one millionth of the total mass of our galaxy. Because mass causes gravity, and gravity causes orbits, the galactic orbital paths of all objects in the galaxy are caused by the total mass of the galaxy and not the mass of the black hole at the center. Consider this analogy. Three girls form a circle and all lock their hands at the center of the circle. These girls now run quickly and steadily around their circle so that they can feel the strain in their arms. Even though they can feel the centrifugal force pushing them outwards, they do not fly off because their linked arms pull inward. The girls are all, in a sense, in orbit around their combined center of mass. They are not orbiting the gold ring on the finger of one of the girls, which happens to be at the center of their circle. If she took off her ring, their motion would not change much. The girls are like the objects in our galaxy, their linked arms are like the gravitational force linking everything together, and the gold ring is like the black hole. Every object in the galaxy is in orbit around the center of the combined mass of the galaxy. The center of mass is often called the "barycenter". In general, small bodies do not orbit large bodies. Instead, large and small bodies together orbit their combined center of mass. The textbook "Orbital Mechanics" by Tom Logsdon notes, "Newton also modified Kepler's first law by noting that if both of the two bodies in question have appreciable mass, the smaller body will not orbit about the center of the larger body. Instead, both of them will orbit around their common barycenter. A similar phenomenon can be observed at a football game. When a majorette tosses her baton into the air, it does not rotate around the heavy end. Instead, the entire baton rotates about its center of mass."

context: tag/brain/ question: Do blind people dream in visual images?

Yes, blind people do indeed dream in visual images. For people who were born with eyesight and then later went blind, it is not surprising that they experience visual sensations while dreaming. Dreams are drawn from memories that are stored in the brain as well as from brain circuitry that is developed while experiencing the outside world. Therefore, even though a person who lost his vision may be currently blind, his brain is still able to draw on the visual memories and on the related brain circuits that were formed before he went blind. For this reason, he can dream in visual images. What is more surprising is the discovery that people who were born blind also dream in visual images. The human experience of vision involves three steps: (1) the transformation of a pattern of light to electrical impulses in the eyes, (2) the transmission of these electrical impulses from the eyes to the brain along the optic nerves, and (3) the decoding and assembly of these electrical impulses into visual sensations experienced in the brain. If any one of these three steps is significantly impaired, blindness results. In the vast majority of cases, blindness results from problems in the eyes and in the optic nerves, and not in the brain. In the few cases where blindness results from problems in the brain, the person usually regains some amount of vision due to brain plasticity (i.e. the ability of the brain to rewire itself). Therefore, people who have been blind since birth still technically have the ability to experience visual sensations in the brain. They just have nothing sending electrical impulses with visual information to the brain. In other words, they are still capable of having visual experiences. It's just that these experiences cannot originate from the outside world. Dreams are an interesting area because dreams do not directly originate from the outside world. Therefore, from a plausibility standpoint, it is possible for people who have been blind since birth to dream in visual images. However, just because blind people have the neural capacity to experience visual sensations does not automatically mean that they actually do. Scientists had to carry out research studies in order to determine if people who have been blind since birth actually do dream in visual images. At this point, you may be wondering, "Why don't we just ask the people who have been blind since birth if they dream in visual images?" The problem is that when you ask such people this question, they will always answer no. They are not necessarily answering no because they actually do not have visual dreams. They are saying no because they do not know what visual images are. A girl with eyesight visually recognizes an apple because at some point in the past she saw the apple and ate it, and therefore is able to connect the image of an apple with the taste, smell, shape, and touch of an apple. She is also able to connect the image with the word "apple." In other words, the visual image of an apple becomes a trigger for all the memories and experiences she has previously had with apples. If a girl has never personally experienced the visual image of an actual apple, then the experience of seeing an image of an apple in a dream for the first time has no connection to anything in the real world. She would not realize that she is seeing an apple. As an analogy, suppose you have never tasted salt. No matter how much people describe salt to you, you do not know what the experience is really like until you experience it personally. Suppose you were all alone your whole life, cut off from all people and all of society, and you came across a bag of very salty potato chips for the first time. When you eat the chips, you would experience the taste of salt for the first time, but you would have no way to describe it, because you would have no other previous experiences or connections with it. Similarly, people who have been blind since birth have no experience of connecting visual sensations with external objects in the real world, or relating them to what sighted people describe as vision. Therefore, asking them about it is not useful. Instead, scientists have performed brain scans of people who have been blind since birth while they are sleeping. What scientists have found is that these people have the same type of vision-related electrical activity in the brain during sleep as people with normal eyesight. Furthermore, people who have been blind since birth move their eyes while asleep in a way that is coordinated with the vision-related electrical activity in the brain, just like people with normal eyesight. Therefore, it is highly likely that people who have been blind since birth do indeed experience visual sensations while sleeping. They just don't know how to describe the sensations or even conceptually connect in any way these sensations with what sighted people describe as vision. With that said, the brain scans during sleep of people who have been blind since birth are not identical to those of sighted people. While people who have been blind since birth do indeed dream in visual images, they do it less often and less intensely than sighted people. Instead, they dream more often and more intensely in sounds, smells, and touch sensations. We should keep in mind that a person who has been blind since birth has never had the experience of seeing images originating from the external world and therefore has never formed visual memories connected to the external world. The visual components of their dreams therefore cannot be formed from visual memories or the associated circuitry. Rather, the visual sensations must arise from the electrical fluctuations that originate within the brain. What this means is that people who have been blind since birth probably do not experience detailed visual images of actual objects such as apples or chairs while dreaming. Rather, they probably see spots or blobs of color floating around or flashing. The spots may even correlate meaningfully to the other senses. For instance, a dream of a police car siren sound traveling from the left to the right may be accompanied by the visual sensation of a spot of color traveling from the left to the right at the same speed. In summary, the current evidence suggests that people who have been blind since birth do indeed dream in images, but we do not know exactly what they see. On a related note, brain scans have found that all humans dream in visual images before they are born. Because the womb is in total darkness, and therefore none of us experienced actual vision before we were born, this means that we all experienced visual dreams before birth despite having no visual memories to draw from. Therefore, the visual dream experiences of a fetus are similar to those of an adult who has been blind since birth.

context: tag/brain/ question: Do scientists have a hard time understanding art?

Actually, scientists tend to be more art-minded than your average person. Science is a highly creative process, and so it tends to attract people who innately understand art, appreciate art, and create art. The old notion goes that one half of the brain is highly creative and the other half of the brain is highly technical/mathematical. According to this old notion, each person has a dominant half, resulting in only two types of people in the world: creative people who can't do math, and technical people who can't make art. First of all, this old notion is simply wrong. The brain is very complex. Creativity and technical ability, along with hundreds of other traits, emerge from a complicated interaction of neurons that involves the whole brain. A study performed by Harnam Singh found that "Notably, the MG [Mathematically Gifted] showed no reliable left-right differences when processing global or local information on unilateral trials." This study found that mathematical ability is more a function of how well the two halves of the brain communicate and work together and not a function of which half dominates. Furthermore, there are creative technical people and non-creative, non-technical people, so the two traits are not mutually exclusive. Also, there are billions of different kinds of people in the world, each with his or her unique blend of traits. Even if half of the brain was solely creative and the other half was solely technical, real science requires both, so scientists would have both halves equally dominant. It is true that science uses mathematics and technical recipes. But those are just the language of science. The meat of science is highly creative: forming hypothesis, building new tools, crafting new models, designing new experiments, interpreting results, and making conceptual connections. The only part of science that is not very creative (running the experiment once everything is designed and setup) is not even done typically by scientists these days. The actual running of an experiment is typically carried out by machines or lab technicians. Forming a hypothesis involves creating an idea about how the world works that may or may not be true. There are no technical recipes for making original hypotheses. When students read biographies of great scientists, they often puzzle, "how did he come up with this idea in the first place?" Often, the only answer is, "He was creative." Some of the best scientists have the craziest ideas for a new model or a new experiment. Most of these ideas don't work out, but some do. Building new scientific tools; such as microscopes, telescopes, particle accelerators, and particle detectors; is an important form of science. People who use established science to build ever better tools are engineers. People who use new science in order to build better tools are scientists. There is not one right way to build a better tool. It requires a lot of creativity to think up ways to apply some new science in order to create a better tool. Similarly, new models, new theories, and new experiments have to thought up by a creative person before they can be tested, applied, and tweaked. When a given experiment spits out a set of resulting numbers, it takes creativity to make connections and interpret the numbers. Scientists are very much like artists: they create new ideas, apply them, and see if they work. For science, an idea "works" if it predicts the behavior of the physical universe. For art, an idea "works" if it pleases the senses and/or successfully conveys a meaningful concept, emotion, or impression. Just as there is not one right way to paint a tree, there is not one right way to make a telescope. (Although, there are more wrong ways to make a telescope than there are wrong ways of painting a tree. In this sense, art is more forgiving than science.) If you have a picture in your mind of a person non-creatively carrying out a predefined list of technical steps, you have not pictured a scientist. You have pictured an assembly-line worker, a repairman, or an accountant. Because science is a highly creative process, scientists tend to be painters, singers, sculptors, composers, illustrators, poets, or novelists in their free time. Einstein played the violin. Max Plank was a good pianist, but gave up a promising career at the conservatory to study physics and become one of the founders of quantum theory. Feynman was secretly a published artist and not-so-secretly an avid drummer. A study by R. S. Root-Bernstein found that the success of a budding scientist is strongly correlated with the degree to which he enjoys musical and artistic hobbies.

context: tag/brain/ question: How bad of an alcoholic do you have to be to have your brain affected?

One drink of alcohol is enough to affect your brain, whether you are an alcoholic or a casual social drinker. Alcohol is a psychoactive drug that interferes directly with the normal functioning of many parts of the brain. Fortunately, much of the effect caused by alcohol consumption can be repaired by the body when the person stops drinking. But ongoing alcohol consumption at heavy levels can cause damage so severe that the body cannot repair it and the damage becomes permanent. The National Institute of Health states, "Difficulty walking, blurred vision, slurred speech, slowed reaction times, impaired memory: Clearly, alcohol affects the brain. Some of these impairments are detectable after only one or two drinks and quickly resolve when drinking stops. On the other hand, a person who drinks heavily over a long period of time may have brain deficits that persist well after he or she achieves sobriety...Alcohol can produce detectable impairments in memory after only a few drinks and, as the amount of alcohol increases, so does the degree of impairment. Large quantities of alcohol, especially when consumed quickly and on an empty stomach, can produce a blackout, or an interval of time for which the intoxicated person cannot recall key details of events, or even entire events." Drinking alcohol (ethanol) is a two-carbon alcohol with the molecular formula CH3-CH2-OH. The hydroxyl group on the end (-OH) is able to participate in hydrogen bonding and accounts for much of its physical properties.

context: tag/brain/ question: How can we unlock the 90% of our brain that we never use?

Healthy humans use all of their brain. There is no part of the brain that goes unused. Certain tasks work certain parts of the brain more, but they all play important roles, as explained by neurobiologist Dr. Eric Chudler. Brain maps, as found in modern anatomy books, indicate that each part of the brain has a specific function essential to a healthy human. If there were a part of your brain that really went unused, then you could safely damage that part in an accident with no ill effects. But decades of medical records show that damage to any part of the brain has severe effects. If 90% of the brain were not used, then 90% of the brain tumors would cause no problem. Imagine brain doctors telling 90% of their cancer patients, "I have good news and bad news. Bad news: you have a brain tumor. Good news: it's in the part of the brain that you will never use." The thought is absurd. If the 10% myth is instead supposed to mean that humans only use 10% of their brain in a given moment, it is still false. The brain is not a collection of independent machines that are turned on or off depending on whether you are reading or singing. Rather, brain functions emerge as a complex interplay of many parts of the brain. Physiologically, nerves are like muscles in that they degenerate when unused. If 90% of the brain went completely unused, then that portion would degenerate significantly. But brain scans of a healthy person reveals all parts to be intact. This myth was propagated by authors trying to sell books on mystical ways to unlock your hidden potential, claiming that unused brain power could be tapped using the methods in their books. The greatest danger to your brain is not the possibility that a large portion is going on unused. Rather, the greatest dangers are stroke, Alzheimer's disease, and tumors. The best ways to protect yourself from such risks include eating healthy, exercising, and getting enough rest. Do you really want to use your brain to its full potential? Then put down your unlocking-hidden-brain-potential book and go on a run.

context: tag/brain/ question: How do I turn on more parts of my brain and get smarter?

Turning on more parts of your brain does not make you smarter. The brain does not really work that way. An example of a brain experiencing high activation is a seizure. During a seizure, a person has severely decreased functioning and mental capability. A person having a seizure can't talk, can't walk, and can't do math problems. This extreme example should make clear that "turning on more of the brain" does not equal "smarter". The brain contains a complex network of neurons that includes many feedback loops. Thoughts, memories, and calculations in the brain are emergent phenomena that arise out of the interplay of many neurons. A new idea is not stored in a single neuron. Rather, an idea is stored in the way many neurons are connected together. More specifically, an idea is stored in the way a brain signal travels through these connections in a cascading, looping, non-linear fashion. The analogy that the brain works like a computer is completely wrong. Standard microchip computers are deterministic, linear, and have localized processing operations. This type of processing is excellent for doing long mathematical calculations and carrying out deterministic actions such as displaying photos or editing text documents. However, this type of processing is terrible at generating creativity, recognizing patterns in complex systems, attaching value to information, extrapolating complex trends, learning new skills, and generalizing data into a small set of principles. In contrast to computers, the brain is non-deterministic, asynchronous, non-linear, and has delocalized processing operations. In this way, the brain is really the opposite of a traditional microchip computer; it is good at generating creativity, recognizing patterns, evaluating information, extrapolating trends, learning new skills, and generalizing data; but it is bad at doing long numerical calculations. Since a computer is linear and deterministic, you can make it more powerful simply by installing more processor cores and more RAM. In contrast, the brain is non-linear and non-deterministic, so turning on more neurons at once won't make the brain smarter. These comments lead to the question, "How does a person get smart?" My response is, "What do you mean by smart?" I do not give this response to be evasive. My response gets at the core of how a brain works. Strictly speaking, your brain is always getting smart in some way, whether you are working at it or not (assuming your brain is generally healthy). Your brain is always processing the external world, finding patterns, forming memories, and learning information. From a neurological perspective, skipping your math homework and watching cartoons instead does not make your brain any less smart. It just makes your brain good at reciting cartoon theme songs instead of being good at solving math problems. If your job is to recite cartoon theme songs on stage, then watching cartoons will make you smarter than completing your math homework will. With this in mind, "smart" does not really mean using your brain more. Assuming you are generally healthy, you always use the optimal amount of brain power whether you try to or not. "Smart" means using your brain to learn the information and skills that are important to you. The fields of mathematics, science, language, and history all contain facts, concepts, and skills that pertain to the real world. These concepts and skills either exist in the real world, are applicable to the real word, or are useful in carrying out professions in the real world. For these reasons, the learning of concepts and skills in mathematics, science, language, and history has become culturally associated with being smart. But a person who drops out of school and spends all of his time enjoying video games, television, movies, and fantasy books is still smart in the neurological sense of learning things and filling his brain with information. He is just smart in areas that are mostly useless, non-productive, and non-employable. Note that making a video game is entirely different from playing a video game. Memorizing where all the treasures are hidden in your favorite video game does not make you more able to get a job designing games. It takes mathematics, language, art, and programming skills to build virtual worlds. That is what I mean when I say that the knowledge and skills gained by playing video games are mostly useless, non-productive, and non-employable. In short, the way to get smart is to decide what field is important to you, and then spend a lot of time working and learning in that field. If you want to get good at math, do math problems again and again. Furthermore, you have to put in the work to make sure you learn to do the math problems correctly. If you want to get good at playing the violin, practice every day. There is no magical way to learn a subject without putting in the hard work and time. With that said, there are more effective ways of learning a subject than others. As mentioned previously, the human brain is constructed so that it is good at recognizing and learning patterns. Therefore, you will learn a subject more quickly if you first learn the rules, and then connect the information into patterns using the rules as you go. For instance, learning how to read by going through a list of 5000 of the most common words and simply memorizing the sound of each word is slow and tedious. In contrast, mastering a few dozen phonics rules is much more effective. As another example, memorizing a list of battle names and dates in a particular war is less productive. In contrast, it is more effective to first learn about war heroes, weapons technology, basic geography, and the war strategies that were used; and then connect the names and dates of the battles to these concepts. Creating maps, charts, timelines, and biographies is far more effective for learning history than memorizing unconnected list of facts because the human brain is optimized for seeing and learning patterns.

context: tag/brain/ question: How do nerves control every organ and function in the body?

Nerves do not control every tissue and function in the human body, although they do play a large role. There are three main ways that bodily organs and functions are controlled: Nerves carry orders from the brain and spinal cord in the form of electrical signals. Nerves also help sense the state of tissues and relay this information back to the brain and spinal cord, enabling us to experience pain, pleasure, temperature, vision, hearing, and other senses. The body uses electrical signals sent along nerves to control many functions because electrical signals can travel very quickly. At the end of each nerve's axon terminals the electrical signals are converted to chemical signals which then trigger the appropriate response in the target tissue. However, the control exerted by the nervous system inevitably resides in the brain and spinal cord, an not in the nerves, which just pass along the signals. Most signals get processed in the brain, but high-risk signals are processed and responded to by the spinal cord before reaching the brain in the effect we call "reflexes". Although the central nervous system plays a large role in controlling the body, it is not the only system that exerts control. The endocrine system is a series of endocrine glands throughout the body that excrete certain chemical signals called hormones into the blood stream. The circulating blood then takes the hormones throughout the entire body where different tissues respond in characteristic ways to the hormones. The response of an organ or system to a hormone depends on how much of that hormone is present in the blood. In this way, endocrine glands can exert control over different organs and functions of the body by varying how much hormone they emit. In contrast to the central nervous system, the pathway of control for the endocrine system is purely chemical and not electrical. For example, the thyroid gland in the neck controls how quickly the body uses energy by secreting varying levels of thyroid hormone. Too much thyroid hormone, and you become restless, jittery, and unable to sleep. Too little thyroid hormone and you become sleepy, lethargic, and enable to think straight. A healthy body constantly monitors the activity level and adjusts the thyroid hormone levels as needed. Other examples of endocrine glands are the adrenal glands, which prepare the body for facing an emergency, and the reproductive glands, which control body mass and reproduction. Hormones in the body control functions as diverse as libido, fertility, menstruation, ovulation, pregnancy, child birth, lactation, sleep, blood volume, blood pressure, blood sugar, the immune system, vertical growth in children, muscle mass, wound healing, mineral levels, appetite, and digestion. Ultimately, much of the endocrine system is subservient to the brain via the hypothalamus, but the endocrine system does operate somewhat independently using feedback loops. Lastly, organs and functions in the body are controlled through local self-regulation. Rather than depend on the brain to dictate every single minute task, organs and cells can accomplish a lot on their own so that the brain is freed up for more important tasks. An organ can communicate regulatory signals through its interior using localized chemical signals such as paracrine hormone signalling. Typically, such hormones do not enter the blood stream, but are transported locally by simply flowing in the space between cells. This approach works because paracrine hormones are only meant to operate on nearby cells. For example, the clotting of blood and healing of wounds are controlled locally through an exchange of paracrine hormones. The organ with the highest degree of self-regulation is probably the liver. The liver hums along nicely, performing hundreds of functions at once without much direction from the rest of the body. An organ can also communicate through its interior electrochemically. For instance, the heart does not beat because a nerve is telling it to. The heart beats on its own through a cyclic wave of electrical impulses. While it is true that the brain can tell the heart to speed up or slow down, the actual beating of the heart is controlled locally. Also, each cell of the body has some degree of self-regulation internal to the cell itself. Some cells exert more internal control than others. For instance, white blood cells hunt down and destroy germs in a very independent fashion, as if they were autonomous organisms. Active white blood cells do not wait for the brain or a hormone to tell them to do their job. Sperm cells are so autonomous that they can continue to survive and function properly even after completely leaving the male's body. In reality, the central nervous system, the endocrine system, and the local regulation systems are not independent, but exert control over each other in a complicated manner.

context: tag/brain/ question: How does ice cream in your stomach cause a headache?

An ice cream headache has nothing to do with your stomach, but is rather the result of the roof of your mouth (your palate) getting cold too quickly. In fact, you can get an ice cream headache before even swallowing the ice cream. Ice cream headaches occur whenever you eat or drink something cold too rapidly, and they last about 20 seconds. Eating cold food slowly can give your palate time to cool down normally and adjust to the low temperatures without causing a headache. According to the Mayo Clinic, the exact mechanism at work in ice cream headaches is not currently known, but it is believed that the headache is a case of referred pain. When the roof of your mouth gets cold too quickly, the pain signal sensed in your mouth is passed on to the trigeminal nerve, which then passes it on to the brain where it is processed and experienced. The trigeminal nerve senses pain from the entire face and forehead, so when it gets overloaded, pain from your mouth seems to be coming from your forehead. Ice cream headaches can be avoided by eating cold foods slowly, letting the cold food warm up before eating it, or by swishing a bit of the cold food around in your mouth before consuming the rest in order to help your palate adapt. If they do occur, these headaches can be alleviated by pressing your warm tongue against the roof of your mouth to warm it up, or by drinking a warm beverage.

context: tag/brain/ question: How long can you use a cell phone before getting a brain tumor?

Cell phones do not cause cancer, no matter how long they are used, because they communicate using radio waves. Radio waves simply don't have enough energy per photon to ionize atoms. The World Health Organization states, "Only the high frequency portion of the electromagnetic spectrum, which includes X rays and gamma rays, is ionizing". Cancer occurs when the DNA in a cell is altered so that the cell grows in an uncontrolled way. DNA can be altered in many ways, but when it comes to cancer due to radiation exposure, an electron must be knocked off a molecule by a bit of radiation. The smallest possible bit of electromagnetic radiation is known as a photon. Electrons are bound to molecules, so it takes a certain minimum energy to knock off the electron. Light with frequencies above ultraviolet (x-rays and gamma rays) are known as ionizing radiation because each photon has enough energy to knock an electron off a molecule. Photons of light with frequencies at and below ultraviolet (radio waves, microwaves, infrared, red, orange, yellow, blue, violet) are non-ionizing. They simply don't have enough energy to remove an electron and cause cancer. Increasing the strength of a radio wave will not help it knock off electrons. Increasing the strength of a radio wave simply introduces more photons into the beam. But each radio photon still has the same energy it always does, which is not enough to ionize. When non-ionizing waves flow past a molecule, they shake up the electrons a bit but don't knock them off. This electron shaking leads to heating. A microwave oven heats up your food in the same way a campfire heats up your skin and in the same way a radio wave ever-so-slightly heats up your body. None of them causes cancer, no matter how long you sit there. You may get burned by sticking your hand in the fire or grabbing a hot dish right out of the microwave, but you won't get a tumor. If a radio wave did cause cancer, then candlelight would cause far more cancer, because visible light has far more energy per photon. In terms of radiation, a candle emits far more energy than a cell phone, and they are both harmless. Infrared waves have more energy per photon than radio waves, and everything with a non-zero temperature emits infrared waves. This means that the infrared waves coming from your chair, your pencil, and your clothes would give you cancer long before radio waves from your cell phone would, but they don't. The world is swamped with natural radio waves and infrared waves. Just ask any infrared camera designer what his main obstacle is in designing a camera and he will tell you his greatest challenge is separating the meaningful waves from all the background waves in the air. Anybody who has struggled with an old-fashioned analog TV antenna in an attempt to get a clear picture can tell you that the natural radio waves in the air can be quite overwhelming compared to the man-made ones. People who are worried about being bombarded with radio waves in our increasingly wireless world must realize that the sun and the rocks have been bombarding mankind with infrared and radio waves long before radio broadcast stations and WIFI routers came along. There are electromagnetic waves with high enough frequency to damage molecules: x-rays and gamma rays. That's why if you get too many x-ray scans taken you will get cancer. In addition, alpha, beta and neutron radiation can cause cancer, but they are only created by nuclear reactions, and not by cell phones. Any machine that produces x-rays is known to be dangerous and is heavily regulated in order to ensure safe use.

context: tag/brain/ question: What part of the brain is hurt when you get headaches?

Most headaches have nothing to do with the brain being damaged or strained. There is more to your head than your brain. Surrounding your brain are meninges, bones, muscles, skin layers, lymph nodes, blood vessels, the eyes, ears, mouth, nose and cavities called sinuses. Most headaches are caused by strain or pressure buildup in these other areas and not in your brain. Brain tumors and strokes can cause headaches, but they usually cause other more serious symptoms such as unconsciousness, seizures, paralysis, and vision loss. For people with tumors or strokes, headaches are typically the least of their concerns. And even when a brain tumor does cause a headache, it does so indirectly by applying pressure to the skull and other parts of the head. In fact, the brain itself lacks pain receptors, so it is literally impossible to have pain in your brain. Surgeons can perform operations on the brain without numbing it for this reason. The brain is the place where we process and experience bodily sensations, so all pain ends up getting experienced in the brain. But all pain originates outside the brain. Sometimes a headache seems to be coming from deep within your head, but that is just a psychological/physiological trick where intense pain seems to spread out and come from other places than where it is really occurring. There are hundreds of types of headaches, all with different causes. For generally healthy people, the most common sources of headache are tension and head colds.

context: tag/brain/ question: Why are human brains the biggest?

The brains of humans are not the biggest compared to all other animals. The average human brain has a mass of about 1 kg. In contrast, the brain of a sperm whale has a mass of 8 kg and that of an elephant has a mass of 5 kg. You may be tempted to think that bigger brains means smarter and that because humans are the smartest animals, we must have the biggest brains. But biology does not work that way. When an animal has a larger overall size, all of its organs must be generally bigger just to keep it alive. A sperm whale weighs 35 to 45 metric tons and stretches about 15 meters long. With so much biological tissue to take care of, the sperm whale needs a giant heart to keep blood pumping to all this tissue, giant lungs to provide oxygen to all this tissue, and a giant brain to coordinate it all. The sperm whale needs a giant brain not because it is intelligent, but because it has so much low-level functions to carry out with such a huge body. Intelligence is instead more strongly linked to the brain size relative to the body size. If a large brain is housed in a small body, there is less low-level signal processing and control that the brain has to carry out. As a result, the brain can be freed up for more higher-order thinking. But even this picture is over-simplified. Intelligence seems to depend on many things: absolute brain size, relative brain size, connectivity, energy expenditure, etc. But as a rough model, relative brain size is indeed correlated to intelligence within main groups of organisms. While humans do not have the biggest brains overall, we do have the biggest brains relative to our body size among the mammals.

context: tag/brain/ question: Why does a person with only one working eye have zero depth perception?

Having only one working eye does not lead to zero depth perception. Although using two eyes does indeed play a large role in depth perception, there are also many other approaches that the human visual system uses to perceive depth. In general, approaches that enable depth perception are called "depth perception cues" or simply "depth cues". Depth perception is the ability to see a scene as a three-dimensional world containing three-dimensional objects that move according to three-dimensional physics. All of the various depth cues are described in the sections below, examining first the two-eye depth cues and then the one-eye depth cues. Note that none of my explanations in this article are meant in any way to minimize the suffering and disability that may be experienced by people who have only one functioning eye. Two-Eye Parallax One of the most important depth cues is two-eye parallax. Because each eye is at a different location in the head, each eye sees a slightly different view of the world. The difference between what your left eye sees and what your right eye sees depends on the three-dimensional shape of each object and its location in the three-dimensional world. The closer that an object is to you, the greater the difference between what your left eye sees and what your right eye sees. The human brain is therefore able to extract depth information from the difference between what your two eyes see. If the image of a chair seen by your left eye and the image of the same chair seen by your right eye are nearly identical, then the chair must be far away. In contrast, if these two images of the chair are very different, then the object must be very close. This effect is demonstrated in the diagram shown on the right. The overall geometric effect is called "parallax". When a human is using two eyes in order to take advantage of parallax, it is called "two-eye parallax" or "binocular parallax". The difference between the left-eye image and the right-eye is called "binocular disparity". The ability of the brain to extract depth information from this disparity is called "stereopsis". Vergence The other two-eye depth cue is vergence. When your two eyes both look directly at the same object, they must both rotate slightly toward each other to do this. How much your eyes rotate depends on how close the object is. When an object is far away from you, your two eyes only rotate toward each other a small amount in order to be both looking directly at the same object. In contrast, when an object is close to you, your two eyes most rotate toward each other a large amount in order to be both looking directly at the same object. This is demonstrated in the diagram on the right. The muscles that are involved in the eyeball rotations send signals to the brain about how much the eyes are rotated. The brain can then extract depth from this information. Note that this is an oculomotor depth cue and not a visual depth cue, so images on a flat screen can never convey this depth cue. One-Eye Depth Cues Although both two-eye depth cues—two-eye parallax and vergence—play significant roles in depth perception, they are not the only depth cues. The nineteen other depth cues need only one eye to work. These cues are therefore called "one-eye depth cues" or "monocular depth cues". If a person only has one functioning eye, he can still see depth using these nineteen other depth cues, which are described below. 1. Motion Parallax If you are moving smoothly as you look out at the world, it is equivalent to you remaining motionless and the entire world moving in a corresponding way. As the whole world appears to move, objects that are closer to you will appear to move at a faster speed because of parallax. Your brain understands that all of the moving objects in your view have the same true effective speed (because it's really just one object—your head—that is moving through space). Therefore your brain can determine how far away an object is from you by how fast it appears to be moving. Geometrically, this arises from the same parallax effect that was described in a previous section, but now the different-perspective images result from you moving your eyes to different viewing locations and not from you using two eyes. The figure below demonstrates the "motion parallax" depth cue. The image on the left shows a random collection of motionless dots in three-dimensional space when no motion parallax is present. In contrast, the animation on the right shows the same collection of motionless dots but now with motion parallax present. The particular type of motion parallax presented in this animation represents you moving your head to the left and to the right repeatedly while gazing continuously forward. Because the dots in the animation above do not have a meaningful shape or size, are positioned randomly, and are not actually moving, the only depth cue present is motion parallax. As you can see, just this one depth cue by itself creates a convincing sense of depth. Note that this animation was generated from scratch using a complete analysis of the physics involved and therefore may be more accurate than if 3D-rendering software had been used. The next figure also demonstrates motion parallax. The type of motion parallax presented in this animation represents moving your head forward and backward repeatedly as you gaze continuously forward. Again, because the dots in this animation do not have a meaningful shape or size, are positioned randomly, and are not actually moving, the only depth cue present is motion parallax. Again, the animation creates a convincing sense of depth. The image on the left shows the same collection of dots without motion parallax present. This animation was also generated from scratch using a complete analysis of the physics involved and therefore may be more accurate than if 3D-rendering software had been used. 2. Kinetic Depth Effect Physical objects tend to move in common ways which your brain understands and can use to extract depth information. For instance, for a rigid object that is rotating, every part of the object travels along a circular path around the same rotational axis in the real world. When viewed, every part will appear to be traveling along an elliptical path around the same rotational axis. Furthermore, the size of a part's elliptical path depends on how far away that part is from the rotational axis. Your brain can detect this and extract depth information. Your brain can also do this type of thing with other common types of motion, such as radial motion, projectile motion, wave motion, and walking motion. In the figure below, there is a random collection of dots with no meaningful size or shape. In the image on the left, the dots are motionless. In the animation on the right, the dots are rotating in unison about a common rotational axis. In both cases, the observer is stationary. The animation therefore shows the rotational version of the kinetic depth effect. Because your brain subconsciously understands and has experience with the physics of rotating objects, the animation below appears to have depth. The kinetic depth effect is different from motion parallax. Motion parallax involves a moving observer looking at a motionless world (and therefore always results in the whole world seeming to move in the same direction), while the kinetic depth effect involves a motionless observer looking at moving objects. In some particular cases, the physics of motion parallax and of the kinetic depth effect may be equivalent. However, this is generally not the case. There is no way in which an observer can move his head that will produce a result equivalent to the kinetic depth effect of waving motion, walking motion, explosive motion, and so forth. The animation above was intentionally created using dots with no meaningful shape, size, or location so that the only depth cue present is the kinetic depth effect of rotational motion. This animation was generated from scratch using a complete analysis of the physics involved and therefore may be more accurate than if 3D-rendering software had been used. The figure below also demonstrates the kinetic depth effect of rotational motion. In this case, the animation involves the rotation of a single object with a three-dimensional shape rather than several objects with no shape. The creature in the image on the left is not rotating and therefore seems to be flat. In fact, you can't even tell which creature it is. In contrast, the animation on the right is rotating, giving you a sense of the three-dimensional shape of the T-Rex. Interestingly, because the animation above involves an extended object with a meaningful shape, the rotational kinetic depth effect not only provides you with positional depth information, it also provides you with shape-related depth information. Note that the two animations above correctly include the size perspective effect that is inherent in the observation of true rotational motion. For instance, the dinosaur's head appears slightly larger when it is closer to you and moving to the right, and then slightly smaller when it is farther away from you and moving to the left. Because of this perspective effect, your brain can only properly see the dinosaur above rotating in one direction (with its head closest to you when it's moving to the right). This is different from bistable rotation animations that intentionally omit size perspective effects in order to create bistable perception illusions. The figure below also demonstrates the kinetic depth effect. In this case, the animation involves three-dimensional wave motion. Because your brain subconsciously understands and has experience with true wave motion, the animation above appears to have depth. Note that the animation above contains only a random collection of dots that have no meaningful shape or size. Therefore, the only depth cue present is the kinetic depth effect of wave motion. The animation above was generated from scratch using a complete analysis of the physics involved and therefore may be more accurate than if 3D-rendering software had been used. The figure below demonstrates a different type of kinetic depth effect. In this case, the animation presents the familiar motion of a person walking past a wall. Because your brain subconsciously understands and has experience with walking motion, the animation below appears to have depth (i.e. the walking person seems to be in front of the wall). Amazingly, the animation below contains no direct shape information of any kind, and yet you can still see a human walking. This animation was specifically constructed so that it contains only one depth cue: the kinetic depth effect of a person walking. The image on the left shows the exact same collection of dots but without any kinetic depth effect. As a result, you don't see on the left a human in front of a wall. Note that for accuracy, this animation was generated using motion capture data. The animation below also demonstrates the kinetic depth effect of a human walking. In this animation, you not only perceive a three-dimensional human figure walking. You also perceive that the human is walking obliquely toward you in three-dimensional space, which gives an added sense of depth. The image on the left shows the same collection of dots without any kinetic depth effect present. Incredibly, the animation above literally shows just twelve black dots with meaningless shapes and sizes moving on a flat white screen. And yet, your brain can perceive a stunningly realistic three-dimensional human figure moving in a stunningly realistic three-dimensional way. This is because your brain is so finely attuned to the detailed three-dimensional physical motion involved when a human is walking and how it appears to the human eye. Note that this animation includes a two-second frozen image at the beginning so that you can effectively recognize that it is motion that gives the sense of depth. As before, the animation above was generated using motion-capture data for the sake of accuracy. 3. Depth from Optical Expansion When an object is moving steadily toward you, its apparent size increases in a specific way. The rate at which it appears to get bigger depends on how far away it is and how fast it is moving toward you. When the object is far away, it will appear to get bigger very slowly. When the object is very close, it will appear to get bigger quickly. This effect is called optical expansion. Your brain can deduce not only the object's motion but also the object's distance. Note that the reverse is also true: an object moving steadily away from you appears to get smaller at a rate that proportional to its distance. When a baseball is thrown toward you, your brain uses optical expansion to keep track of its distance. This helps you properly catch the ball at the right time. The optical expansion depth cue is similar to the kinetic depth effect cue, except that in the kinetic depth effect cue, your brain is analyzing the apparent speed at which the object changes location in space. In contrast, in the optical expansion depth cue, your brain is analyzing the rate at which the apparent size of the object changes. A train that is traveling directly toward you would have zero kinetic depth effect but would have significant optical expansion. The figure below demonstrates the "optical expansion" depth cue. The animation on the right shows a baseball that is correctly experiencing the optical expansion that results from being thrown toward you. For comparison, the image on the left shows the same baseball without optical expansion. (Also included in the animation is a little bit of spin and kinetic depth effect because the animation would look strangely unnatural without these.) To be completely clear, optical expansion does not only involve an object appearing to get bigger as it moves toward you. It also involves the object getting bigger in a specific, non-linear way that is dictated by the physics. Your brain subconsciously understands and has experience with this physics and can therefore extract depth information. Note that this animation was generated using physically accurate calculations of the baseball's location, speed, and apparent size as a function of time for a baseball undergoing curveball projectile motion. 4. Familiar Shape If an object has a familiar shape, your brain can recall from memory the apparent shape of that object that corresponds to viewing the real object, and then extrapolate from there. In this way, the three-dimensional shape of the object can be perceived without needing any other depth cues. The figure below demonstrates the "familiar shape" depth cue. The image on the right contains in reality a collection of straight black lines and gray areas on a flat white screen. However, the lines are arranged in the familiar shape of what you see when you look at a real table. You therefore perceive depth. The image on the left shows the exact same number of straight lines attached at the same places as the right image, but it does not seem to have depth because the angles of the lines are all "wrong". In other words, the object on the left does not have the correct familiar shape that occurs when viewing a real table. Note that for the image on the right, I have intentionally drawn the table without perspective effects so that the only depth cue is the familiar shape cue. 5. Relative Size If two objects in your field of view are the same type of object, then your brain assumes that their true physical sizes must be the same. Therefore, your brain assumes that the difference in their perceived sizes must be solely caused by perspective effects. Your brain can therefore extract depth information based on how much the perceived sizes of the two objects differ. For instance, if two single-story houses are in view, then the house that appears to be five times taller than the other house must be about five times closer to you. The figure below demonstrates the "relative size" depth cue. For the image on the right, your brain notices that each of the four objects has the same shape and therefore assumes that they all have the same true size. Therefore, your brain perceives that the smaller objects must be farther away. In contrast, the objects in the image on the left all have the same size and therefore appear to be at the same distance. I have intentionally chosen here an object with an unfamiliar size and shape so that only the depth cue present is the relative size depth cue. 6. Familiar Size If a certain object has a known size, then its perceived size corresponds to how far away it is, even if there are no other objects in the field of view to compare it to. Your brain can therefore extract depth information from the perceived size of the object. For instance, an apple is usually a few inches tall. An apple that appears to be much smaller than this must therefore be far away. The figure below demonstrates the "familiar size" depth cue. The image on left includes two non-specific, unfamiliar objects so that no depth cues are present. As a result the two objects in the left image appear to be the same distance away. In contrast, the image on the right includes two familiar objects. Because you are familiar with baseballs and soccer balls, and you know that the true size of a baseball is smaller than the true size of a soccer ball, your brain perceives that the soccer ball must be farther away. In order to get this effect to work well while looking at this figure, try to visualize the balls as real objects in a real scene. 7. Estimated Size Amazingly, even if you see an object that has nothing to compare it to and has an unfamiliar shape and size, your brain can still extract depth information from its perceived size by estimating its true size. In other words, your brain estimates the most probable true size of the object and then uses this as if it were a familiar size depth cue. The estimated size depth cue is not particularly effective because the estimated size will typically not be very accurate. The figure below demonstrates the "estimated size" depth cue. Although the objects in the image on the right are unfamiliar, your brain may assume that cylindrical objects in everyday life (like soup cans) tend to have a small true size while conical objects in everyday life (like Christmas trees) tend to have a large true size. Therefore, your brain may assume that the conical object in the right image is much bigger in true size and therefore must be farther away from you than the cylindrical object because it does not look that much bigger. If you have a hard time seeing depth in the placement of the two objects in the right image, don't worry because this depth cue is not particularly effective. 8. Uniform Size For a single, extended object that is known to be roughly constant in size along its length, the parts of the object that appear to be smaller must be farther away because of perspective effects. For instance, a baseball bat is roughly constant in width along its length. Therefore, the end of the baseball bat that appears to be much smaller than the other end must be much farther away. In art, this effect is called "foreshortening". The figure below demonstrates the "uniform size" depth cue. A cylindrical rod in the real world has a uniform size along its length. Therefore, when one end of the rod appears larger than the other end, your brain correctly sees the larger end (the red end in this case) as the closer end. When looking at the image on the right, notice how the red end of the rod seems to be sticking out of the screen. In contrast, the image on the left shows the same rod but without the "uniform size" depth cue present. 9. Parallel Lines This cue can be thought of as a general case of the uniform size depth cue. This is because when two lines are parallel to each other in the real world, this is equivalent to a single overall object having a uniform size along its length. For instance, a straight road extending away from you has a uniform size along its length, but can be thought of as two parallel lines (i.e. the two sides of the road). Two lines that are parallel to each other in the real world will be perceived as converging toward each other as they stretch farther away from you. If your brain knows that the two lines are parallel in the real world then it can extract depth information based on how close the lines appear to be. The places where the lines appear closer to each other must be farther away from you. The figure below demonstrates the "parallel lines" depth cue. The image on the right shows a scene involving two roads on a flat ground plane with this depth cue at work. Therefore, these roads appear to be stretching away from you into the distance. In contrast, the image on the left shows the same scene but without this depth cue, leading it to look flat. For a set of parallel lines that all extend exactly away from you, they will all appear to meet at one vanishing point at the center of your field of view, as shown in the figure above. In contrast, if a set of parallel lines extends away from you at an oblique angle, then these lines will all appear to meet at one vanishing point that is not at the center of your field of view. This effect is shown in the figure below. The image on the right shows two sets of parallel lines on the ground that each has its own non-central vanishing point. The image on the left shows the same scene but without any depth cues. In general, every set of parallel lines has its own vanishing point. Therefore, there could be an infinite number of independent vanishing points in an image. Interestingly, if all the sets of parallel lines in a scene are all parallel to the ground plane, all of their vanishing points will lie on the horizon line (which is where the sky appears to meet the ground). This may seem like a rare case, but humans love to build things with surfaces parallel to the ground, so it is actually quite common. It is so common, in fact, that some people mistakenly think that vanishing points must always lie on the horizon. In every day life, humans tend to build objects that are a box shape or a collection of box shapes, such as buildings, desks, shelves, cabinets, books, tables, beds, and so forth. The edges of a box form three sets of parallel lines. Therefore, a collection of boxy objects that have their faces aligned will have only three vanishing points. First instance, a row of houses has most of its edges appear as lines converging at one of the three vanishing points. For such cases, artists speak of drawing in "three-point perspective". Sometimes in art, the vertical vanishing point is ignored (so that all lines that are vertical in real life are drawn as vertical on the paper). For a collection of aligned boxy objects, this reduces the situation down to two vanishing points, which artists call "two-point perspective". If there is a collection of aligned boxy objects and two of the dimensions are drawn without perspective, then there is only one vanishing point, which artists call "one-point perspective". These concepts are shown in the figure above. Note that the parallel lines depth cue is not a special case of the horizon effect depth cue. The perception of depth established by parallel lines arises from the lines converging at a vanishing point and not from objects being close to the horizon. In fact, the parallel lines depth cue works even if there is no horizon at all. The figure below shows a situation where there is no horizon. In the image on the right, all of the lines that are running along the length of the tunnel meet at the central vanishing point. In contrast, the image on the left shows the same tunnel without the parallel lines depth cue present, insofar as that is possible. Note that if there is not a horizon but there are vanishing points, the horizon effect still occurs in the sense that the closer an object appears to be to a vanishing point, the farther away it seems to be. However, the vanishing point horizon effect still has nothing to do with parallel lines directly. 10. Texture Gradient Similar to how objects that are closer to you appear larger, parts of the pattern in a texture that are closer to you will appear larger. Your brain can therefore extract depth information from how the different parts of a texture compare to each other in perceived size. Additionally, the texture of a surface can indicate the tilt of the surface, which can help portray the three-dimensional shape of objects. The figure below demonstrates the texture gradient depth cue. In the image on the left, all of the spots of the textured surface are perceived as being the same size, the same shape, and at about the same spacing, making this image appear flat. In contrast, the image on the right shows that the dots near the top of the image are smaller, closer together, and more distorted than the other dots, giving the impression that they are farther away. Note that the left image and the right image in the figure above show the exact same textured surface with the dots in the same places. The texture gradient effect works not only on flat ground planes. It can also portray the three-dimensional shape of complicated objects and scenes. For instance, the figure below is the same as the figure above, except that a canyon has been cut in the ground. The three-dimensional shape of the canyon is made apparent in the image on the right by the texture gradient depth cue. Note that there are no other depth cues present in this image (except for a small amount of recess shading). The image on the left shows the same texture and the same canyon but without the texture gradient depth cue. Another example of the texture gradient depth cue is shown in the figure below. As this figure demonstrates, a texture gradient does not have to consist of a pattern that has been painted on a flat surface. It can also consist of a large collection of three-dimensional objects that are situated so that they approximately form a flat surface. An additional example of the texture gradient depth cue is shown in the figure below. As this figure demonstrates, a texture gradient does not have to consist solely of independent features or objects. Rather, it can consist of an interconnected pattern. The image on the right includes the texture gradient effect. As a result, the top of the image appears to be farther away from you than the bottom of the image. In contrast, the image on the left shows a texture but without the texture gradient effect, making it look flat. 11. Horizon Effect For an object sitting on the ground, the physics dictates that the closer the object's center appears to be to the horizon, the farther away the object is from you. Your brain can therefore estimate how far away an object is from how close its center appears to be to the horizon line. The figure below demonstrates the "horizon effect" depth cue. In the image on the left, all three objects are at the same height in the image. In contrast, the image on the right shows the same three objects but at different perceived heights. Your brain sees the blue cone as visually closer to the horizon and therefore perceives that it is farther away from you than the other objects. Interestingly, the horizon can also take the form of a vanishing point. For instance, for objects sitting in a tunnel, the closer that an object appears to be to the tunnel's vanishing point, the farther away it seems to be. The figure below demonstrates the "horizon effect" depth cue when there is a vanishing point instead of a horizon line. In the image on the right, the blue cone appears to be closer to the vanishing point and therefore is perceived to be farther away from you. The image on the left shows the same scene, insofar as it is possible, without the horizon effect depth cue or the parallel lines depth cue. 12. Occlusion When a near object is roughly in the same line of sight as a distant object, the near object will partially or completely block the view of the distant object (assuming it is not transparent). Therefore, the object that is being blocked from view must be farther away from you. This effect can be called occlusion, interposition, eclipsing, or overlapping. Your brain understands this effect and can use it to determine the relative distances of objects. The figure below demonstrates the "occlusion" depth cue. In the image on the left, the three objects are all clearly visible with no occlusion and therefore you cannot tell which object is closer. In contrast, the image on the right shows the same objects but includes occlusion. You are therefore able to perceive the red cylinder as being closer to you and the blue cone as being farther away from you. (A small amount of horizon effect had to be included in order to prevent the objects from unnaturally penetrating each other.) Note that the occlusion depth cue can only tell you which object is closer to you. It cannot tell you the absolute distance of an object. The occlusion effect does not have to involve three-dimensional shapes. Even with flat pieces of paper, you can tell which piece of paper is farther away because it is the one being occluded. This is shown in the figure below. The image on the right shows one paper occluding another paper, in two different configurations. In both configurations, the paper that is being partially blocked appears to be farther away. The same two papers are shown in the image on the left but without the occlusion depth cue, making it impossible to tell which one is farther away. The figure below also shows occlusion. However, in this case, there is a single object with its front face occluding its back face, rather than one object occluding a separate object. The figure on the right shows a box that is defined by its edges, presented in two different configurations. The occlusion effect gives you a sense of which face of the box is closest to you. In this way, occlusion can help give a sense of depth to an object. In contrast, the figure on the left shows the same box without occlusion information. As a result, you can't tell which configuration the box is in or which face is closest to you. The figure below shows another example of occlusion. In this case, for dramatic effect, the occlusion depth cue has been combined with the optical expansion depth cue and the kinetic depth effect. When the baseball is partially hidden by the bars, you perceive it to be moving behind the bars. When the baseball partially hides the bars, your perceive it as moving in front of the bars. Because the white bars are visually part of the frame, the baseball seems to fly out of the image at the end. 13. Surface Shading The way that light falls on an object depends on the three-dimensional shape of the object. Therefore, your brain can extract depth information from the shading on an object. The parts of an object that are darker tend to be the parts that are titled away from the light source. Therefore, if the position of the light source is known (or can be estimated), the tilt in three-dimensional space of each part of an object's surface can be deduced from its level of shading. The figure below demonstrates the "surface shading" depth cue. Note that in this case, we are not focusing on the depth perception related to the position of each object but on the depth perception related to each object's three-dimensional shape. In the image on the right, the surface shading enables you to see the circular object as a three-dimensional sphere and the other object as a three-dimensional cylinder. The fact that the shading varies smoothly along the surfaces enables you to perceive the sides of the cylinder and the entire sphere as smoothly round. In contrast, the image on the left shows the exact same objects but without surface shading. As a result, the two objects look like flat paper cutouts. 14. Recess Shading The points on an object or landscape that are recessed will appear darker because light has a harder time reaching down into the recess. The recess shading therefore conveys the depth and shape of the recesses. Through this depth cue, your brain is able to perceive the presence, the shapes, and the depths of holes, recesses, cracks, corners, inlets, and narrow spaces.The figure below demonstrates the "recess shading" depth cue. The image on the left contains three holes that have no recess shading. As a result, they don't even look like holes. In contrast, the image on the right shows the same holes but now with recess shading included. As you can see, the shading enables you to see the holes and to see their three-dimensional shapes. The figure below also demonstrates the "recess shading" depth cue, this time combined with the "parallel lines" depth cue. As a result of the depth cues, the image on the right appears to show an arched tunnel that stretches away from you into the distance. As you can see, including two depth cues instead of one makes the image's sense of depth even more convincing. For comparison, the image on the left shows the same tunnel without any depth cues, insofar as it is possible. The figure above shows the same tunnel as in the previous figure, but now including only the "texture gradient" and "recess shading" depth cues, instead of the "parallel lines" and "recess shading" depth cues. For comparison, the image on the left shows the same tunnel without any depth cues, insofar as it is possible. 15. Shadow Shape The shape of a shadow depends on the three-dimensional shape of the object that is casting the shadow. Therefore, your brain can partially deduce three-dimensional shape information from an object's shadow. The figure below demonstrates the "shadow shape" depth cue. In the image on the left, you see the outline of some creature, but it is hard to see the three-dimensional shape of the creature or even what kind of creature it is. In contrast, the image on the right shows the same creature but now being illuminated from the side so that its shadow falls on the left wall. This shadow reveals the creature to be a T-Rex and partially reveals the three-dimensional shape of this T-Rex. In general, this depth cue works even if the illumination is not aimed directly toward a wall, as demonstrated in the figure below. The image on the right involves a shadow that is cast obliquely on the ground. This shadow reveals that this structure is townhouses. This shadow also enables your brain to more effectively see the townhouses as three-dimensional objects. In contrast, the image on the left shows the same structure without a shadow, which causes it to appear as a non-descript blob of black. 16. Shadow Size, Location, and Blurriness The size, location, and blurriness of an object's shadow all depend on how far away the object is from the shadowed surface. In general, the farther away an object is from the shadowed surface, the larger, the blurrier, and the more shifted its shadow will be. Your brain can therefore deduce distance information from the size, location, and blurriness of shadows. The figure below demonstrates this depth cue. The image on the right shows three balls and their shadows. The shadow of the rightmost ball is larger, blurrier, and more shifted, indicating that the rightmost ball is farther away from the ground and closer to you. In contrast, the image on the left contains the same three balls but without shadows so that there is no depth to the scene beyond the roundness of the balls. The figure below also demonstrates these shadow effects. The image on the right shows the shadow location depth cue at work but does not include differences in shadow blurriness or shadow size. Even with just this one type of shadow depth cue at work, your brain can still perceive that the rightmost paper is farther away from the checkered surface and closer to you. 17. Atmospheric Effects When an object is very far away, the air between you and the object changes its appearance. Air is not perfectly transparent. The nitrogen and oxygen molecules that make up 99% of atmospheric air give a distant object a slight white-blue tint under daytime lighting conditions. As an additional effect, the water droplets in the air give the air a slight white or murky grey appearance. These effects also cause the final image to diminish in contrast, color saturation, and sharpness. The end result is that the farther away an object is, the more it will have a muted blue-white color and a softer, blurrier appearance. Your brain can therefore deduce the distance of an object based on how much its image is degraded by atmospheric effects. Note that atmospheric effects only become significant when the light from an observed object travels through large amounts of air. Therefore, this visual cue only works for objects that are very far away (unless it's an extremely foggy day). You probably use this visual cue more than you realize. Astronauts who have walked on the moon reported that because the moon lacked an atmosphere, all of the distant hills looked much closer than they actually were, which was disorienting. They reported that as they walked toward a hill, it seemed to recede at the same rate. The figure below demonstrates the "atmospheric effects" depth cue. In the image on the right, a series of mountains at different distances are observed to have different shades and colors because of the intervening air. In contrast, the image on the left shows the exact same mountains but without any atmospheric effects. As a result, all the mountains visually merge together and look flat. Note that the figure above was intentionally drawn as simple as possible in order to clearly demonstrate the effects. The figure below also shows atmospheric effects, but now using an actual photograph of the real world. The image on the right is a raw photograph of a mountain landscape, without any photo editing. The blue-white tints in this photograph are completely natural. This photo demonstrates that the farther away a mountain is, the more it appears blue-white, unsaturated, and contrast deficient. Note that the sky is blue-white for the same reason that the distant mountains are blue-white, because of the effect of the atmosphere on the light passing through it. The image on the left shows the exact same photo but without any atmospheric effects. To create the image on the left, I took the raw photograph and removed the atmospheric effects using photo editing software. (This involved removing the blue tint and increasing the saturation and contrast one layer of mountains at a time.) Notice how all of the mountains in the left image seem to merge together into one indistinct mass without much depth. Interestingly, the image on the left looks like it came from a video game that failed to properly include atmospheric effects. 18. Accommodation and Pupil Response In order for the human eye to properly focus on objects that are at different distances from it, the ciliary muscles in the eye must change the shape of the eye lens by changing the amount of muscle contraction. To bring a distant object into focus, the ciliary muscles relax, which allows the lens to flatten. To bring a near object into focus, the ciliary muscles contract, which pushes the lens into a rounder shape. The human eye has sensory mechanics to detect how much the ciliary muscles are contracted. In this way, your brain can deduce the distance of an object by focusing on it and then sensing the contraction level of the ciliary muscles. Interestingly, this depth cue depends on muscle contraction information rather than image information, so I can't demonstrate how it works using images. Pupil response also helps accommodation. The size of the pupil slightly effects how much an object appears to be in focus. The shape of the lens in your eye gives rise to optical aberrations. As a result, the more of the lens that is used, the blurrier the image. Therefore, your pupil works along with the ciliary muscles to bring objects into focus. Your brain uses pupil constriction information along with ciliary muscle contraction information to determine the object's distance. 19. Depth from Defocusing When the human eye brings a certain object into focus, objects that are at a different distance will appear blurrier. The amount of observed blur depends on how far away in the forward direction the other objects are from the object that is in focus. Specifically, the farther away an object is in the forward direction from the object in focus, the blurrier it will appear. Your brain can therefore deduce distance from the amount of defocusing blur. The figure below demonstrates the "depth from defocusing" depth cue. The image on the left shows three strawberries without defocusing blur. As a result, they all appear to be the same distance away. In contrast, the animation on the right shows the same strawberries with defocusing blur included. (A small amount of the relative size depth cue has also been included to prevent the image from looking unnatural). In the animation, the point of focus repeatedly shifts between the left strawberry and just in front of the middle strawberry. As a result, the left strawberry appears to be closer to you. Summary As you can see, the human visual system uses about twenty-one different depth cues! The exact number will depend on how you decide to group special cases into categories. Of the twenty-one depth cues, only two require using both eyes. The other nineteen depth cues work perfectly with only one eye. Movie theaters that present two-eye parallax depth cue information in addition to the traditional one-eye depth cues require wearing special glasses to properly see the images. These movies are commonly, but incorrectly, called "3D movies". They do not include all of the depth cues and are therefore not fully 3D. These movies are more accurately called "stereoscopic movies". Also, traditional movies that don't require wearing the special glasses contain most of the one-eye depth cues and are therefore already very close to being 3D-realistic. Because of this, "3D movies" are not that much more 3D-realistic than traditional movies. This is perhaps why "3D movies" have not replaced traditional movies, despite having existed for over a hundred years. Neither traditional movies nor "3D movies" contain accommodation cues, pupil response cues, true depth-of-focus cues, vergence cues, or true motion parallax cues. Despite all of this, traditional movies and "3D movies" can still appear convincingly three-dimensional because they include many of the other depth cues. In summary, the human visual system is quite capable of seeing depth even if one eye is not functioning. Thankfully, this also means that humans can see depth for objects and scenes displayed on a flat computer screen or movie screen. It also means that artists who understand the one-eye depth cues can create a convincing sense of depth when painting or drawing on paper, canvas, or another flat surface.

context: tag/cancer/ question: Did cancer exist before man-made chemicals were around to create it?

Yes, cancer existed long before humans were able to create synthetic substances that cause cancer (cancer-causing agents are called "carcinogens"). Unfortunately, cancer is a natural part of life. Every multicellular creature on earth can get cancer, even in the remotest locations (yes, even sharks get cancer). While some man-made chemicals can indeed cause cancer, they are not the only causes. Strictly protecting yourself from every man-made carcinogen will not guarantee that you will never get cancer. First of all, cancer is an umbrella term that encompasses hundreds of different diseases. What all of these diseases have in common is that normal cells are changed so that they begin to reproduce in an abnormal way, spreading through the organism and harming it. There is a vast array of mechanisms which can lead to cells reproducing abnormally. As with other biological processes, cell reproduction is ultimately controlled by the DNA of the cells. When a cell's DNA is changed, the cell is said to be mutated. Mutations are often harmless. However, if a mutation occurs in a portion of the DNA that controls cell reproduction, it can cause the cell to reproduce abnormally and pass on its mutation to the daughter cells. Any agent that leads to the mutation of DNA has the potential to cause cancer. Cancer-causing agents include (see the Notes Section at the end for more details): All of these carcinogens existed before humans developed the technology to create synthetic chemicals. Although human industrial activity can increase a person's exposure to natural carcinogens, this does not change the fact that these carcinogens are natural. Even if you were somehow able to protect yourself from 100% of the carcinogens; whether natural or man-made; you could still get cancer. Cancer can form even when no harmful agents are present. Every time a cell reproduces, it must make copies of its DNA for its daughter cells. Due to the random fluctuations present in all molecular movements, copy errors can result during DNA replication even when no carcinogens present. In this way, DNA mutation is a natural part of cell replication. Many mutations are harmless. Some mutations are even beneficial and help drive evolution. However, mutations sometimes lead to cancer. Because of the presence of natural carcinogens in the environment, and because mutation is a natural part of cell replication, cancer happens even in the absence of man-made chemicals. Despite the fact that cancer is natural, we can certainly reduce our risk of cancer by avoiding tobacco, alcohol, arsenic, radon, ionizing radiation and other carcinogens; by eating more fruits and vegetables; by exercising regularly; by getting vaccinated; and by using sunscreen. NOTES Alcohol (ethanol), such as found in beer and wine, is the natural byproduct of yeasts fermenting sugar and can thus be found anywhere that yeast is found, including in overripe fruit and in palm tree sap. Alcohol is known to cause cancers of the mouth, throat, laryx, esophagus, liver, colon/rectum, and breast (source). Arsenic is a chemical element found in many natural minerals. Natural arsenic is known to seep into groundwater, contaminating water sources used by humans, especially in regions of the world where natural arsenic levels are high. Arsenic has been found to cause cancers of the lungs, bladder, kidney, and skin (source). Asbestos is a naturally-occurring silicate mineral that contains small, strong, needle-like fibers. When asbestos dust is inhaled, the fibers irritate and puncture lung cells, causing scarring and chemical imbalance that can lead to cancer. Asbestos is a mechanical carcinogen rather than a chemical carcinogen since it is the small size and hard, needle-like shape of the asbestos that enables it to damage cells rather than its chemical makeup. All minerals that have small, hard, needle-like fibers are suspected to be carcinogenic. Asbestos is known to cause cancers of the lungs, pleura, larynx, and ovaries (source). Cosmic rays are high-energy particles emitted by distant supernovae outside of our solar system. Cosmic rays constantly rain down on the earth and have enough energy to ionize the atoms in biological cells, thereby potentially causing cancer. The earth's atmosphere provides somewhat of a shield against cosmic rays. However, people who spend a lot of time on airplane flights do not benefit as much from atmospheric shielding and have increased exposure to cosmic rays and solar radiation (source). Helicobacter pylori (H. pylori) is a bacterium that grows in the mucus layer of the stomach. H. pylori has been found to cause stomach cancer (source). Hepatitis B virus is a virus that affects humans who are exposed to infected bodily fluids. This virus has been found to cause liver cancer (source). Human papillomavirus (HPV) is a virus that has been found to cause about 70% of cervical cancer cases. Additionally, HPV has been found to cause cancers of the vulva, vagina, penis, oropharynx, and anus (source). Lead is a chemical element that occurs naturally in minerals, soil, plants, and animals. Since lead is a stable, solid metal under standard conditions, exposure to lead usually occurs through ingestion or inhalation. Lead likely causes lung cancer and stomach cancer, although the evidence is not yet conclusive (source). Radon is a natural radioactive chemical element that is produced by the decay of radium in soil and rocks. As a gas, radon tends to collect in enclosures close to the ground, such as basements. Radon decays to other radioactive isotopes which decay and emit ionizing radiation. Radon is known to cause lung cancer (source). Solar radiation contains ultraviolet radiation which damages skin cells. Ultraviolet radiation is also emitted by tanning beds. Prolonged exposure to ultraviolet radiation is known to cause skin cancer (source). Tobacco, such as found in cigarettes and cigars, comes from the leaves of the tobacco plant. Tobacco is known to contain at least 50 different chemicals that cause cancer. Tobacco is the leading cause of preventable cancer. It has been found to cause lung cancer as well as cancers of the mouth, lips, nose, sinus, larynx, throat, esophagus, stomach, pancreas, kidney, bladder, uterus, cervix, colon/rectum, ovary, and blood (source).

context: tag/cancer/ question: How do 5G cell phone signals harm humans?

The electromagnetic waves from all cell phone systems, including 5G systems, cause zero harm to humans. All cell phones and cell phone towers use radio waves to communicate. Radio waves have been in use for wireless communications for over a hundred years with no credible evidence of adverse health effects. The radio waves from cell phones do not mutate DNA, do not damage cells, and do not interfere with biological functions - not even a little bit! Furthermore, all objects naturally emit radio waves as part of their regular thermal emission. The radio waves emitted by a cell phone are physically no different from the radio waves continuously emitted by rocks, trees, desks, and all other physical objects. We are continuously immersed in radio waves emitted by rocks, trees, dirt, buildings, pencils, watermelons, physics professors, etc. Furthermore, the world has been immersed in these radio waves since the dawn of time, long before radio antennas came along. Radio waves are also what are used by AM/FM music radio stations. Note that I already effectively answered this question ten years ago. You can read my previous answer here. This article is actually nearly identical to that article that I wrote ten years ago. I am writing this article, despite being redundant, because of several requests from my readers to address 5G technology specifically. 5G cell phone communication networks may have more sophisticated engineering protocols than previous generations of cell phone communication networks, but they still use just plain old radio waves. "5G" stands for fifth generation. 5G radio waves are physically identical to 4G radio waves in terms of their overall physical characteristics. They are different in that 5G uses more clever ways to encode and process the information carried by the radio waves. The engineering is different but the physics is not. There are two ways that electromagnetic radiation in general can damage biological tissues, cells, and functioning: through ionization events and through bulk heating. An ionization event is when a bit of electromagnetic radiation knocks a single electron off of an atom or molecule. This can cause the chemical bond between two atoms to break, or can cause an atom to become a reactive ion which can then go on to break chemical bonds. If this happens too much, it can lead to mutations, cancer, radiation sickness, and/or tissue damage. However, only ionizing radiation has enough energy per photon to create an ionization event. The only forms of electromagnetic radiation that are ionizing are high-frequency ultraviolet rays, x-rays, and gamma rays. Radio waves have far too little energy per photon to ionize atoms. We are not just talking about a low amount of ionization from radio waves. Radio waves cause exactly zero individual atomic or molecular ionization events. They simply have far too little energy per photon to ionize atoms. In contrast, x-rays, some ultraviolet rays, and gamma rays do have enough energy per photon to cause individual ionization events. That is why there must be radiation-safety protocols, radiation shields, and radiation-safety-trained professionals in order for an x-ray machine to be used at a dentist's office, while none of this is needed at all to sit on a bench or walk by a rock or stand on the sidewalk, even those these things are continuously naturally emitting radio waves. The non-ionizing nature of radio waves is why you can listen to your favorite FM music radio station in your car and need zero radiation-safety equipment, zero radiation-safety protocols, and zero radiation-safety-trained professionals. Radio waves have far less energy per photon than visible light. If radio waves caused cancer then the regular visible light coming from a candle would cause thousands of times more cancer. But they don't. They do not have enough energy per photon to be ionizing. Note that all of this is very basic, very well-established, mainstream physics that has been known and verified countless times throughout many decades. I am not just guessing about this or sharing an opinion. The other way that electromagnetic radiation can affect biological tissue, cells, and functioning is through bulk heating. Bulk heating does not require a high energy per photon. It just requires a high total intensity of the electromagnetic wave. For this reason, all forms of electromagnetic radiation can cause bulk heating, including radio waves. This is why the microwave radiation in a microwave oven can heat up food while still having zero ability to cause individual ionization or cancer. Bulk heating from non-ionizing electromagnetic radiation is also what causes you to feel warm when you are standing near a campfire. The campfire emits radio waves and infrared waves with high enough intensity to warm you. If cell phone signals were powerful enough to heat you significantly, it would be no different from being warmed by a campfire. Bulk heating can indeed damage biological tissue by burning it. However, radio waves must have a very high total intensity to cause burns and thereby damage tissue. The radio waves used by cell phones, AM/FM radio channels, WIFI, and all other forms of radio communications are far too low in intensity to cause burns. Furthermore, radio waves that do indeed have high enough total intensity to cause physical burns through bulk heating are hard to generate and rare. For this reason, almost all types of burns that the typical person encounters; such as sunburns, burns from fires, burns from toasters, burns from irons, burns from electrical shocks, burns from acids, burns from heaters, burns from over-heated appliances, and so forth; are not caused by radio waves. The amount of radio waves emitted into the air by 5G telecommunications equipment is a drop in the bucket compared to the radio waves already present in the air from other sources. Furthermore, unlike the early stages of cancer and radiation poisoning, a physical burn from electromagnetic-wave bulk heating causes immediate pain, instantly triggering the person to back off from the source. If the radio waves from a 5G cell phone were powerful enough to cause burns (which they are not), it would feel just like getting burned from touching a hot iron. You would immediately pull your hand away before too much damage was done. Non-ionizing electromagnetic radiation can only heat and burn tissue in a non-specific, bulk-material manner; and cannot make individual changes at the cell level or molecular level. Note that the atoms and molecules that make up cells, cell parts, fluids, and other biological matter do indeed interact through the electromagnetic interaction. In fact, all chemical bonds and chemical reactions are ultimately caused by electromagnetic field effects. However, the atoms and molecules of biological matter interact through static atomic-scale electric fields, which are completely different from radio waves. Surprisingly, some people feel passionately that 5G signals are harmful to humans, despite there being zero credible evidence and zero physical plausibility. Some people believe that the secret purpose of installing 5G technology is to harm humans. The problem with this type of conspiracy theory (i.e., the widespread poisoning of public spaces by conspiring organizations) is that political and technology leaders are also human and also use public spaces. If the people responsible for implementing 5G technology were really causing harmful electromagnetic radiation to be broadcast into public spaces, they would also be harming themselves, their children and their loved ones. While there may indeed be a handful of evil or mentally ill people that are happy harming themselves and their loved ones while harming others, it is absurd to think that effectively all political and technology leaders, at all levels, in all countries, are happy harming themselves and their loved ones. In summary, 5G cell phone systems use radio waves to transmit information. These radio waves are non-ionizing and too weak to cause burns, and therefore cause zero harm to humans.

context: tag/cancer/ question: How much extra radiation am I exposed to if I stick my hand in the microwave right after it turns off?

You are actually exposed to less radiation if you stick your hand in a properly functioning microwave oven right after it turns off. First of all, the microwaves emitted by a microwave oven are not harmful beyond their ability to heat you. Microwaves are non-ionizing, meaning that they do not have enough energy per photon to rip electrons off of atoms or break chemical bonds, which is what leads to cancer and radiation sickness. In fact, microwaves have far less energy per photon than the light from a candle or even the infrared thermal radiation from your hand. Microwave ovens emit electromagnetic waves in the frequency range of gigahertz (GHz). These are the same types of waves used by radars, cell phones, and WIFI routers. Microwaves can burn you if they are powerful enough and hitting you for long enough, but this is fundamentally no different from being burned by a campfire's thermal radiation. Sticking your hand in the microwave while it is on (which would require breaking the oven's safety features) and leaving it there is a bad idea because you will get burned. Also, a properly functioning microwave oven automatically turns off the moment you open the door. The last bit of microwaves emitted by the oven bounce around inside and are absorbed within microseconds, long before you have even finished opening the door. By the time you stick your hand in the oven, the last bits of microwaves are long gone. Microwaves are a form of electromagnetic waves, just like visible light. The microwaves in the oven disappear when the door opens just as quickly as a room gets dark when you turn off the light. The walls of a microwave oven are metal, which keep microwaves inside from leaking out. The oven is constructed to avoid leaking microwaves not because they cause cancer, but because that would be a waste of energy. The oven's job is to cook food; a job it would not do very well if its energy were leaking out into the room. Interestingly, the metal walls of a microwave oven also block a lot of external radiation (from the sun, stars, rocks, storms, etc.) from getting in. Because of this shielding effect, your hand inside a non-running microwave actually receives less radiation than your hand out in the open air. Either way, the radiation you are exposed to is so low-energy that there is nothing to be concerned about.

context: tag/cancer/ question: Is ionizing radiation always harmful?

No, ionizing radiation is only harmful to an organism as a whole when its amount gets too high. We are constantly bombarded with very small amounts of ionizing radiation that occur naturally, and we get along just fine with our lives without being seriously harmed by this radiation. There are trace amounts of naturally-occurring radioactive atoms in the air, in the rocks, in our food, and inside our bodies. When these atoms radioactively decay, they emit ionizing radiation. By its nature of being ionizing, such radiation can damage individual molecules, even at low intensity. But if the amount of ionizing radiation exposure is very low, our bodies can handle a few damaged molecules without any problem, so that there is no net harm done to our bodies. Ionizing radiation is radiation that has enough energy per particle to rip electrons off of atoms and therefore break chemical bonds. In contrast, radiation types such as microwaves, radio waves, and visible light are non-ionizing, meaning that they do not have enough energy to permanently damage molecules beyond simple heating effects. Ionizing radiation includes X-rays, gamma rays, neutron radiation, proton radiation, and high-speed electrons. Natural sources of ionizing radiation include the radioactive decay of unstable atoms that exist everywhere and cosmic rays from space. Man-made sources include medical scans such as X-ray images as well as nuclear power plants, nuclear weapons testing, and any industrial or scientific process that involves nuclear reactions or high energies. Advocating that humans cease all nuclear activity in order that our exposure to ionizing radiation will be reduced to zero makes no sense since we will always be exposed to some amount of ionizing radiation from natural sources. The more logical approach is to allow nuclear research and technology to proceed, but put strong regulations and safety procedures in place so that humans are never exposed to ionizing radiation amounts that are above the safety threshold. The amount of total harm that ionizing radiation can cause a human depends on the total amount of radiation received, which is a function of the intensity of the radiation and the length of time that the person is exposed to the radiation. The total amount of ionizing radiation received by a body is termed the "dose". Since different tissues react differently to ionizing radiation, of more importance is the "effective dose", which is the total amount of ionizing radiation received that is able to do biological damage. A person that is exposed to higher-than-normal levels of radiation, but only for limited amounts of time, will not receive a significantly higher effective dose and thus may still be in the safe zone. For instance, employees can safely work in nuclear reactor facilities as long as they monitor their radiation exposure and limit their time in the facilities so that their dose does not exceed safe levels. It is hard to set one standard threshold above which radiation exposure becomes seriously harmful since the definition of "seriously harmful" is subjective. Medium-low doses of ionizing radiation can still cause nausea and may still cause a miniscule increase in the chance of getting cancer, although this increase may be too small to be considered significant. Despite the complexity of this field, general safety thresholds can still be set. Experimentally, cancer risk has only been found to increase for doses above 100 mSv per year according to the World Nuclear Association. A good safety threshold should therefore be set at a value that is well below 100 mSv per year. The U.S. Nuclear Regulatory Commission sets the occupational safety limit for ionizing radiation exposure to be 50 mSv per year. For comparison, natural background radiation provides a dose of 3 mSv per year, a single typical banana provides 0.0001 mSv, a single set of dental X-rays provides about 0.005 mSv, a single set of barium X-rays provides about 5 mSv, and a single full-body CT scan provides about 20 mSv. As you can see, a single medical scan is too weak to cause harm even though it may involve ionizing radiation. On the other hand, undergoing several full-body CT scans in a short period of time can make the radiation add up to a total dose that is at a harmful level. For this reason, medical doctors are trained to avoid ordering too many radiation-intensive scans for a single patient in a short amount of time (unless the benefits outweigh the risks, e.g. if the scan helps save a patient from imminent death, it is worth the increased cancer risk). When the dose is high enough, ionizing radiation causes two types of harm to humans: direct tissue damage and cancer. Direct tissue damage happens when enough molecules are broken apart that the cells simply can no longer function. This can lead to radiation burns, radiation sickness, organ failure, and even death. In contrast, cancer results when the cells receive a small enough amount of damage that they can still live and function, but damage to the genes causes the cells to pursue aggressive and uncontrolled growth.

context: tag/cancer/ question: Why are cancer mortality rates rising?

Cancer mortality rates are not rising. They are dropping. For instance, data from the World Health Organization shows that 26% fewer U.S. middle-aged men died of cancer in 2010 than in 1975. For U.S. middle-aged women, the cancer mortality in the same time period has dropped 19%. Surprisingly, the deaths due to coronary heart disease in the U.S. have dropped even more, around 80% for both genders in the last 30 years. In fact, the mortality rate in the U.S. due to almost every cause has dropped significantly over the years. The nation is a healthier, safer place than it was decades ago, thanks in part to technology and modern medicine.

context: tag/cancer/ question: Why don't dark-skinned people get sunburns?

Dark-skinned people do get sunburns. While it's true that the higher pigment levels that make certain people's skin look dark helps protect against sunlight, the pigments do not block 100% of the light. The skin pigment melanin is produced by special skin cells called melanocytes to protect the body from the damaging effects of ultraviolet light. Higher levels of melanin means less sunburn and less skin cancer. But even the darkest-skinned person is not protected 100% from sunlight. A 2010 CDC study found that 13% of black women and 9% of black men reported getting at least one sunburn in the past year. Furthermore, 38% of Hispanic women and 32% of Hispanic men reported getting at least one sunburn in the past year. For comparison purposes, the average value across all ethnicities and genders was 50.1%. While dark-skinned people definitely get fewer sunburns, they still get them. When a dark-skinned person gets a sunburn, it may not be visually noticeable, but the damage is still there. A dark-skinned person with a sunburn still experiences the skin tightness, pain, sensitivity, heat, and peeling that light-skinned people experience. Because dark-skinned people get sunburns, they also get skin cancer from sun exposure. The rates of skin cancer for dark-skinned people are far below that of light-skinned people, but the rates are not zero. Another CDC study found that about 1 in every 100,000 black people gets skin melanomas each year and about 4 in every 100,000 Hispanic people get skin melanomas each year. Dark-skinned people have a higher chance of getting skin cancer from sun exposure than of winning the lottery. But, unlike the lottery, dark-skinned people have a way to drastically alter their odds. Sunscreen protects dark skin against sunlight damage just as well as it protects light skin. About 1 in every 200,000 black people dies from skin cancer each year and 2 in every 200,000 Hispanic people die from skin cancer each year. For comparison purposes, about 6 in every 200,000 white people die from skin cancer each year.

context: tag/cancer/ question: Why isn't there just one cure for cancer?

There is not one cure or treatment for cancer because cancer is not a single disease. The word "cancer" is an umbrella term that includes hundreds of different diseases. Furthermore, cancer is typically harder to battle than infectious disease because there is no foreign agent attacking the body which can be distinguished from the healthy parts of the body. Instead, cancer involves the body's own cells doing the damage. Therefore, there are just as many different types of cancer as there are different types of cells in our bodies. This means that the type of treatment that is most effective will depend on 1. where the cancer exists in the body, 2. the type of cancer, and 3. how far along the cancer is. In an introduction to the publication titled "Nature Insight: Cancer", the senior editor Bernd Pulverer states, "Cancer is an umbrella term covering a plethora of conditions characterized by unscheduled and uncontrolled cellular proliferation. As the average age in many countries steadily rises, so do cancer-related deaths, so that cancer will be one of the most common causes of death in the 21st century. Almost any mammalian organ and cell type can succumb to oncogenic transformation, giving rise to a bewildering array of clinical outcomes." For example, thyroid gland cells take in the dietary element iodine more than other cells in order to produce thyroid hormones, which contain iodine atoms. Papillary thyroid cancer cells can therefore be destroyed by giving the patient radioactive iodine (iodine-131). When the patient drinks the radioactive iodine, it enters the bloodstream and then becomes concentrated in the thyroid cells. When this radioactive version of the element undergoes radioactive decay inside the thyroid cells, radiation is released which destroys the cells. In this way, iodine-131 can be used to treat certain thyroid cancers. At the same time, radioactive iodine cannot be used to treat prostate cancer, because prostate cells do not significantly take in iodine.

context: tag/centrifugal/ question: Do I weigh less on the equator than at the North Pole?

Yes, you weigh less on the equator than at the North or South Pole, but the difference is small. Note that your body itself does not change. Rather it is the force of gravity and other forces that change as you approach the poles. These forces change right back when you return to your original latitude. In short, a trip to the equator is not a viable long-term weight-loss program. Your weight is the combination of all the large-scale, long-term forces on your body. While the earth's gravity is by far the strongest large-scale force, it is not the only one. What you experience as "something pulling you down" is actually the total of all the forces and not just gravity. The four dominant large-scale, long-term forces are: Note that although earth's Coriolis force plays a major role in shaping hurricanes and ocean currents, since it is not a static force, it does not contribute to your overall weight. Also, additional forces appear when you ride a roller-coaster, an elevator, a swing, or another vehicle, but these forces are transient, so they do not contribute to your overall, long-term weight. Finally, electromagnetic and nuclear forces are either too small-scale, or too short-term to contribute to your overall weight. For our purposes, we want to consider the forces that differ significantly at the equator versus the poles. While the sun's gravity is strong enough to keep us and the earth in orbit, the sun's position relative to a spot on the equator versus the poles is constantly changing. As a result, averaged over a few days, the gravitational force of the sun on a spot on the equator is the same as the gravitational force of the sun on a spot on the poles. The same situation applies to the moon. This leaves only earth's gravity and earth's centrifugal force as the two forces that contribute to your weight differing at the equator versus the poles. As we learned in high school, earth's gravity is approximately constant all over the surface of the earth. But this is only an approximation. If the earth were perfectly spherical and if its density were perfectly uniform, then the strength of earth's gravity would be exactly constant at all points on its surface. But it's not. There are three major complications to earth's gravitational field. First the earth is not a sphere. The earth is spinning, causing it to slightly flatten like a pizza crust thrown and spun in the air. As a result, the earth is an oblate spheroid and not a perfect sphere. If you stand at sea level on the equator, you are 6378 km from the center of the earth. In contrast, at each pole, you are only 6357 km from the center of the earth. Since the strength of gravity weakens as you get farther away from a gravitational body, the points on the equator are farther and have weaker gravity than the poles. The other two complications to earth's gravitational field; non-uniform internal density and local surface mass variations such as mountains; are small enough factors that we will neglect them here. Therefore, assuming the entire mass of the earth is located at its center, we can calculate the force of earth's gravity at the equator and at the poles. Using Newton's law of gravity, we find that the force of earth's gravity on your body at the equator is 9.798 m/s2 times the mass of your body, whereas at the poles it is 9.863 m/s2 times the mass of your body. The earth's centrifugal force also varies with latitude. The centrifugal force is the outward force felt whenever you are in a rotating reference frame. While the centrifugal force is a non-fundamental force caused ultimately by the inertia of bodies, it is very real for the body in a rotating reference frame, such as your body on the surface of the rotating earth. The centrifugal force is proportional to the tangential speed of the rotating reference frame. The equator is moving quickly as the earth's spins, so it has a lot of centrifugal force. In contrast, the poles are not spinning at all, so they have zero centrifugal force. Since centrifugal force points outwards from the center of rotation, it tends to cancel out a little bit of earth's gravity. If the earth were not spinning, you would be heavier as you would feel the full force of gravity. Since there is more centrifugal force at the equator to cancel gravity, your overall weight at the equator versus at the poles is even less. The centrifugal force on your body at the equator is 0.034 m/s2 times the mass of your body. The centrifugal force at the poles is zero. Your total weight at sea level at the equator (gravity minus centrifugal force) is therefore 9.764 m/s2 times your mass, whereas your weight is 9.863 m/s2 times your mass at the poles. If we use a more accurate model (such as taking into account the shape of the continents) these numbers will be slightly different, but the overall point will be the same: you weigh about 1% less at the equator than at the poles. If you weigh 200 pounds at the North Pole, you will weigh 198 pounds at the equator. Note that we have focused on the equator and the poles as the extremes, but the same effect applies to all latitudes. You weigh slightly less in Mexico City than in New York City, as Mexico City is closer to the equator.

context: tag/centrifugal/ question: How do space ships make artificial gravity?

Despite the fact that outer space is brimming with gravity, the lack of solid ground in space means that objects without thrust are in a continual state of free fall, and free fall feels just like zero gravity. To stop all objects in a space ship from floating around due to their free fall motion, you would need artifical gravity. In the conventional sense, artificial gravity connotes a system aboard a ship that makes all objects fall to the floor and be held there as if they were on earth's surface, but still allows people to walk around freely. In this sense, straps my hold a weightless astronaut to the floor, but they would not quality as artificial gravity because they would not allow the astronaut to walk around. Similarly, magnetic belts would not quality as artificial gravity because objects that the astronaut releases would still float around. The only physically possible way to create a force as strong as earth's gravity that acts on all objects in a ship is through acceleration. Acceleration always creates inertial forces. Inertial forces such as the centrifugal force or Coriolis force are very real in the accelerating reference frame. They are not imaginary or fictional, but are simply non-fundamental in that they arise from the movement of the frame itself. If the acceleration is held constant and at the right value, the inertial force will behave identically to earth's gravity and will, in fact, be equivalent to earth's gravity. This fact is actually a basic tenet of General Relativity. There are two kinds of accelerations, rotational and linear. A ship could achieve artificial gravity by rotating about its axis. To be practical, the radius of rotation would have to be quite large. Additionally, a ship could create artificial gravity by constantly accelerating forwards. Shows that portray artificial gravity without rotation or constant forward acceleration are simply non-physical. Incorrect artificial gravity is often used in movies because of budgeting concerns. It is very expensive to make actors sitting on earth look like space voyagers floating in a space ship, or alternatively, to construct a space ship set that is constantly rotating.

context: tag/centrifugal/ question: Why is the centrifugal force talked about so much if it's not real?

The centrifugal force is very real if you are in a rotating reference frame. It causes objects in a rotating frame of reference to accelerate away from the center of rotation. Washing machines, uranium enrichment centrifuges, and biology lab centrifuges all depend on the reality of the centrifugal force . However, the centrifugal force is an inertial force, meaning that it is caused by the motion of the frame of reference itself and not by any external force. If I stand on the ground and watch children spinning on a playground toy then in my stationary frame of reference their outward acceleration is caused simply by their inertia. In my frame, which is external to the rotating frame, there is no centrifugal force at work. But in the rotating frame of reference of the children, there is a centrifugal force. This oddity arises from the fact that forces only take on their expected meaning in Newton's laws when we are in non-rotating (inertial) reference frames. In rotating reference frames, Newton's laws take on a more complicated, non-intuitive form. But Newton's laws in the rotating frame can be made to look like the regular Newton's laws if we treat the extra pieces in the equations as inertial forces. In other words, the intuitive nature of pushes and pulls in everyday life can be extended to rotating reference frames if we call the effects of the rotation inertial forces. The centrifugal force is one of these inertial forces. The Coriolis force is another one. As an analogy, consider friction. The force of friction is fundamentally due to the electromagnetic forces between molecules. Even though friction itself is not fundamental, that does not make it any less real. A block of wood sliding on the ground feels an opposing force that is real. We call it friction. In a similar way, the centrifugal force has very real effects on objects in a rotating reference frame and is therefore real. But the centrifugal force is not fundamental. Rather it is caused by the rotation of the reference frame. The centrifugal force is not some psychological oddity humans experience. It affects everything in a rotating reference frame, not just the humans. The earth bulges out at the equator because of the centrifugal force. Geosynchronous satellites (those that hover constantly over the same spot on earth) depend on the centrifugal force exactly canceling gravity so that the satellite remains stationary in the earth's reference frame. The eye of a hurricane (the calm spot in the middle) is caused because the centrifugal force cancels the air pressure gradient force at that point. When the hurricane air that is spiraling inwards due to the pressure difference hits the point where it gains enough centrifugal force, it stops.

context: tag/cold/ question: How do scientists cool objects to absolute zero?

Nothing can be cooled to a temperature of exactly absolute zero. The temperature of an object is a measure of the average random motion energy (kinetic energy) of its atoms. Absolute zero is the temperature at which all of an object's atoms have been brought to a dead stop relative to each other. This temperature is denoted by the number zero on absolute temperature scales such as Kelvin. Absolute zero is more of a fundamental limit than a reachable temperature. Absolute zero can never be perfectly reached because of quantum fluctuations. Perfectly stopping every atom at a distinct point would require fixing the exact location and momentum of the atom, which is not physically possible according to the Heisenberg Uncertainty Principle. Every quantum system has a non-zero ground state energy which is the lowest possible state. This is nature's way of enforcing the Heisenberg Uncertainty Principle. Additionally, thermodynamics states that perfectly cooling an object to absolute zero would require an infinite number of steps. Despite the inaccessibility of a temperature at exactly absolute zero, scientists have been able to get very close. The Low Temperature Laboratory at Aalto University cooled a piece of rhodium metal to 0.0000000001 K using a nuclear demagnetization refrigerator. A nuclear demagnetization refrigerator cools a material by aligning the spin of nuclei using a strong magnetic field.

context: tag/cold/ question: What makes space so cold?

Space is not always cold. It depends if you are facing the sun or not. And even if you are in shadow, space is not cold in the sense that it will cool you down quickly. The part of an astronaut facing the sun becomes blazing hot while the side in shadow remains a moderate temperature due to the suits internal machinery. Think of how you feel standing close to a campfire on a cold night, and you get some of idea of the temperatures effects in space. The dark parts of the moon reach -410° F while the sunlit parts reach 240° F, as measured by the Lunar Reconnaissance Orbiter. We don't experience these cold and hot extremes on earth because the atmosphere mixes around and evens out the temperatures. Planets without atmospheres, however, experience these extreme variations. A related question would be: what makes space so cold when in shadow? The answer is that the sun is the ultimate source of heat in our solar system, so if you are not in sunlight, and there is no air or ground (or portable heater) to carry sunlight's energy to you, then you have no source of heat. Cold is simply the absence of heat. Any spot in the universe that is sufficiently blocked from all heat sources will eventually cool down to freezing temperatures. But points in space removed from heat sources aren't cold in the sense that they would quickly make you cold. Quick heat transfer requires contact or air, both lacking in space. As a result objects cool very slowly through the much slower mechanism of thermal radiation. A human exposed to outer space in shadow without a space suit does not instantly freeze to a block of ice. It takes a lot longer than movies show for a non-heated body in space to cool down to deep space temperatures, which are about -280° F.

context: tag/cold/ question: When I step out of the shower, what makes the tile floor so much colder than the bathroom mat?

Assuming you don't plug in your bathroom mat to turn on an internal heater, the mat is the same temperature as the tile floor. Everything in the room is at about the same temperature: room temperature. The tiles conduct heat much better than the fluffy mat, which means they suck heat out of your feet much faster. This is not just a psychological effect. Your feet actually get colder on the tiles. But it's because of high thermal conductivity, not low temperature. Granite tiles have a thermal conductivity that is 100-150 times higher than that of cotton bathroom mats, according to The Engineering Toolbox. That means your warm feet lose heat 100 times faster to the room-temperature tiles than to the room-temperature mat. The sensation of cold you experience is your feet at a reduced temperature due to rapid heat loss, not the tiles at a reduced temperature. Three of the best thermal conductors are diamond (which loses heat 1 million times faster than cotton), gold (11,000 times faster than cotton), and copper (14,000 times faster than cotton). So think twice before lining your bathroom floor with diamond, gold, or copper and throwing out the mat.

context: tag/cold/ question: Why do cold temperatures give you a cold?

Cold temperatures can affect your health, but they don't directly give you an infection like the common cold. Infections are caused by bacteria, fungi, and viruses. Rhinoviruses are the usual cause of the common cold, as laid out by Ronald Eccles in his book "Common Cold". In order to make you feel sick, an infectious agent must get inside your body, and then get the upper hand on your immune system. While cold temperatures do not directly make you sick, the effect of cold on health is often under-emphasized my media reports anxious to debunk a myth. Dr. E. G. Mourtzoukou found that cold temperatures have significant effects on infection rates, as published in the journal article titled "Exposure to cold and respiratory tract infections." Cold air inhaled causes your respiratory tract to underperform. Additionally, cold temperatures force your body to divert more energy to staying warm, leaving less energy available to fighting germs. You may already have been exposed to an infection but not know it. Upon spending too much time in the cold, your body typically responds to the acute stress by signaling to your immune system to downgrade its activity, which may give the infection the chance to flourish. As a result, you start feeling sick. While the cold did not directly cause the sickness, it can make things worse. The bottom line? Don't roll around in the snow with your coat off if you're starting to feel sick. And don't spend a long time outdoors in very cold weather if your respiratory system feels irritated.

context: tag/cold/ question: Why don't trees freeze and burst in the winter like cold pipes?

In many cases, trees do partially freeze in the cold of winter and burst like plumbing pipes in an unheated home. When liquid water freezes to ice, it expands in volume because of the way the water molecules spread out to form a solid crystalline lattice. If the water is contained in a closed vessel, it can press so hard as it freezes and expands that it bursts the container. This explosive effect is common in insufficiently heated houses, where the cold leads the plumbing pipes to burst. The fluid transport tissue in trees (xylem and phloem) can be seen as little pipes carrying water and nutrients throughout the tree. They too can freeze and burst, causing the tree to crack and/or explode. The crackling sound or gun-shot pop you hear in the forest in the winter is the sound of trees freezing and bursting. The bursting is usually not as violent or as deadly to the tree as you may first expect for a few reasons. A tree has hundreds to tens of thousands of these fluid channels. If one bursts, the tree has plenty of other ones to rely on. Furthermore, each channel is small, so that an individual channel bursting does not do much damage. In addition to the fluid channels, each cell of the tree is itself a little bag of water that can pop upon being frozen. If the water inside the cells freeze, it is instantly fatal to the tree. While many trees can withstand freezing of the fluid transport channels outside the cells, none can survive intracellular freezing. Trees that survive in cold climates must therefore protect their cell interiors from freezing. The cold hardiness of trees is caused by many factors:

context: tag/color/ question: Can humans ever directly see a photon?

Yes. In fact, photons are the only things that humans can directly see. A photon is a bit of light. Human eyes are specifically designed to detect light. This happens when a photon enters the eye and is absorbed by one of the rod or cone cells that cover the retina on the inner back surface of the eye. When you look at a chair, you are not actually seeing a chair. You are seeing a bunch of photons that have reflected off of the chair. In the process of reflecting off of the chair, these photons have been arranged in a pattern that resembles the chair. When the photons strike your retina, your cone and rod cells detect this pattern and send it to your brain. In this way, your brain thinks it's looking at a chair when it's really looking at a bunch of photons arranged in a chair pattern. Your eyes can see bunches of photons, but can they see a single, isolated photon? Each rod cell in your eye is indeed capable of detecting a single, isolated photon. However, the neural circuitry in your eye only passes a signal along to the brain if several photons are detected at about the same time in neighboring rod cells. Therefore, even though your eye is capable of detecting a single, isolated photon, your brain is not capable of perceiving it. If it could, an isolated photon would just look like a brief flash of brightness at a single point. We know this because a sensitive camera sensor is indeed able to detect and process an isolated photon, and the photon just looks like a brief flash of brightness at a single point. A photon has several properties, and each of these properties carries information about the source that created the photon or the last object that interacted with the photon. The basic properties of a photon that carry information are color (i.e. frequency), spin (i.e. polarization), location, direction of propagation, and wave phase. There are also many other properties of a photon; such as energy, wavelength, momentum, and wavenumber; but these are all dependent on the frequency and therefore do not carry any extra information. Additionally, when many photons are present, information can be carried by the number of the photons (i.e. brightness). When a group of photons reflects off of a chair, the photons form patterns of color, spin, location, direction, wave phase, and brightness that contains information about the chair. With the proper tools, each of these patterns can be analyzed in order to gain information about the chair. The human eye is designed to detect the color, location, direction, and brightness patterns of a group of photons, but not the spin or wave phase. Color information is detected in the eye by having three different types of cone cells that each have a different range of color sensitivity. One of the types has a sensitivity range centered on red, another type has a range centered on green, and another type has a range centered on blue. The eye can see almost all of the colors in the visible spectrum by comparing the relative activation of these three different types of cone cells. For instance, when you look at a yellow tulip, yellow photons stream into your eye and hit your red, green, and blue cone cells. Only the red and green cone cells are triggered by the yellow photons, and your brain interprets red plus green as yellow. In contrast to cone cells, there is only one type of rod cell, and so the rod cells can only detect brightness and not color. The rod cells are primarily used in low lighting conditions. Location information is detected in the eye by having the cone and rod cells spread across different locations along the retina. Different photons existing at different locations will trigger different cells. In this way, the spatial pattern of photon location is directly detected by the retina. Note that photons can come from many different directions and blur together. For this reason, the eye has a stack of lens in the front which focuses only the light to a certain cell which comes from a single point on the object being viewed. The lens plays an essential role in extracting location information about the object being viewed from the location information of the photons on the retina. If the lens malfunctions, photon location on the retina no longer corresponds exactly to point locations on the object being viewed and the image ends up blurry. Note that the human optical system can only directly image two dimensions of the photon location information. Information about the third dimension is indirectly extracted by humans using a variety of visual tricks (called "depth cues"), the main trick being the use of two eyes that are slightly offset from each other. Direction information is only crudely detected by humans by having the brain keep track of which way the eyes are pointed, and by having the eyes look at an object from many different angles. For instance, a room with one wall painted red and the opposite wall painted blue has red photons from the wall shooting in one direction and blue photons from the other wall shooting in the opposite direction. At a given spot in the room, the bunch of photons at that spot includes red photons and blue photons traveling in opposite directions. However, a human can only deduce that the red and blue photons are traveling in different directions (and therefore deduce that the red and blue walls are at different locations) by turning his head and analyzing two different views while his brain tracks the orientation of his head. Brightness information is directly extracted by the retina by measuring how many photons strike a certain region of the retina in a certain time increment. Both the rod cells and the cone cells can collect brightness information. Since the human eye ultimately only sees photons, a light-generating machine can make a physical object seem to be present by recreating the correct patterns of photons that would come off of the object if it were really present. For instance, we can make it look like a chair is present if we create a collection of photons with the same patterns as the collection of photons that is present when a chair really is there. This is what computer display screens do. A camera captures the patterns in the photons coming from a chair and stores the information as bits of electricity. A computer screen then uses this information to recreate the photon collection and you see a picture of the chair. However, standard computers screens can only specify the color, brightness, and two-dimensional location of the photons they create. As a result, the image of a physical object on a computer screen is two-dimensional and not completely realistic. There are many tricks that are used to try to convey the third dimension of information to humans, including the polarization glasses used in 3D cinemas and the lenticular lenses used on some book covers. However, such systems are usually not entirely realistic because they do not actually recreate the full three-dimensional photon field. This means that such "3D" recreations of objects can only be viewed from one look angle and are not entirely convincing. Some people find that because such "3D" systems use visual tricks rather than a full three-dimensional photon field, these systems give them headaches and nausea. In contrast, a holographic projector comes much closer to recreating the full three-dimensional photon field coming from an object. As a result, a hologram looks much more realistic and can be viewed from many different angles, just like a real object. However, true holograms are currently not able to effectively reproduce color information. Note that many color-accurate images that are claimed to be holograms are actually flat images with tricks added in to make them look somewhat three-dimensional. A fully-realistic photon recreation of a physical object will not be possible until holograms are able to accurately recreate color information. The two properties of photons that human eyes cannot see are spin (i.e. polarization) and wave phase. Note that under the right conditions some people can detect the overall polarization state of an entire light beam; but no naked human eye can directly see the polarization pattern. By looking through rotatable polarization filters, which convert polarization information to color intensity information, a trained human can learn to indirectly see the polarization pattern of the photons coming from an object. An example of this is the photoelasticity method which allows people to see mechanical stresses in certain objects. In contrast to humans, some animals such as honeybees and octopuses can indeed directly see the polarization pattern of a collection of photons. For instance, honeybees can see the natural polarization pattern that exists in the daytime sky and use it for orientation purposes. Photon wave phase can also not be directly detected by humans but can be detected by machines called interferometers. Phase information is often used to determine the flatness of a reflecting surface. In summary, humans can indeed see photons. Humans can see all of the properties of photons except for spin and wave phase. Since photons travel in patterns dictated by the source that created them or the last object that the photons interacted with, we usually don't realize we are looking at photons at all. Rather, we think we are looking at the physical objects that are creating and scattering the photons. Now, perhaps you meant to ask, "Can humans ever see a photon in the same way we see a chair?" Again, we can see a chair because photons bounce off of it in a certain pattern representative of the chair and enter our eyes. In order to see a photon in the same way you see a chair, you would have to have a bunch of photons bounce off of the one photon you are trying to "see" and then have this bunch enter your eye. However, photons never directly bounce off of each other, so this could never work. Even if photons could bounce off of each other, you would not see anything special from this setup. You would still just see a flash light at one point when the small bunch of photons strikes your retina. When you think you see a light beam sitting out in space, such as coming from a flashlight, you are in reality seeing the dust particles along the path of the beam because of the photons bouncing off of the dust particles.

context: tag/color/ question: Can you make a sunset in a cup of milk?

Yes, you can make a sunset in a cup of milk. The same orange and red pattern of colors that you see when the sun goes down can be created in your cup of milk if you set up the situation properly. The physics that makes your cup of milk orange and red is the exact same physics that makes the sky at sunset orange and red. In this sense, you can literally make a sunset in your cup of milk. You don't even need the sun to do it. Let's look at the basic physics first and then we'll understand how to make a sunset in a cup. When light scatters off of an object that is much larger than its wavelength, the light acts just like a little marble. Because of this, the different colors of light all bounce off a large object at the same angle. This type of scattering is called "geometric scattering". It is the type of scattering that we are most familiar with in everyday life. Red light has a wavelength of 630 nanometers. In contrast, the diameter of an apple is about 8 centimeters, which is about 130,000 times larger than the wavelength of red light. Therefore, red light definitely bounces off an apple geometrically. Since white light consists of all visible colors, shining white light on an object that is much bigger than the wavelength of the light causes the different colors to all reflect at the same angle. This leads to two effects when a big object is illuminated by white light: 1) the object has the same color no matter what angle it is viewed at, and 2) the overall color of the object is largely determined by which colors are and are not absorbed. For instance, a maple leaf is much larger than the wavelength of visible light and thus causes light to scatter geometrically. A healthy maple leaf absorbs red, orange, yellow, blue, and violet light from the full spread of colors that are present in the incident white sunlight. Therefore, the leaf only reflects back green light. We see the leaf as green since this is the only color of light that reaches our eyes. Furthermore, the leaf looks green from all viewing angles. Since the color of a large object is mostly determined by its absorption spectrum, which is typically constant for all objects made out of the same material, the color of a large object is the same for all objects in the same class. For instance, all the healthy leaves on an oak tree are green. Because color is constant across all viewing angles and across all objects in a class when optical scattering is at work, humans tend to think of color as an innate property of an object, which is a helpful but inaccurate oversimplification. In contrast to geometric scattering, Rayleigh scattering involves the scattering of light off of objects that are much smaller than the wavelength of the light. When light scatters off of such an object, the light does not act like a marble striking and bouncing off a point on the surface of the object. Rather, the light acts like a vibrating uniform electric field that completely encompasses the object. As a result, the light scatters in all directions to some extent. Furthermore, the amount of light that scatters in a certain direction depends on the color of the light and not on the object's surface geometry. This leads to two effects when a small object (smaller than about 100 nanometers) is illuminated by white light: 1) the object has a different color depending on what angle it is viewed at, and 2) the color of the object is not determined by the shape or surface material properties of the object. What is the pattern of color generated by Rayleigh scattering? An object displaying Rayleigh scattering scatters mostly blue and violet colors in the sideways direction, leaving red, orange, yellow, green, and reduced amounts of blue and violet to continue traveling in the forward direction. Since small objects don't scatter very much light, and since humans can't see small amounts of light, it takes a large collection of small objects in order for humans to see the light produced by Rayleigh scattering. Furthermore, the objects have to be fairly spread out so that they act like independent objects. If a collection of small objects are closer to each other than the wavelength of light, they will just act like one giant object. So, where can we find a large collection of nanoscale objects that are somewhat dispersed? In the atmosphere and suspended in liquids. When you think of small objects dispersed through the atmosphere, you probably think of dust particles, bits of pollution, raindrops, droplets of mist, and the small droplets of liquid water that make up clouds. It turns out that compared to the wavelength of visible light, all of these objects are far too big to participate in Rayleigh scattering. Instead, these objects mostly generate geometric scattering, which tends to scatter all colors equally in all directions. For this reason, dust, pollution, rain, mist, and clouds tend to be white, or variations of white such as gray or brown. The objects in the sky that are small enough to display Rayleigh scattering are the air molecules themselves, which are mostly nitrogen molecules (N2) and oxygen molecules (O2). Each air molecule scatters blue and violet colors the most in the sideways directions and lets the other colors continue on in the forward direction. That is why the daytime sky is blue (the daytime sky does not look violet for several reasons, the main one being that human eyes do not see the color violet very well). Around sunset, there is so much air between the sun and the observer that the blue colors have already been scattered to other parts of the earth, leaving mostly the red and orange colors. Milk is mostly a collection of tiny protein-coated blobs of oil suspended in water. These blobs are small enough to generate Rayleigh scattering. Therefore, by shining light through a glass of milk, you can get the same color effects as in the sky. However, regular milk has such a high concentration of these oil blobs that each light ray scatters many times before exiting the cup. Each series of multiple scattering events tends to randomize and average away the color effects of Rayleigh scattering. As a result, a cup of milk at regular concentration just looks white. In order to see the color effects, you need to dilute the milk. This will cause the oil blobs to spread out enough that the light rays only scatter once. Take a clear glass cup with a smooth surface and fill it almost to the top with water. Next, add milk to the cup one drop at a time. After adding each drop, mix everything together and look at a bright light bulb through the cup. Keep adding the drops of milk until the light bulb appears red or orange when viewed through the cup. Presto! You have a sunset in a cup. To heighten the effect, do this at night with all the lights turned off except the one light bulb you are looking at through the cup. Next, position yourself so that you are looking at the side of the cup relative to the line connecting the cup and the light bulb. You now see a blue color. Presto! You have the daytime sky in a cup.

context: tag/color/ question: Does the back of a rainbow look the same as its front side?

A rainbow does not have a back side. If you were to walk completely to the other side of the mist cloud that is creating the rainbow and turn around, you would not see a rainbow. You have to realize that a rainbow is not a stationary physical object. Instead, it is a pattern of light that becomes a stable image only when you look at it from the right angle. You may not have noticed it, but every time you look directly at the center of a rainbow, the sun is directly behind your head. This is the only angle at which the light pattern that constitutes the rainbow can enter your eye and therefore lead you to see it. The sun is always in the opposite part of the sky from the center of the rainbow. This is because a rainbow is actually just sunlight which has been refracted and reflected. Refraction occurs when the sunlight enters and leaves the small spherical water droplets that constitute the mist. This refraction is what causes the rainbow's spread of colors and arching shape. However, the overall location of the rainbow is determined mostly by the step in the process where the sunlight is reflected off of the inner back surface of the water droplets. The sunlight that reflects only once off the inner back side of the mist droplets is what constitutes the primary rainbow. Additionally, a small amount of the sunlight reflects twice off the inner back surface. This light consequently comes out of the mist droplet at a slightly different angle, leading to the secondary bow. The secondary bow is always there, but it is so dim that humans can only see it during clear viewing conditions. This second reflection only changes the light's direction a small amount. As a result, both the primary bow and the secondary bow can only be seen when looking away from the sun. Furthermore, neither one can be seen from the back side, i.e. when looking toward the sun. This point brings up other interesting questions. Can't some of the sunlight pass through the back side of the water droplet without being reflected? Wouldn't this light be visible from behind the rainbow? The answers are yes and yes. Although it is only a small amount, some of the sunlight that enters a mist droplet indeed continues through the back side without being reflected. Therefore, if you were to walk to the other side of the cloud of mist that is creating the rainbow and turn around, you would indeed see a pattern of light (if the viewing conditions are favorable). However, it would not be a rainbow. It would be a pattern called an "atmospheric solar corona", as shown below. A solar corona still includes an arcing shape and a spread of colors, but the size and color sequence of a solar corona are different that of a rainbow. Since most of the sunlight that enters the mist droplet is reflected and not transmitted through, rainbows are very bright and common while solar coronas are dim and rare. Additionally, since you are behind the rainbow, you are now looking directly toward the sun. The sun's glare therefore makes it hard to see the solar corona.

context: tag/color/ question: Have astronomers ever observed a violet shift like they have blue shifts and red shifts?

Violet shifts happen all the time. We call them blue shifts. When a star emits light, the color of its light as observed on earth depends on its motion relative to earth. If a star is moving towards the earth, its light is shifted to higher frequencies on the color spectrum (towards the green/blue/violet/ultraviolet/x-ray/gamma-ray end of the spectrum). A higher frequency shift is called a "blue shift". The faster a star moves towards the earth, the more its light is shifted to higher frequencies. In contrast, if a star is moving away from the earth, its light is shifted to lower frequencies on the color spectrum (towards the orange/red/infrared/microwave/radio end of the spectrum). A lower frequency shift is called a "red shift". The faster a star moves away from the earth, the more its light is shifted to lower-frequency colors. This effect is known as the "Doppler shift". It is the same principle at work when an ambulance siren coming towards you is high pitched and then switches to a lower pitch sound once the ambulance passes you and is traveling away from you. The Doppler shift is also used in police radar guns to measure how fast your car is going based on how the radio wave shifts in frequency when it bounces off your car. The book titled The Handy Astronomy Answer Book by Charles Liu states, "When an object emitting light – or any kind of electromagnetic radiation, for that matter – moves toward someone, the wavelength of its emitted light is decreased. Conversely, when the object moves away, the wavelength of its emitted light is increased. For visible light, the bluer part of the spectrum has shorter wavelengths, and the redder part of the spectrum has longer wavelengths. Thus, the Doppler effect for light is called a ‘blueshift' if the light source is coming toward an observer, and a ‘redshift' if it is moving away. The faster the object moves, the greater the blueshift or redshift." Simply by looking at the colors of light from a star, astronomers can figure out how fast that star is moving relative to earth using the Doppler shift. Any spectral color of light can shift to any other spectral color if the motion of the source is right. For this reason, if an orange light beam has been blue shifted, that does not mean that its final color is blue. It just means that its final color has been shifted towards the blue end of the spectrum; i.e. its color has been shifted up in frequency. An orange light that has been shifted so that it ends up as a yellow light has been "blue shifted". An orange light that has been shifted so that it ends up as a violet light has also been "blue shifted." In contrast, a violet light that is shifted so that it ends up as an orange light has been "red shifted". When it comes to the Doppler effect, "red shifted" should be heard as "down shifted" and "blue shifted" should be heard as "up shifted". For instance, if an ultraviolet ray (which is higher in frequency than blue) is shifted up in frequency so that is ends up as an X-ray, we still call it a blue shift, even though it actually has shifted away from the blue. This case only makes sense if you interpret "blue shift" to mean "up shift" and not "towards blue". Now an interesting question arises. We call a down shift a "red shift" because red is the color at the bottom edge of the visible part of the electromagnetic spectrum. Red is the first color of a rainbow. With this type of reasoning, we should call an up shift a "violet shift" because violet is the color at the top edge of the visible part of the electromagnetic spectrum. Violet is the last color of a rainbow. But we don't. We call an up shift a blue shift and not a violet shift. Why? The reason is that humans do not see violet very well. Even though humans can technically see violet, and therefore it is the color at the upper edge of the visible spectrum, we do not see it very well. As a result, blue is the de facto upper edge of the visible spectrum. This is why an up shift is called a blue shift. Such a confusing state of affairs seems worth avoiding, but the fact is that having blue be the upper edge of the visible spectrum is part of the human experience. The sky contains all colors of light at all times, put its light peaks in one color at different times of the day. Scientifically, the peak color of the sky shifts from sunrise to noon in the order: infrared, red, orange, yellow, green, blue, violet, ultraviolet. But humans experience the color of the sky from sunrise to noon as: red, orange, yellowish-white, white, blue. The difference between the scientific version of the sky's color and what humans experience are due to three facts: 1) our eyes can't see ultraviolet or infrared, 2) our eyes can't see violet very well, and 3) our eyes experience a nearly even mix of all colors as white. In terms of how we experience the sky, the color spectrum seems to start at red and end at blue. The same goes for incandescent flames such as in a campfire or on a candle. Scientifically, flame colors go from coldest to hottest in the order of: infrared, red, orange, yellow, green, blue, violet, ultraviolet. But humans experience the color of flames from coldest to hottest as red, orange, yellow-white, white, blue. From everyday experience, blue seems to be the upper end of the visible color spectrum, even though we can technically see violet. That is why an upwards Doppler shift is called a blue shift. A "violet shift" would therefore mean the same thing as a blue shift if the phrase were ever used: an upwards shift. Use of the Doppler shift has allowed astronomers to make some interesting observations. On average, the light from all stars outside our local group of galaxies is red shifted. Also, the farther away a star is, the more its light is red shifted. This fact indicates that our universe is expanding and all of the stars outside our local group of galaxies are moving away from us. Also, when a star rotates, one edge of the star is moving towards us relative to its center while the other edge is moving away. As a result, light from one edge of a star is slightly red shifted while light from the other edge is slightly blue shifted. Astronomers can use these two shifts in order to calculate how fast a star is rotating. The same approach can be used to calculate how fast a galaxy is rotating.

context: tag/color/ question: How do projectors project the color black?

Projectors do not project the color black. This makes sense since black is really the absence of light, and you can't project something that does not exist. When a projector sends a beam of light on to a wall or a projector screen so that an image is formed on the wall or screen, the parts of the image that look black are really a very dim white color (which we sometimes call gray). The projector sends some light to all parts of the image, including the parts that we perceive as black. Some white light is indeed beamed to the parts of the image that are supposed to be black, but the light is typically dim enough in these regions that they look black to our eyes when surrounded by areas of the image that are receiving much more light and therefore are much brighter. Our human eyes and brains are designed to evaluate a color based on how it looks relative to the colors of the surrounding objects, rather than based on the absolute spectral content of the color. For instance, look at the image below. Directly surrounding both black dots are patches of blue that are the exact same color. They look different to humans because of the way our brains perceive color based on the color of surrounding areas. You can convince yourself that the areas right around the two black dots are the same color blue by holding a thick piece of paper up to this image with two holes cut out right over the black dots, allowing you to see the colors directly around the dots without seeing the rest of the image. The tendency of our brains and eyes to evaluate a color based on its relation to nearby colors is actually beneficial. Consider a standard yellow banana. A ripe banana placed next to an unripe banana is always more yellow and less green than the unripe banana, no matter what type of light is shining on it. The relative color difference between the two bananas does not change, even if the light source does change. In contrast, the absolute color of the bananas does change as the light source changes. For instance, objects that are in shade on a sunny day are more blue than objects in direct sunlight, because they are being illuminated by the whitish blue sky rather than the white sunlight. Therefore, a ripe banana sitting in direct sunlight has an absolute color of yellow, while the same ripe banana has an absolute color of yellowish green when in the shade on a clear sunny day. If humans were only able to evaluate colors according to their absolute spectral content, then we would bizarrely conclude that ripe bananas become unripe every time they go in the shade. By basing our color perception on relative color differences rather than absolutes, our brains are able to link object color to intrinsic properties of the object (such as ripeness), rather than assume that object color only tells us the color content of the light source. Returning to the case of the image cast on the wall by the projector, in order for our brains to perceive a part of the image as black, it just has to be less bright than all the other parts of the image. In this way, a projector can throw some white light on a wall and convince you that it is black. There are a few ways that you can convince yourself of this concept. First of all, turn off the room lights, turn on the projector, and then send an image from your computer to the projector that is completely black everywhere. Look closely and you will see that the area of the wall on which the projector is projecting is brighter than an area of the same wall that is out of the projector's reach, despite the fact that the projector is supposed to be projecting perfect blackness. Now turn off the projector. You will notice that the wall darkens when the projector turns off, because it has stopped sending the dim white light that is supposed to represent the black colors in the image. As another way to convince yourself of this, turn the projector on and have it display an image that has black and white regions. Now turn on and off the room lights. You will notice that when the room lights are on, the black regions in the image no longer look very black. They still look black in comparison to the other parts of the image, but they look like a washed-out, muted black color. This is because our color perception depends on relative color differences. When you turn on the room lights, you flood both the black and white regions of the projected image with white light, thereby decreasing the contrast between dark and light in the image. This decreases your eyes' ability to use relative color differences to perceive a dim white color as a black color. For this reason, a projected image is most vivid and convincing when the image has a high contrast between the brightness of its white regions and the darkness of its black regions. High contrast is achieved by building a projector that can send out a lot of light to some regions and send out very little light to other regions. High contrast is also achieved by simply turning off the room lights and shuttering the windows. The fact that we watch projected movies in darkened rooms is direct evidence that projectors can't emit literal black, but instead emit dim white light which is interpreted as black when the contrast is high enough.

context: tag/color/ question: How does plasma make a campfire flame orange?

The de-excitation of plasma (charged gas) is not the source of the light given off by a campfire's flame. The incandescence of solid soot particles billowing up on an updraft of hot air is what creates the light seen as a flame. Let us look at the typical ways materials emit light and then apply it to the campfire. Electrons in molecules can exist in various orbital states. When electrons absorb energy, they are lifted to a higher energy state in the molecule. This is roughly similar to lifting a ball onto a high shelf where it can fall back down from. The absorbed energy of the electron is stored as potential energy as it sits in the higher energy state. Many things can excite an electron in a molecule to a higher energy state, including chemical reactions, collisions with other particles, and absorption of light. Eventually, excited electrons fall back down to their lower states, like a ball on a shelf rolling and falling back to the ground. When the electrons fall, their potential energy is lost and given off. There are many ways that a falling (de-exciting) electron can give off its energy, but the most common way is for it to emit a bit of light called a photon. As a result, any process that excites electrons leads to light being created when they de-excite. Often, this light has a non-visible color or is too dim to be seen by human eyes, but the light is still being created. Because of the law of conservation of energy (which states that energy cannot be created or destroyed), all of the energy that the electron loses in transitioning from a higher state to a lower state must be given to the photon that it creates. Electron states only exist at fixed levels that are constant for a given molecule, so a certain isolated molecule can only emit photons of specific energies. Because a photon's frequency (the light's color) is directly proportional to its energy, certain isolated molecules can only emit certain colors of light. The specific set of colors that a molecule emits (its spectrum) is unique to the molecule and acts like a fingerprint. Astronomers can tell what chemicals a star is made out of by simply looking at what colors are present in its starlight. By plotting the few, distinct colors present in the light given off by a single molecule, a "line spectrum" is formed, which looks like a collection of thin lines. In addition to electrons getting excited, the whole molecule itself can get excited by spinning more or vibrating more. There are therefore three ways a molecule can emit light: an electron falls to a lower state (electron transition), the molecule spins less (rotational transition), and the molecule vibrates less (vibrational transition). Now something interesting happens if you have more than one molecule involved. If you have a collection of molecules, they tend to bump into each other. When they collide, some of the energy in the excited electrons, excited spinning states, and excited vibrating states is converted simply to movement (kinetic energy). As a result, there is less energy present for the photon that is emitted when the molecule transitions, leading to a photon with a color that is different from if there had been no collision. Because the collisions are random, the color changes of the light given off are random. Where there was just one color (a line) in a certain portion of the spectrum of the light emitted, there are now many colors. Collisions between molecules therefore tend to smear the nice crisp lines of the light's color spectrum into bands with many colors. The more collisions there are and the harder they are, the more colors there are in the light given off. If there is a very high amount of strong collisions between molecules, all of the light given off by molecular transitions gets smeared into one continuous band of colors. In such a case, all colors of the rainbow are present in the light and the light is therefore white. The light is typically not pure white, but is whitish red, whitish orange, etc. depending on the nature of the collisions. Light with this broad arrangement of colors is called "thermal radiation" or "blackbody radiation" and the process that creates this light is called "incandescence". In every day life, we refer to incandescence as "glowing hot". A material with zero collisions therefore emits a line spectrum, which is a collection of a few perfectly defined, unsmeared colors. On the other extreme, a material with an infinite number of collisions emits a blackbody spectrum, which is a perfectly smooth collection of all colors in a very distinct distributional shape called a "blackbody curve". The two opposite extremes; the line spectrum and the blackbody spectrum; are idealizations. In the real world, each spectrum is somewhere between the two extremes. When we say that the color distribution of light is a line spectrum, we mean that it is close to a line spectrum, and not that it is exactly a line spectrum. There are two things that determines how fast molecules collide with each other. The first is the density of the molecules. The closer the molecules are together, the more chance they have to collide. Solids have their molecules very close together and therefore collide enough to emit all colors of light. Solids typically emit a spectrum that is close to a blackbody spectrum. On the other hand, a dilute gas has its molecules much farther apart, so the color distribution of its emitted light looks more like a line spectrum. The other thing that affects the collision rate is the temperature of the object. As an object gets hotter, its molecules shake around more and move faster, which leads to more collisions. More collisions means more spectral line spreading, and it also means that more energy is being exchanged in each collision. The thermal spectral curve shifts to higher energies for higher temperatures, which is the same as higher frequencies. Room temperature objects are constantly emitting light in a color we can't see (infrared). As we heat up an object such as an electric stove element, the collisions between molecules become more intense and more frequent. The light given off therefore has a spectral thermal distribution that shifts to higher frequencies. The peak of the light given off by a heated object shifts from infrared, to red, to orange, and so on as it gets hotter. But remember, that thermal radiation contains all colors, so that hot metal does not glow red, it actually glows whitish red, and then whitish orange as it gets hotter, etc. (Remember that "white" is just how we experience an equal mixture of all colors.) Let us look at some examples. A neon sign contains a gas at normal temperature getting excited by electricity. The light emitted by a neon sign is therefore close to a line spectrum (just a few, sharply defined colors). In contrast, the sun contains gas, but the gas is at such a high temperature that the light emitted is close to a blackbody spectrum. The metal elements in a toaster, electric stove, and incandescent light bulb are all solid and therefore glow when heated such that the light they produce is close to a blackbody spectrum, which contains all colors and therefore is whitish. Now let us apply this to a campfire's flame. At first thought, you may think that the space where the flame exists only contains gases, therefore the light emitted should be a line spectrum with only a few distinct colors. This is clearly not the case. The flame has a broad range of colors, from red to orange to yellow and all the colors in between. So there must be more going on than heated gases. The best general description of a campfire flame's color is: whitish orange. This description matches exactly what we would expect from the thermal radiation of incandescence. It turns out that a campfire flame contains small solid particles of half-burnt wood called "soot" that are so hot that they glow. They glow in the exact same way that electric stove elements glow. When you look at a whitish-orange flame, you are looking at a cloud of little, hot, glowing, solid bits of half burnt fuel. The hot air that is flowing upwards catches this glowing soot and carries it upwards. As the soot rises, it cools and its thermal spectrum shifts to ever lower colors. That is why the bottom of a flame is whitish-yellow, and as you look higher, it changes to whitish-orange, and then whitish-red, and then to whitish-infrared (which we can't see, but can feel with our hands). The flame of a campfire actually extends meters into the air, we just can't see the top portion because the soot has cooled enough to emit mostly infrared colors. Interestingly, the presence of half-burnt soot in a flame, and therefore the flame's whitish color, is a result of there not being enough oxygen. If oxygen is pumped fast enough into the fire, or is premixed properly with the fuel, all of the fuel gets burned all the way and there is no soot. Adding enough extra oxygen therefore makes the whitish-orange flame go away. The color that remains in the flame (usually blue) is from the electron transitions involved in the chemical reaction itself. Fuels that don't need much oxygen to burn, such as the natural gas in a gas stove, can burn without giving off whitish-orange glowing half-burnt particles.

context: tag/color/ question: Is a quadruple rainbow possible?

Yes, although very rare, it is possible for a human to see four natural rainbows at once in the sky. A rainbow occurs when white sunlight scatters off of raindrops in the air. Because of the dispersive properties of water, the different colors of light in the sunlight bend (refract) different amounts when entering and leaving the raindrop. As a result, the different colors leave the raindrop at different angles, making you see the different colors at different locations in the sky. Because this scattering is a geometric effect that depends on the direction of the original incoming sunlight, the rainbow forms as a circle (or part of a circle) that is centered on the point exactly opposite of the sun. The main rainbow (called the "primary rainbow") involves sunlight entering the raindrop, reflecting once off the inner back surface of the raindrop, and then exiting the raindrop. Additionally, light can bounce twice off the inner surface of the raindrop before exiting. The second reflection causes these light rays to exit at an angle that is very different from that of the light rays that only reflect once. Therefore, a secondary rainbow forms that has a larger radius than the primary rainbow. The secondary rainbow is created by the same sunlight and the same refraction process as the primary rainbow, so it is also centered on the point exactly opposite the sun. Because of the additional reflection, the colors in the secondary rainbow are reversed in order compared to the primary rainbow. Since some light is lost out of the raindrop with every reflection, the secondary rainbow is much fainter than the primary rainbow. In principle, the secondary rainbow is always present. However, the secondary rainbow is often so faint that humans can't see it. When viewing conditions are right (i.e. it is an unusually sunny day and there are an unusually high number of raindrops in the air), the secondary rainbow can be seen quite distinctly. As you may have guessed, light can bounce three times off the inner surface of the raindrop before exiting, creating a third-order rainbow; or bounce four times, creating a fourth-order rainbow; and so on. However, the third-order and higher-order rainbows are so faint that they are almost never seen by the naked eye. By using a camera and image enhancements techniques, the third-order rainbow can indeed be imaged. This task is difficult enough that successfully capturing and analyzing the third-order rainbow earned Michael Grossman, Elmar Schmidt, and Alexander Haussmann a publication in the academic journal, Applied Optics. Considering the near impossibility of seeing third- and fourth-order rainbows, how are people able to see four rainbows in the sky at once? The answer is that some of the sunlight can be reflected before entering the raindrops. Recall that rainbow formation is a geometric scattering effect leading the rainbow to be centered on the point opposite the sun. What would happen if there were two suns at two different locations in the sky? Then sunlight would hit the raindrops at two different angles, and two primary rainbows would result, each centered on a different point. If viewing conditions are favorable enough that the secondary rainbows are visible, then there would be four rainbows: a primary rainbow and a secondary rainbow centered on the point opposite the one sun, as well as a primary rainbow and a secondary rainbow centered on the point opposite the other sun. On planets with rain and two suns, quadruple rainbows are common. However, we don't have two suns, so why am I mentioning this? A large, flat, shiny surface can reflect enough sunlight that the situation acts like two suns. A calm lake does exactly this. The sunlight coming directly from the sun is at an angle that is different from that of the sunlight reflecting off the surface of a lake, and they therefore form different rainbow sets centered on different points. The rainbows formed from the sunlight coming off of the lake are called "reflection rainbows". In summary, although it is very rare, it is possible for the naked human eye to see four natural rainbows at once in the sky, consisting of: In principle, the naked human eye could see six rainbows if there were two different large reflecting surfaces, creating the effect of three suns. For instance, if a large, flat, smooth glacier was sitting next to a lake and its surface was tilted compared to the lake's surface, then if conditions were just right, you could see six rainbows at once. Although physically possible, this situation is exceedingly rare.

context: tag/color/ question: Is human blood ever any color other than red?

Yes, human blood is green in the deep ocean. We have to be careful about what we mean by color. Objects don't really have an intrinsic color. Rather, the color of an object is determined by three factors: 1) the color content of the incident light that is illuminating the object; 2) the way the object reflects, absorbs, and transmits the incident colors of light; and 3) the way in which the detector such as your eye or a camera detects and interprets the colors of light coming from the object. In everyday life, the incident light (such as from the sun or from a light bulb) typically contains all colors of visible light in nearly equal proportions. Furthermore, the healthy human eye can detect all colors of visible light. For these two reasons, in typical circumstances, we can treat the color of an object as only depending on the properties of the object itself. However, once we move away from typical circumstances, we have to use the more complete description of color, which involves the light source, the object, and the detector. With this in mind, let's turn to the color of blood. As reported in the journal Applied Spectroscopy, Martina Meinke and her collaborators measured the diffuse reflectance of human blood and found the spectrum which is shown below. This particular spectrum is for blood with a hematocrit (the percent of the blood's volume taken up by red blood cells) of 33% and oxygen saturation of 100%. These researchers also measured the reflectance spectrum of blood for other hematocrit values and oxygen saturation values. They found that although the spectrum slightly changes for different hematrocrit and oxygen saturation values, the overall trend shown below remains the same. Therefore, in terms of the overall trend, the image below is a good representation of the reflectance of any human's blood. (Note that even deoxygenated blood follows these trends and is dominantly red, not blue.) As we see in the image above, blood mostly reflects red light. Interestingly, though, blood also reflects a little bit of green light. If we shine white light (which contains all colors) onto the blood, blood looks red since it reflects so much more red light than green light. However, if we use a light source that contains all of the visible colors except red and shine it onto the blood, the blood will be green. With no red light present in the first place, the blood can't reflect any red light. The only thing left that it can reflect is the green light. The blood is therefore green. Note that this is not a trick of the eyes. The blood is literally green. In fact, human blood is always a little bit green. We usally don't notice the green color of blood because there is typically so much more red light being reflected by the blood. But if you shine a light on the blood that contains green light but no red light, the green color of blood becomes obvious. This is exactly what happens deep in the ocean. Water is naturally very slightly blue colored because it absorbs some of the red light passing through. The deeper you go in the ocean, the less red light there is in the sunlight that reaches you. Without red color in the sunlight, only green light reflects from the blood. This fact can be startling to divers who get a cut while diving. Again, the blood does not change when in the deep ocean. Rather, the green color of blood that is always there becomes obvious once the brighter red color is no longer present. Since the green reflectance peak of blood is always there, blood can be obviously green anytime you have a light source with no red color, and not just in the deep ocean.

context: tag/color/ question: What is it about red that makes bulls so angry?

The color red does not make bulls angry. In fact, bulls are partially color blind compared to healthy humans, so that they cannot see red. According to the book "Improving Animal Welfare" by Temple Grandin, cattle lack the red retina receptor and can only see yellow, green, blue, and violet colors. Color vision in mammals is accomplished by a collection of cone cells on the back of the eye (the retina). There are three kinds of cone cells: one kind that detects predominantly red colors, another kind that detects mainly green, and the last kind that detects mainly blue. Although cone cells respond most strongly to their main color, they can still respond to other close colors. This color overlap of the cones' sensitivity is what allows us to see so many colors. For instance, a pure yellow color stimulates both the red cone and the green cone, and we experience the combination as yellow. If instead of looking at a pure yellow dot of light, you looked instead at a red dot very close to a green dot with the right balance, you would still experience yellow because the red cones and green cones are being stimulated in the same way. This fact makes the manufacture of computer screens very practical. Instead of implanting a million pixels into a computer screen at every point, each with a different color, the manufacturer only has to construct a grid of red, green, and blue pixels. Humans are actually looking at an array of red, green, and blue dots on a screen but perceive millions of colors. Most mammals, including bulls, are dichromats. This means that they only have two different kinds of cones, as opposed to the three in humans. Bulls lack the red cones, but still have the green and blue cones. A bull's vision is very similar to the vision of a human with red-cone color blindness, known as protanopia. To them, a red cape looks yellowish-gray. It is perhaps the threatening, waving motion of the matador's red cape that enrages a bull, and not the color.

context: tag/color/ question: What is it about the ocean that makes it look blue when it reflects the sky?

The ocean is not blue just because it reflects the sky. The ocean is mostly blue because water itself is blue. In a Journal of Chemical Education paper titled "Why Is Water Blue?" by Charles L. Braun and Sergei N. Smirnov, water is shown to have a slight intrinsic blue color. It takes a large quantity of water, like in the ocean, for humans to notice the blueness. In household uses, like when drinking a glass of water, we use far too little water to notice its blueness. As a result, we think it is always clear. According to Braun and Smirnov, water absorbs red light due to vibrational transitions of the molecules, leaving the blue light to reflect back. For the same reason, large volumes of snow and ice also have a blue tinge. Sensitive laboratory equipment verifies the faint blue color of water even when only a cupful is present. Note that if the ocean's surface is calm and you look at in from a low viewing angle, then some of the blue color on the water's surface is the reflection of the sky. But the reflection of the sky does not totally account for the blueness of water.

context: tag/color/ question: What is the color of the sun?

The color of the sun is white. The sun emits all colors of the rainbow more or less evenly and in physics, we call this combination "white". That is why we can see so many different colors in the natural world under the illumination of sunlight. If sunlight were purely green, then everything outside would look green or dark. We can see the redness of a rose and the blueness of a butterfly's wing under sunlight because sunlight contains red and blue light. The same goes for all other colors. When a light bulb engineer designs a bulb that is supposed to mimic the sun, and therefore provide natural illumination, he designs a white bulb, not a yellow bulb. The fact that you see all the fundamental colors present in a rainbow (which is sunlight split by mist) and no colors are missing is direct evidence that sunlight is white. The sun emits all colors of visible light, and in fact emits all frequencies of electromagnetic waves except gamma rays. This includes radio waves, microwaves, infrared waves, visible light, ultraviolet waves, and X-rays. The sun emits all these colors because it is a thermal body and emits light through the process of thermal radiation. Just like a hot coal or an electric stove element that glows, the sun glows in all colors because of its temperature. That is why incandescent light bulbs emit light that mimics sunlight so well: they contain metal filaments that are heated until they glow in the same way the sun does. It may be tempting to examine the color content of sunlight and identify the brightest color (the peak frequency) as the actual color of the sun. The problem with this approach is that peak frequency does not have a concrete meaning. The peak frequency is different depending on whether you are in frequency space or in wavelength space, as shown in the images below. In wavelength space, sunlight peaks in the violet. In frequency space, sunlight peaks in the infrared. Which is right? They are both right. They are just two different but perfectly valid ways of measuring color content. And this shows us why giving special meaning to the peak frequency is rather meaningless. Furthermore, astronomers like to model the sun as a perfect blackbody, which it is not. According to the wavelength-space blackbody model, the sun peaks in the green! When astronomers say the sun is green, they mean that their inexact model peaks in wavelength in the green. Unfortunately, "The sun is Green!" makes for more exciting headlines than, "The sun is white and would peak in the green if it were a perfect blackbody and if you measure in wavelength space." Although not as exciting, the ultimate truth is: the sun is white; its spectrum peaks in the violet in wavelength space, in the infrared in frequency space, and in the green according to the wavelength-space blackbody approximation. Note that the plots below show sunlight as it is measured in space before entering earth's atmosphere (data from the ASTM Terrestrial Reference Spectra). This is the true color content of the sun. The sunlight that we experience on the surface of earth has been filtered by the atmosphere and is slightly different. The atmosphere tends to scatter out blue and violet more than the other colors. As a result, direct sunlight on the surface of the earth is slightly redder than sunlight in space. Around sunrise and sunset, when the sunlight travels through a lot more atmosphere than usual, sunlight on earth's surface becomes even more red. But the sun itself is white.

context: tag/color/ question: Why are red, yellow, and blue the primary colors in painting but computer screens use red, green, and blue?

Red, yellow, and blue are not the main primary colors of painting, and in fact are not very good primary colors for any application. First of all, you can define any colors you want to be the "primary colors" of your color system, so that other colors are obtained by mixing the primary colors. Although there may be an infinite number of color systems, they are not all equally useful, practical, or effective. For instance, I am free to create a color system where I define light blue, medium blue, and violet as my primary colors. Even though I am free to define my primary colors as such, this color system is not very useful in general because no amount of mixing of these primary colors will produce red, orange, yellow, etc. Therefore, we should make a distinction between a color system and an effective color system. The effectiveness of a color system is best measured as the number of different colors that can be created by mixing the primary colors of the system. This set of colors is called the "color gamut" of the system. A color system with a large gamut is more able to effectively represent a wide variety of images containing different colors. The most effective color systems are those that closely match the physical workings of the human eye, since it is ultimately the human eye which experiences the color. The human eye contains a curved array of light-sensing cells shaped like little cones and rods. Colored light is detected by the cone cells. The cone cells come in three varieties: red-detecting, green-detecting, and blue-detecting. They are so named because the red cone cells mostly detect red light, the green cone cells mostly detect green light, and the blue cone cells mostly detect blue light. Note that even though a red cone cell predominantly detects the color red, it can also detect a little bit of some other colors. Therefore, even though humans do not have yellow cone cells, we can still see yellow light when it triggers a red cone cell and a green cone cell. In this way, humans have a built-in color decoding mechanism which enables us to experience millions of colors, although we only have vision cells that predominantly see red, green, and blue. It should be obvious at this point that the most effective color systems are ones that closely match the human eye, i.e. color systems that mix red, green, and blue light. There is a slight complication because there are really two main ways to create a light beam. We can either create the light directly using light sources or we can reflect white light off of a material that absorbs certain colors. A system that creates light directly is called an "additive" color system since the colors from the different light sources add together to give the final beam of light. Examples of additive color systems are computer screens. Each image pixel of a computer screen is just a small collection of light sources emitting different colors. If you display an image of a pumpkin on your computer screen, you have not really turned on any orange-emitting light sources in the screen. Rather, you have turned on tiny red-emitting light sources as well as tiny green-emitting light sources in the screen, and the red and green light add together to make orange. In contrast to an additive system, color systems that remove colors through absorption are called "subtractive" color systems. They are called this because the final color is achieved by starting with white light (which contains all colors) and then subtracting away certain colors, leaving other colors. Examples of subtractive color systems are paints, pigments, and inks. An orange pumpkin that you see printed in a newspaper is not necessarily created by spraying orange ink on the paper. Rather, yellow ink and magenta ink are sprayed onto the paper. The yellow ink absorbs blue light and a little green and red from the white light beam, while the magenta ink absorbs green light and a little blue and red, leaving only orange to be reflected back. There are therefore two equally-valid methods for creating color: additive systems and subtractive systems. With this in mind, there are thus two color systems that are most effective (i.e. most able to match the human eye): (1) an additive system that creates red, green, and blue light and, (2) a subtractive system that creates red, green, and blue light. For an additive system, light is created directly. This means that the primary colors of the most effective additive color system are simply red, green, and blue (RGB). This is why most computer screens, from iPods to televisions, contain a grid of little red-, green-, and blue-emitting light sources. For a subtractive color system, a certain reflected color is obtained by absorbing the opposite color. Therefore, the primary colors of the most effective subtractive system are the opposites of red, green, and blue, which happen to be cyan, magenta, and yellow (CMY). This is why most printed images contain a grid of little cyan, magenta, and yellow dots of ink. Cyan is the opposite of red and is halfway between green and blue. Magenta is the opposite of green and is halfway between blue and red, and yellow is the opposite of blue and is halfway between red and green. In summary, the most effective color systems are red-green-blue for additive color systems and cyan-magenta-yellow for subtractive color systems. So where did the red-yellow-blue color system come from that they teach in elementary school? Typically, students first encounter color concepts when painting in an art class in grade school. Paint is a subtractive color system, and therefore the most effective primary colors for painting are cyan, magenta, and yellow. Note that high-quality paintings typically do not use just three primary colors since more vivid scenes can be achieved using dozens of primary colors. But when teaching art, it's easier to start more simply; with just three primary colors. Now, to a little grade-schooler, the words "cyan" and "magenta" don't mean much. Furthermore, to an undiscerning youngster's eye, cyan looks awfully close to blue and magenta looks awfully close to red. Therefore, cyan-magneta-yellow becomes corrupted to blue-red-yellow. Elementary art teachers either ignorantly perpetuate this less effective color model (because that's how they were taught as children), or intentionally perpetuate it (because it's just too hard to teach six-year-old's the difference between cyan and blue). Historical tradition was also a prime driver of the red-yellow-blue color system since it was historically thought to be effective before the details of human vision were understood. Since the red-yellow-blue color system is less effective, it is not really used anywhere these days except in elementary school art.

context: tag/color/ question: Why are there only six fundamental colors: red, orange, yellow, green, blue, and violet?

There are an infinite number of fundamental colors, if by "fundamental" you mean "spectral". Spectral colors are also known loosely as rainbow colors. A spectral color is composed of a single fundamental color on the visible part of the electromagnetic spectrum, as opposed to a mixture of colors. Spectral colors such as red or green are composed of light waves of a single frequency. Non-spectral colors such as brown and pink are composed of a mixture of spectral colors. Simple lasers, by their very nature, only emit one frequency of light (to an excellent approximation). This means that simple lasers can only generate spectral colors (compound colors from laser systems can be made by mixing the light from several lasers, or by running the spectral color through a material that converts it to a mixture of colors). Even though the spectral colors are a subset of all colors, there are still an infinite amount of spectral colors. This fact becomes obvious when looking at the spectrum from a prism, which is simply a spread of spectral colors. Note that an atmospheric rainbow is not a spread of pure spectral colors. Although it is close to a pure spectrum, a rainbow in the sky really consists of mixed colors. For this reason, it is incorrect to refer to a pure spread of spectral colors as a "rainbow" even though they look similar. A pure spread of spectral colors can be produced by passing perfectly white light through a prism or a diffraction grating. A pure spectrum does not have six solid bands of color. Rather, a pure spectrum has a smooth variation of colors. Red and orange are spectral colors, but so is the color half-way between red and orange. You don't have to make this color by mixing red and orange. It exists on its own as a fundamental color with its own frequency. The same is true of all the in-between spectral colors that don't have common names. Because the physical color spectrum is continuous, the naming of colors is purely a societal affair. For instance, Americans identify the color "yellow" as the color next to green with no orange tint. But Germans use the word "gelb", which translates to "yellow", to identify colors that span from yellow to yellow-orange. If a yellow-orange truck drove by, the American would call it orange and the German would call it yellow. Who is right? They are both right because they are just using names differently. It is a good thing that fundamental frequencies (colors) come in a continuous spectrum and not just in a few discrete options. Otherwise, we would not be able to enjoy so many radio stations. Each radio station broadcasts radio waves at a different frequency to avoid interference with each other. If there were only six "colors" to the radio part of the spectrum, we would be stuck with six radio stations. This principle also allows many cell phones in the same room to communicate with a cell phone tower without interfering with each other.

context: tag/color/ question: Why are veins blue?

The veins themselves are not blue, but are mostly colorless. It is the blood in the veins that gives them color. Furthermore, the blood in human veins is also not blue. Blood is always red. Blood that has been oxygenated (mostly flowing through the arteries) is bright red and blood that has lost its oxygen (mostly flowing through the veins) is dark red. Anyone who has donated blood or had their blood drawn by a nurse can attest that deoxygenated blood is dark red and not blue. The blood in your veins appears blue because you are looking at your veins through layers of skin and fat according to Alwin Kienle in his paper "Why do veins appear blue? A new look at an old question" published in the Journal of Applied Optics. Skin scatters a lot of the red portion of white light before it can reflect off the blood, leaving the blue light to reflect off the blood and back to our eyes. It is a similar effect to how the white sun appears red at sunset due to the blue colors being scattered away by the atmosphere.

context: tag/color/ question: Why can't color blind people see any colors?

Most people with color blindness can see almost all colors. They are typically only blind to a few colors. Furthermore, they are not completely blind to those colors, but are only partially blind. The human eye contains three types of color detecting cells: red-detecting cones, green-detecting cones, and blue-detecting cones. Each cone cell can detect many colors in addition to its main color. For instance, the cone cells in the eye that can see red light can also see orange and yellow. The red cone cells just see red the best, and that is why they are called the red cones. Similarly, the green cones can also see orange, yellow and blue. This overlap is what allows humans to see so many different colors. Humans can see the color yellow, for instance, even though there are no yellow receptors, because of this overlap. When you look at a yellow flower, the yellow light stimulates the red and green cells in your eyes, and your brain interprets a little bit of red plus a little bit of green as yellow. Typically, a color blind person has only one type of light cell absent or non-functioning. The textbook Color Vision, edited by Werner Backhaus, Reinhold Kliegl, and John Simon Werner, states that about 1% of Caucasian males have partial color blindness with one of the three color receptor types absent or non-functioning. This book further states that people who are completely color blind; those with all three types of receptors absent or non-functioning; are "extremely rare". People with protanopia color blindness lack the red detecting cone cells or pigments. As a result, they do not see red or orange colors as well. But they see all the other colors just fine. People with deuteranopia color blindness lack the green detecting cones or pigments, but have their other cones working just fine. People with tritanopia color blindness lack the blue detecting cones. People with these three types of partial color blindness only have problems with a few colors and can see most colors just fine. You should keep in mind that the response of the different color-detecting cells overlap. When a person with full vision looks at a pure blue wall, it is not just his blue cone cells that are stimulated. His blue cone cells are stimulated a lot and his green cone cells are stimulated a little. His brain therefore experiences pure blue as a large blue cell signal plus a small green cell signal because of the way their responses overlap. Now apply this concept to a color blind person. A person with tritanopia is missing the blue-detecting cells in his eyes. When he looks at a blue wall, his blue cells don't see anything because he is missing the blue cells. But the green-detecting cells are still there, and they see blue a little bit. In this way, a typical color blind person is not really completely blind to any color. But his brain experiences a small green signal as dark green. A person with blue color blindness therefore experiences pure blue as dark green. His color perception is not complete and not quite right, but he does experience color, even in the color range where he has greatest difficulty. Whereas a person with full vision sees the colors green, blue, and violet, a person with blue color blindness sees these colors as green, dark green, and darker green. He sees color. He just sees the colors slightly wrong and has a harder time telling certain colors apart. Another point to keep in mind is that most colors that we see in everyday life are a mixture of fundamental colors. For instance, in terms of fundamental colors, pink is a mixture of a lot of red plus a little of orange, yellow, green, blue, and violet. When a person with blue color blindness looks at pink, only the blue part of the color mixture is faulty, which is a small part of the color pink. As a result, pink looks nearly normal to a person with blue color blindness. The same goes for other colors that are a mixture of fundamental colors. Also, there is more to an object than its color. An object also has brightness, shading, texture, and shape. If a chair that is painted pure red is placed in front of a person with red color blindness, she would have no problem seeing the chair. She would still see the shape, shading, texture, and brightness of the chair. She would still see some color in the chair too. The chair would just look dark yellow instead of red. In this way, people with color blindness are not really "blind". They can see all objects just fine. Better phrases would be "incomplete color vision" or "less sensitive color vision". Finally, people with color blindness learn to use mental tricks to compensate for the deficiency. For instance, oranges are always orange and stop signs are always red. A person with red color blindness has a harder time telling apart red and orange, but he can sure tell the difference between an orange and a stop sign. Even though the color difference between these two objects is very subtle for him, he feels that the color is very different because he associates the object identity with its color, and stop signs are very different from oranges. If you place a pure red posterboard next to a pure orange posterboard, a person with red color blindness may have a hard time identifying the red object. But place a stop sign next to an orange and ask him to identify the red object, and he will have no problem pointing to the stop sign as red. He may even think he is actually seeing red in this case, when he is just mostly seeing an object he has associated with the word red. Similarly, a color blind person typically has no problem driving and obeying traffic lights. You may think that he can't tell red lights from green lights and won't know when to stop or go. In reality, he may not see these lights in their full color, but he can see enough color to tell them apart. Additionally, he comes to associate the top light with red and the bottom light with green without even realizing it. He may tell you he is "remembering the color", when in reality he is remembering the location of the traffic light that means "go" and "stop". In this way, color blind people sometimes think they are fully seeing colors even though they are not because they are unconsciously relying on mental tricks and associations to compensate. This post is meant in no way to trivialize or underestimate the burden that color blind people suffer in having incomplete color vision. Rather, this post is meant to simply uncover the science behind color blindness. Because of their sensitivity to most colors, the overlap of their cone cell response, and the other visual cues connected to an object, most people with color blindness have a nearly complete color experience. (Personal note for those curious: I am not color blind but personally know people who are).

context: tag/color/ question: Why does a rainbow contain a pure spread of spectral colors?

A rainbow does not contain a pure spread of spectral colors, although it is somewhat close. A spectral color is a color that contains only one wavelength component of its electromagnetic wave. In contrast, a non-spectral color contains many wavelengths and is therefore a mixture of spectral colors. Simple lasers produce effectively pure spectral colors. The visible "electromagnetic spectrum" is a continuous spread of all of the spectral colors, arranged according to wavelength (i.e. red, orange, yellow, green, blue, and violet). Furthermore, the complete electromagnetic spectrum is a continuous spectrum and contains an infinite number of spectral colors. Just because we do not have a common name for the spectral color between red and orange does not mean that it is not a spectral color. When we display the spectrum of a certain light beam (or the spectrum of the light from a certain object), we are really just showing the spectral colors contained in that light, as well as their intensities and locations on the wavelength scale. Natural white light, such as from the sun, contains all spectral colors and therefore displays a continuous spread when separated into a pure spectrum of spectral colors (ignoring the narrow absorption lines). For instance, when white light enters and exits a glass prism, the different spectral color components of the light bend different amounts due to the dispersive nature of the glass. The different colors exit the prism at different angles, leading to a pure spectrum that becomes visible when it reflects off a wall or screen (strictly speaking, a prism only creates a pure spectrum if the original beam of light is very thin). Unlike the spread of colors created by a prism, the spread of colors created by a spherical raindrop is not a pure spectrum. (By the way, raindrops are round and not tear shaped.) While the brightest part of a rainbow (the colorful outer edge) is close to a pure spectrum, each point in the spread contains a mixture of spectral colors. The more you look inwards from the outer edge of a rainbow, towards the arc's center, the more spectral colors there are mixed together, until finally the entire interior region of a rainbow is faint white, indicating a complete mix of all colors. The reason that a point in a rainbow contains a mix of spectral colors is ultimately because the front surface of a raindrop is round. This means that different parts of the original light beam encounter the raindrop's curved surface at different angles and bend different amounts, even for a single color. The diagram above shows how each color gets bent into many angles. Although pure red is mostly bent by a raindrop into a 42.1° viewing angle to form the outer edge of a rainbow, some of the red is bent into all angles between 0° and 42.1° because of the curved surface of the raindrop. Similarly, pure orange is mostly bent into the 41.9° viewing angle, but some orange is bent into all lower angles as well. The color in a rainbow at 42.1° is therefore red, the color at 41.9° is orange plus a little bit of red, the color at 41.7° is yellow plus a little bit of orange and red, etc. The end result is that the colors in a rainbow tend to blur together and wash each other out. The extended shape of the sun also sends light into the raindrop at slightly different angles and further blurs the colors together. A prism and a raindrop are in principle very similar. They both spread white light out into a span of colors through refraction. The main difference though is that a prism has flat surfaces, leading to a pure spectrum, while a raindrop has a round surface, leading to an impure spectrum. Unfortunately, in everyday language, the phrases "rainbow" and "visible spectrum" are used to mean the same thing, even though scientifically, they are not exactly the same.

context: tag/color/ question: Why does a rainbow exist only in a narrow band?

A rainbow does not exist only in a narrow band. A primary rainbow is the light pattern resulting from sunlight entering a spherical water drop, bouncing once off the back of the drop, and exiting. Defined this way, a primary rainbow exists at all viewing angles from 0° to 42° and therefore forms a wide disc. The viewing angle is the angle formed between the point exactly opposite the sun and the direction you are looking. It's true that the most vivid and spectrally pure part of the rainbow exists only in a narrow band from about 41° to 42°, but this is only one part of the rainbow. There are three main parts to a primary rainbow: All of these parts of the rainbow arise from the exact same thing: light rays refracting (bending) upon entering a spherical water drop, reflecting once off the drop's back surface, and then refracting again upon exiting the drop. The different parts of a rainbow look so different simply based on the way the colors are mixing. Using the laws of reflection and refraction, it is easy to derive a relationship between the viewing angle at which you see a light ray, and the associated height of the corresponding original light ray with respect to the drop center's. The result is shown in the image for the colors red (640 nm), yellow (585 nm), green (530 nm), blue (475 nm), and violet (420 nm). The x-axis plots the height at which the original rays of sunlight hit the spherical water drop relative to the drop's center. A height of 0 corresponds to a direct, dead center hit on the drop and a height of 1 corresponds to an incident ray just glancing off the side. The y-axis plots the viewing angle at which we see light exit the water drop and is measured relative to the antisolar point. A viewing angle of 0° corresponds to looking directly away from the sun, and a viewing angle of 180° corresponds to looking directly towards the sun. The curves on the plot relate the incident ray height and final viewing angle for rays of different colors. White sunlight contains all colors. After the white sunlight scatters off the water drops in the air, different colors end up at different viewing angles. Analyzing this graph, we find that at a viewing angle between 41° and 42°, the colors are well separated. An observer therefore sees this part of the rainbow as a vivid, virtually pure spread of colors. Between 39° and 41°, these colors begin to blend incompletely, leading to odd colors. Finally, for viewing angles between 0° and 39°, there is almost an equal amount of all colors. When all colors of light are mixed together evenly, the color you get is white. The part of the rainbow extending from 0° and 39° is therefore white. As this graph should make obvious, the color red is scattered into all angles from 0° to 42°. But because of the mixing in of other colors, the only angle at which we see almost pure red is at 42°.

context: tag/color/ question: Why does water make a shirt darker?

Getting a spot wet on your shirt does not make that spot permanently darker or intrinsically different. Water just makes fabric appear darker by making the fabric more transparent, allowing you to see darker objects behind the fabric. Let's first consider a white shirt. The individual fibers that make up a white shirt are really not that white. They are actually mostly transparent. However, transparent materials do reflect a small amount of light. For instance, a glass window is transparent (allowing us to see the outside world) but still reflects some of the light that hits it. This is why you can see your reflection in a window at night. Shirt fibers are the same. They are mostly transparent and slightly reflective at the same time. A single fiber therefore looks mostly clear, not white. However, each fiber is surrounded by more fibers which give the light more chances to be reflected. It's important to realize that the reflection of light happens at the interface between the fiber material and air. Therefore, a material that is made out of many small fibers has a large surface area, giving the light many opportunities to be reflected. Furthermore, the weave of the fibers creates many layers of reflecting surfaces. Therefore, the portion of the light that successfully makes it through the first fiber without being reflected can still be reflected by the lower layers. As a result, most of the white light falling on an undyed shirt eventually gets reflected back away from the shirt in a random fashion. The shirt is therefore seen as a diffuse white color. This also means that very little light makes it all the way through the shirt. We therefore see the material as opaque. The main factor that turns a mostly transparent material (the individual fibers) into a mostly opaque material (the shirt) is the array of multiple reflecting surfaces created by the weaving pattern. This effect is quite common. Take any solid chunk of mostly transparent material, introduce small structures that create little pockets of air, and you end up with a white material. For example, a solid block of ice is clear, but ice formed into microscopic structures (snow) is white. A solid block of salt (halite) is transparent, but salt ground down to a pile of grains (table salt) is white. A solid block of quartz is clear, but quartz filled with air bubbles (milky quartz) is white, and quartz ground down to a pile of grains (sand) is white. A cup full of water is clear, but a collection of tiny liquid water droplets (a cloud) is white. In each case, it is the existence of a microscopic material structure filled with tiny air pockets that leads to multiple reflecting surfaces, and therefore to strong reflection. Spill some water on your white shirt and the situation changes. In the spot that is wet, the water fills all the little pockets in the fabric that used to be filled with air. In terms of its interaction with light, water behaves very similar to fiber material. Adding water to fabric effectively removes the multiple reflecting surfaces and effectively turns the shirt into a solid chunk of material which is mostly transparent. Water does not really make the fabric chemically darker. Instead, water just temporarily makes the fabric more transparent. Underneath the shirt is usually something that is darker than white, such as skin or another shirt. Therefore, you perceive a wet spot of fabric that has become more transparent as a wet spot of fabric that has become darker. If you wear a white shirt over a bright orange shirt, spilling water on your belly will make your belly look more orange and not more black. If you hold a white shirt with a wet spot right in front of a bright light, the wet spot will appear brighter than the rest of the shirt because it is more transparent and lets more light through. For the same reason, getting any white, micro-structured material wet will make it look darker, more colorful, or lighter; depending on what's behind it. Pour water on a pile of snow, granulated sugar, or table salt that is sitting on a dark table and the pile will become more transparent and appear darker. There is another effect at work which contributes to the change in visual appearance of a wet spot. When light reflects off of a rough surface, the light reflects in all directions. As a result, the surface looks bright when viewed from any direction. This is called diffuse scattering. In contrast, when light reflects off of a very smooth surface, the light reflects only in the mirror direction. As a result, the surface only looks bright when viewed from a single specific direction, i.e. when viewed at the mirror angle. At all other angles, the surface looks dark. This is called specular scattering. Since there is a low probability that you will just happen to be looking at a smooth surface at exactly the right angle, this means that smooth surfaces generally look darker than rough surfaces. When water spills on a shirt, it fills in all the little pockets of air and turns the shirt effectively from a rough surface to a smooth surface, leading the wet spot to generally look darker. If you add dye to a shirt the situation gets more complicated. Dyes strongly absorb certain colors and strongly reflect other colors. However, the surface of each individual dyed fiber still reflects a little bit of all of the colors. This means that a dry blue shirt is actually whitish blue (which we call "lighter blue"), but the same shirt when wet is just blue (which we call "darker blue"). Adding water to a dyed fabric inhibits the multiple reflections, so more of the light has a chance to be absorbed by the dyes. Therefore, adding water to a heavily dyed shirt does not make it more transparent. It makes the shirt have a darker, more saturated color.

context: tag/color/ question: Why doesn't the outside world appear blue even though so much light comes from the blue sky?

Actually, a lot of the outside world on a sunny day is tinted blue because of the blue sky. The blue tinting of the outside world is quite striking if you know how to look for it. Sunlight that has just left the sun contains a relatively equal mix of all spectral colors and is therefore white. When the sunlight passes through the air in earth's atmosphere, some of the light is scattered sideways by the air molecules. The air molecules scatter a little bit of all colors out of the forward-traveling beam of sunlight, but blue and violet colors are scattered the most, giving the sky a whitish-blue appearance. Since more blue and violet light have been removed from the forward traveling beam than other colors, the direct sunlight that reaches earth's surface is slightly tinted orange-red compared to the sunlight given off at the sun's surface. But this orange-red tint is so small (because the total amount of light scattered out by the atmosphere is small compared to the sunlight that continues on in the forward direction) that direct sunlight that reaches earth's surface is still white. The white direct sunlight is much brighter than the blue skylight, and this is the main reason that the entire outside world is not completely blue. White light contains all colors, so a scene illuminated by white light will show all the innate colors of the objects in the scene. When illuminated by white light such as sunlight, we are able to see the grass as green, the flowers as red and wood as brown. The world shows its usual mix of colors when illuminated by white light. White sunlight is so bright compared to blue skylight that when an object is in direct sunlight, it just looks like its innate color. But, when an object is in shadow so that direct sunlight cannot reach it (and there are not too many clouds that block the skylight), the object is now being illuminated only by blue skylight. Shadows on a clear day are therefore blue tinted. The more shadows there are, the more the world around you is tinted blue. When the sun is low in the sky and behind the trees, the entire landscape can end up being shadowed from direct sunlight and therefore be blue tinted. We often don't notice the blueness of the outside world because our human eyes are better at seeing relative color than absolute color. This means that our eyes are better at picking out color differences than at picking out an overall color on an absolute scale, as shown in the image above. But the blue tint of the outside world becomes obvious if you look into an area of deep shadow on a bright, clear day. The blue tint of shadows can make for dazzling scenery worthy of being painted or photographed.

context: tag/color/ question: Why is the sky blue?

If you look in any popular science book on this topic, it will tell you that the sky is blue because of Rayleigh scattering in the atmosphere. While Rayleigh scattering is a very important part of the answer, it is not the only part. If the only effect at work were Rayleigh scattering, then the sky would be violet, not blue. In fact, there are four factors involved; all required to give the final answer of blue: Rayleigh scattering is what happens when light bounces off an object that is much smaller than its wavelength. Physical derivations show that for Rayleigh scattering, the higher frequency colors such as blue and violet are scattered much more strongly than low frequency colors such as red and orange. Mathematically, the intensity of scattered light is proportional to f 4, where f is the frequency of the light. This means that a color that is twice the frequency of another color will be 16 times brighter than that other color after scattering if they both were equally bright to start out with. In the atmosphere, the objects doing the scattering are mostly nitrogen molecules (N2) and oxygen molecules (O2). These molecules are much smaller than visible light (oxygen molecules are about 0.1 nanometers wide and visible light has a wavelength of about 500 nanometers), so the scattering in the atmosphere is Rayleigh scattering. But if this were the end of the story, the sky would be violet and not blue because violet is the highest-frequency visible color. Let us look at the other effects. Most people think that the sunlight traveling through space before it hits our atmosphere is a perfectly white color. Perfect white is an equal mix of all colors. In reality, the sunlight hitting the atmosphere does not have an equal distribution of colors, but has a thermal (black-body) distribution. The sun is a big ball of incandescent gas very similar to a candle flame. The light that our sun sends out into to space is created by its heat, and therefore it has a thermal distribution of colors. The exact nature of a thermal distribution depends on the temperature of the glowing object (i.e. red-hot toaster elements vs. white-hot coals). The sun's surface temperature is at about 5800 Kelvin, which gives a thermal distribution that peaks in the infrared (in frequency space). The sunlight that hits the atmosphere is therefore not an equal mix of all colors, but is a mix of all colors with red-orange dominating, and with more blue than violet (at least in frequency space). This helps explain why the sky is blue and not violet. But it turns out to still not be enough. Bulk attenuation means that as sunlight travels through the thick atmosphere, it becomes progressively weaker because it is being partially scattered all along the way. The rate at which it becomes weaker is faster for higher-frequency colors. In other words, because blue and violet scatter the strongest, they are also removed the quickest from the forward traveling beam as it travels down through atmosphere. Bulk attenuation is what makes sunsets red and orange. At sunset, the sunlight is approaching an observer at a low angle, so it has to travel through much more atmosphere. It travels through so much air in fact, that by the time the light reaches the layer of air closest to the grounded observer, all of the green, blue and violet colors have long since been scattered out by higher layers of atmosphere, leaving just red and orange. While it is still true that the air in the layer of sky closest to the observer scatters blue and violet light the strongest, there is no blue or violet light left in the light beam to scatter at sunset because of bulk attenuation. Even during the day when the sun is high in the sky, bulk attenuation has an effect. The sunlight in the forward beam near the bottom of the atmosphere has less violet than blue as compared to the beam when it was in the top of the atmosphere because the violet color scatters more quickly our of the forward beam. While this helps explain why the sky is blue and not violet, it is not enough. Finally, human eyes perceive color in a non-linear fashion. For instance, if a blue spot on the wall sits next to a violet spot on the wall, and a laboratory tool measures them both to be equal brightness, our eyes will perceive the blue spot to be brighter. In a loose sense, our eyes are more sensitive to blue light than violet light. But the situation is more complex than this statement makes it seem. Our eyes have three color receptors: red, green, and blue, but these receptors overlap quite a bit. Our brains then mix the three signals non-linearly to give us the experience of color. In a simplified view, equal parts of red, green, and blue are perceived as white, whereas a little red plus a little green plus a lot of blue-violet is perceived as whitish-blue. Our brains mix together the spectrum shown above to give us the sky blue shown below. All of the effects mentioned above are needed to explain why humans see the sky as blue.

context: tag/color/ question: Why was color invented by humans?

Color was not invented by humans. Color is a fundamental physical property of light that exists independent of humans. How color is perceived by a certain person is of course human dependent. For example, a standard helium-neon laser always emits a specific red color (scientifically, its color is the color with a wavelength of 632.8 nanometers). Whether a person perceives this color as sharp, dull, alarming, vibrant, evocative, romantic, muddy red, brick red, orange-red, or even yellow (if he is color blind) depends entirely on the human perception. But this does not change the fact that helium-neon lasers emit light with the color 632.8 nm red. A machine can measure the color of helium-neon laser light and will find it to be 632.8 nm red every time, no matter which humans are running the machine. In fact, even a blind person that has never experienced sight can be trained to run a machine that measures the color of helium-neon laser light to be 632.8 nm red. The blind person will not a have a personal conception of what red actually looks like, but he should have no great difficulty measuring and scientifically describing red. The same is true of all colors and not just 632.8 nm red. The blind person's experience is similar to that of a human with sight looking at an infrared color. None of us can see an infrared color. Therefore, none of us has any concept of what infrared really "looks like" in the human sense of the word. But infrared still exists physically and can be created and detected using machines. An infrared camera helps a human "see" infrared colors by converting the infrared colors to red or green. An infrared camera, therefore, does not really allow you to see infrared colors. It allows you to see red colors that are in the same spatial pattern as the infrared colors. An intelligent alien on a distant planet that has never had sight can still measure the color red using tools in a similar way to how humans measure infrared colors without being able to see them. Physically, there are two kinds of colors: pure spectral colors and mixed colors. A pure spectral color consists of a beam of light that is a simple sine wave with a single wavelength. The wavelength of the light is the color of the light. Light is a waving of the electromagnetic field. The distance in space between the peaks of an electromagnetic wave is its wavelength, and hence its color. When you run a narrow beam of white light through a glass prism, it spreads the light out into its spectral colors. The resulting red, orange, yellow, green, blue, and violet pattern (and all the colors in between) shows the span of possible visible spectral colors. Red has a relatively long wavelength (compared to the other visible colors). On the other end of the visible spectrum, violet has a relatively short wavelength. All of the visible colors have wavelengths that are on the scale of hundreds of nanometers (or tenths of microns). A mixed color is a combination of spectral colors. Exactly how much of each spectral color is added to the mix determines what the final color is. As before, the exact nature of a mixed color is a physical property that exists independent of humans. How a mixed color is perceived is human-dependent. We can say scientifically that 10 Watts of 700 nm red light added to 5 Watts of 530 nm green light and 5 Watts of 470 nm blue light makes a mixed color that exists physically and is independent of humans. Whether humans call this mixed color "pink", "fushia", "peach", "salmon", "romantic", or "George" makes no difference to the fact that it exists scientifically. Human sight is actually quite limited. Healthy humans have only three color receptors: red, green, and blue. While it's true that the red receptors can also pick up orange and yellow, all these colors get collapsed down to a single "red" nerve signal. In the same way, the green receptors can detect yellow, green, and blue colors, but they all get collapsed down by the green receptor to a single "green" nerve signal. The same case holds for the blue receptors as well. What this means is that the color that you get when you mix red light and green light looks to a human the same as a pure spectral yellow light, even though pure yellow and red+green are scientifically different. If a certain explosive chemical can only be ignited by spectral yellow light, then red+green light will never make the stuff explode, even though human eyes can't tell the difference. This limitation of human eyes is called "metamerism". Scientifically speaking, the human eyes are very bad detectors for measuring the true nature of a certain mixed color. A typical mixed color has many different spectral color components, and not just red, green, and blue. We want to therefore use machines and measure all the spectral colors present in a mixed color in order to accurately describe it. Each spectral color can be present in large or small amounts. In order to completely describe a mixed color scientifically, we have to tell the power and wavelength of each spectral component. The resulting plot of a certain mixed color is called its "spectral power distribution" or just its "spectrum" for short. If you ask a random intelligent human, "What is the color of the sun?", you will get the inexact answer "white". But if you ask a solar scientist or a spectrometer machine this same question, the answer will be the solar spectrum.

context: tag/conservation-of-energy/ question: Can momentum be hidden to human eyes like how kinetic energy can be hidden as heat?

Yes and no. In a regular mechanical system with macroscopic parts, momentum can not be "hidden" to human eyes. But in other systems, momentum can be hidden. For instance, in an electromagnetic system, momentum can be transferred to the electromagnetic field, which is invisible to human eyes at most frequencies. Therefore, momentum can be "hidden" in the electromagnetic field. Let us explore this topic more in depth. Every isolated system obeys the law of conservation of energy which states that if no energy is externally extracted or inserted into the system, its total energy will remain constant in time. For example, consider two hockey pucks sliding towards each other on ice. If you sum the kinetic (motional) energy of both pucks before they collide, you will find that it equals the sum of both energies after they collide. We can in fact use the conservation of energy along with some other information to predict what will happen in a simple collision like this. But now suppose we fire the two pucks directly at each other at the same speed and cover their sides with perfect glue. What happens? Of course, when the pucks make contact, they stick together and both end up with zero speed, and therefore zero kinetic energy. This means that the total kinetic energy of the two-puck system at the beginning was some big number, but the total kinetic energy at the end was zero (at least according to human eyes). Has the glue broken the law of conservation of energy? No. If you did a careful analysis of this event, and measured everything you could think of, you would find that the heat (thermal energy) in the two pucks increases after the collision by the exact same amount as the kinetic energy that seemed to disappear (neglecting the friction of the ice). The macroscopic kinetic energy has therefore been converted into heat. The law of conservation of energy still holds as long as we add heat as one of the things that contributes to the total energy. Such a collision is called an inelastic collision. But what is heat? On the atomic level, we find that hotter substances have their atoms vibrating faster and moving around at higher speeds. Thermal energy is therefore just the kinetic energy of microscopic, randomly-moving particles. When the two pucks with glue stick together, their kinetic energy is not really converted to some mysterious thing called "heat". Their macroscopic, ordered kinetic energy (ordered in the sense that all the atoms in the puck are moving along with the puck as it zips across the ice) is simply converted to microscopic, random kinetic energy. The atoms at the surface of each puck smash together, get displaced, smash into other atoms, and so on, creating vibrations. Macroscopic, ordered kinetic energy is obvious to the human eye (we see the puck moving), but microscopic, random kinetic energy is invisible to the human eye (we cannot directly see the atoms jiggling). In this sense, an inelastic collision causes some of the kinetic energy to be "hidden" from human eyes in the form of heat. If we include only the forms of energy visible to the human eye, glue seems to defeat the law of conservation of energy. But if we include hidden kinetic energy (heat), the law still holds. Now there is another law called the law of conservation of momentum. It states that the total momentum of an isolated system before an event must be equal to the total momentum of the system after the event. What is the difference between this law and the law of conservation of energy. The difference is that energy is just a number, while momentum has a direction. Conservation of momentum therefore tells us things like: two pucks initially traveling to the right that collide (say, because one is going faster and overtakes the other) must still be traveling to the right after the collision. The total momentum of both pucks is to the right in the beginning, so it must be to the right in the end. The law of conservation of energy cannot tell us this because it says nothing about directions. Or similarly, if one puck traveling east approaches another puck traveling north, the total momentum is in the north-east direction. Conservation of momentum tells us that after the collision, the total momentum must still be in the north-east direction. Since momentum obeys a conservation law just like energy, and since kinetic energy can be hidden from human eyes in the form of microscopic motion, it is natural to wonder whether momentum can also be hidden. Does the two-puck-glue system convert momentum to some hidden form upon collision, and therefore some momentum seems to get "lost" to human eyes? No. The reason the answer is no is because momentum is directional. The atoms do indeed jiggle faster after an inelastic collision, but the total momentum of atoms jiggling in place is always zero. Put simply, when the atomic motion is random, for every atom going left there is another atom going right, so that their directionality adds to zero. Since momentum describes the directionality of motion, the momentum for simple thermal motion is zero. In order for momentum to not be zero, the atoms have to all be traveling more or less in the same direction. But when all the atoms are traveling in the same direction, the macroscopic object itself is traveling somewhere, which is quite visible to the human eye. Because momentum is directional, it cannot be hidden from the human eye in the form of random atomic motion. Therefore, even if we cover the sides of the pucks with glue so that they stick, the total visible momentum before will equal the total visible momentum after. This makes sense because if the two pucks are flying at each other at the same speed from opposite directions (and have the same mass), their total momentum before is zero (east plus the same amount of west equals zero), so their total momentum after sticking together is zero (zero plus zero equals zero). This idea extends beyond pucks colliding. Any macroscopic mechanical system cannot hide momentum. But, if the system has parts that are so small that they are invisible to the naked eye, and these parts can undergo ordered motion, momentum can be transferred and hidden in these parts. For instance, if a tennis ball is covered with dust grains that are so small you can't see them, when the tennis ball gets hit, the dust can fly off to one side and carry away some of the momentum. If we don't notice the dust, and don't include it in our calculations, we would find that some of the total momentum becomes hidden after the collision. Richard Feynman states in his book, The Feynman Lectures on Physics, the following: "Are there also hidden forms of momentum-perhaps heat momentum?" The answer is that it is very hard to hide momentum for the following reasons. The random motions of the atoms of a body furnish a measure of heat energy, if the squares of the velocities are summed. This sum will be a positive result, having no directional character. The heat is there, whether or not the body moves as a whole, and conservation of energy in the form of heat is not very obvious. On the other hand, if one sums the velocities, which have direction, and finds a result that is not zero, that means that there is a drift of the entire body in some particular direction, and such a gross momentum is readily observed. Thus there is no random internal lost momentum, because the body has net momentum only when it moves as a whole. Therefore momentum, as a mechanical quantity, is difficult to hide. Nevertheless, momentum can be hidden-in the electromagnetic field, for example. Let us move beyond mechanical systems. Electromagnetic waves such as light or radio waves carry momentum. Since all electromagnetic waves are invisible to humans except the narrow range of colors from red to violet, momentum can be hidden in the electromagnetic field. In practice, the momentum carried by electromagnetic waves is so small that you have to use very sensitive instruments to measure it, but in principle it is always there. For instance, if you turn on a radar gun, radio waves come out the front of the gun, carrying momentum with them. Because of momentum conservation, the radar gun itself must therefore recoil in the opposite direction when it is turned on. This recoil is typically too small to notice. But for the sake of the argument, suppose we have a giant radar gun that emits a large amount of radio waves. Since radio waves are invisible to the human eye, all that we would see would be the gun jump backwards upon being turned on, clearly violating conservation of momentum (if we only include visible momentum). We would therefore conclude (and rightly so), that there must be momentum hidden somewhere. In this way, momentum can be "hidden" in the electromagnetic field. Although, it is really only hidden to human eyes. We can detect radio waves just fine with an antenna.

context: tag/conservation-of-energy/ question: How can we travel to the past?

Mainstream science dictates that time travel to the past is impossible. The basic problem with time travel to the past is conservation of energy. For example, pretend I made a time travel portal that sends objects back three seconds in time to the exact same location, and I mount this portal a few inches above a trampoline. I turn the portal on and one second later drop a charged battery into the portal. It falls, appears three seconds earlier, before the portal was turned on, and bounces off the trampoline to a point above the portal. Three seconds later, there I am dropping the original battery and next to it is the battery that time traveled and bounced. The portal turns on and both batteries now fall through the portal. The cycle repeats itself automatically without human intervention. All I had to do was turn on the portal and drop the first battery. Two batteries become four, which become eight, and so on. A businessman might say, "Great! Free batteries. Let's sell them and make a fortune!" But the point is that once I flick on the portal and drop the first battery, the rest is entirely automatic. This means there would be no way to stop it. Keep in mind that this chain reaction is not happening at progressively later points in time. It is all happening in the same three-second loop of time. That one battery would instantly become an infinite number of batteries. They would fill the entire universe, crush us to death, and explode with all of this runaway energy. The law of local conservation of energy is universal, and it's a good thing. It's what keeps our universe from exploding in an instant into an infinite fireball of energy. It's also what rules out time travel to the past. If the presence of a human in our scenario bothers you, we could do without him. We could just as easily have an acorn fall from a tree through a three-second time-travel portal created by a cosmic ray and bounce off the trampoline without any humans around. The explosion of the universe would still occur. The fact that the universe is still around is ample evidence that time travel to the past is impossible. Note that we have not used any ethical or social arguments involving killing your own grandfather. Time travel to the past is forbidden by simple physics even in the absence of humans. While movies about time travel to the past can be creative and thought-provoking, they are simply wrong. There are many speculative theories about time travel to the past, some made by respected physicists, but mainstream physics does not allow it. On the other hand, time travel to the future is allowed and indeed has been measured experimentally. Relativity dictates that the rate of the forward march of time is dependent on both the speed of your reference frame as well as the strength of the gravitational field in your frame. We can travel into the future by either spending time in a vehicle traveling very fast relative to earth or by spending time in a very strong gravitational field. In both cases, time travel does not allow skipping to some future time. All reference frames must experience all seconds in the day. Rather, time just goes slower in such reference frames. If your local time runs fast on account of high speed or gravity, you can experience an hour while earth experiences two. Such time travel is verified experimentally every day by equipment that deal with precise time measurements, such as GPS satellites. Unfortunately, the time dilation effect is very weak. The highest speeds and strongest gravity that men can currently attain only transports them milliseconds into the future.

context: tag/conservation-of-energy/ question: How do free energy machines work?

Free energy machines do not work. No machine can create energy out of nothing, as this would violate the law of mass-energy conservation, which is fundamental and universal. The law of mass-energy conservation states that mass-energy can never be created or destroyed. It can only be redistributed throughout space and transformed into different states. Mass can be converted to energy, and energy can be converted to mass, but together they must be conserved. For instance, when a positron from the tracer liquid of a medical PET scan hits an electron in the patient's body, the positron and electron completely destroy each other and all of their mass is converted into energy. This energy is emitted as two gamma particles (high energy light) that fly off in nearly opposite directions. The PET machine detects the gamma rays, uses them to pinpoint the location of the positron-electron annihilation event, and therefore discovers where in the patient's body the tracer liquid is congregating. Nuclear bombs and nuclear reactors also convert mass to energy, but the conversion is very inefficient and only a fraction of the bomb's mass is converted to energy. Mass is also converted to energy during radioactive decay. In contrast, energy is converted to mass in particle accelerators such as the LHC. In particle accelerators, large tracks of magnets speed up particles such as electrons and protons to incredible speeds. The particles have therefore gained a high amount of kinetic energy from the magnets. The particles are then guided to collide with a stationary target (or collide with other particles that have been accelerated in the opposite direction). Upon collision, the kinetic energy is lost because the particles are stopped. But energy cannot just be destroyed; it must go somewhere. As a result, the energy is converted to mass and hundreds of new particles are created in the collision. These new particles are detected and give physicists clues about what types of particles can exist. Every time a particle accelerator is used, a nuclear reactor is turned on, or a medical PET scan is taken, the conservation of mass-energy is experimentally verified. In fact, the energy taken in or given off by ordinary chemical reactions results from the transformation of energy to mass and mass to energy. In chemical reactions, the mass of the system before the reaction is different from the mass of the system after the reaction. The mass difference is miniscule, but measurable, and is the source of the energy. Because of this fact, every chemical experiment ever done is a validation of the conservation of mass-energy. Out of all the scientifically-sound, repeatable experiments ever performed, a violation of the conservation of mass-energy has never been observed. If the law was broken, and energy was created out of nothing, then the first place it would be observed would be in particle accelerators. Particle accelerators have huge stacks of sensitive detectors that can track the movement of every last bit of mass and energy in the system; electrons, protons, photons, etc. Additionally, the accelerators pump incredible amounts of energy into the particles, so that exotic and rare phenomena are readily observable. If a bit of energy did appear that was unaccounted for, the detectors would see it, but they never have. Beyond overwhelming experimental verification, the law of conservation of mass-energy is required by theory. If energy could pop into existence out of nothing, then in such a big, old universe, energy would eventually pop out of nothing. With the limiting mechanism of conservation out of the way, the energy that pops out of nothing could be as large as infinite. As the age of the universe becomes large, the probability that an infinite energy will pop out of nothing would become 100%. The problem is that an infinite energy (or even a non-infinite one that is large enough) would destroy our universe. The fact that our universe is still around is direct evidence that the law of conservation of mass-energy is fundamental and universal. If this law applied on earth, but not on Alpha Centauri, then infinite energy would pop out of nothing on Alpha Centauri and destroy the universe. The universality of mass-energy conservation is literal and strict. People who believe in free energy machines must also logically believe that the universe does not exist. Proponents of free energy may argue that conservation of mass-energy is usually obeyed, but can be broken in exotic experiments. The center of stars and supernova are far more exotic environments than a tinkerer's basement. Violation of mass-energy conservation would be observed far sooner and far more easily in a star than in an inventor's table-top contraption. And yet, it has never been observed. Free energy can be tempting to people who want something for nothing. If you could build a machine that created energy out of nothing, then you could sell the energy and everyone would get rich without ever doing any work. Free energy machines that seem like they should work are always the product of wishful thinking and sloppy science. If you build a machine and underestimate the amount of mass-energy you have to put into the machine to get it going, and overestimate the amount of mass-energy it will output once going, then your calculations predict that mass-energy has been created out of nothing. But this end result came from poor estimations, and not from ground-breaking science. Most people who "feel" like a certain free energy machine should work simply don't grasp how much mass-energy it takes to get the machine going. For instance, magnetic free energy machines are essentially spinning electromagnetic motors. The machine is plugged into an electrical source, which gets the motor's wheel spinning. The machine is then unplugged and the wheel keeps spinning under its own inertia. Then electrical energy is extracted from the spinning wheel. This energy was not created out of nothing. It was put in the wheel by the original electrical power inputted to the motor. The electric power extracted from the wheel in the end will always be less than the electrical power put into the wheel in the first place. Energy is simply converted from electrical to kinetic (the spinning motion of the wheel is a form of energy), and then back to electrical, while some of the energy is converted to waste heat due to friction. When the inventor of a "free energy" or "over unity" machine claims that his invention really creates energy out of nothing, he is either deluding himself or is outright lying to take advantage of others. The self-delusion usually happens because the inventor does not realize the large amount of external energy he has put into his machine to turn it on, which is more than he could ever get out. A straight-forward measurement of all the energy being inputted to his machine and all the energy being outputted would quickly reveal no actual free energy. But doing real science is hard, so countless "inventors" tinker in their garage and think that "pretty spinning wheel" = "free energy" without doing any actual measurements. For those who do take actual measurements, they think they are always just one step behind from reaching over-unity performance; believing that adding just one more complicated gadget to their machine will put them over the top, when in fact they never actually reach a free-energy result. Consider a canal of water that flows through a turbine. The turbine generates electricity. The electricity is then used to pump all of the water from the bottom of the canal to the top of the canal where it can enter into the turbine again and repeat the cycle. This seems like a closed system that could run forever and continually output electricity; it's free energy! But if you actually do measurements or calculations, you will find that the electrical energy generated by the turbines will never be enough to pump all the water back to the top of the canal. It would require externally provided energy to get the water back to the top and thereby run continuously. But at that point, it's not a a free energy machine. It's just a complicated wheel running on external power. River turbines do extract energy from rivers, but this energy does not come out of nowhere. The river water gained its gravitational potential energy when it was placed at the river head by the evaporation-precipitation process. River water was once ocean water that absorbed sunlight energy from the sun and converted it to gravitational potential energy as it evaporated. The energy outputted by the sun results from mass being converted to energy in its core. The mass of the sun was created by slow accumulation of intergalactic dust that was created by the Big Bang. Because mass-energy cannot be created or destroyed, every bit of mass-energy in the universe can be traced back to its creation in the Big Bang. Real river turbines don't generate energy from nothing. They are simply extracting energy created by the Big Bang and converting it to a useful form. Some people misunderstand vacuum energy and believe it is a form of free energy that can be extracted. An absolute vacuum indeed contains quantum fluctuations, but these do not constitute usable energy. The effects of vacuum energy are already factored into everyday reactions. In the strict sense, you are already using the effects of vacuum energy every time you light a candle or drive your car, but there is still no permanent removal of energy from the vacuum. Every particle is "dressed" or surrounded by a cloud of quantum fluctuations in addition to its regular fields. If you were somehow to remove the cloud, the particle would be left naked and would interact very differently with the world. The mass-energy gained by removing the cloud would cancel the mass-energy lost by changing the way the particle interacts with the world, so in the end mass-energy would still be conserved. You would still end up with the total energy created from nothing equaling zero. For instance, in the Casimir effect, two plates are placed very close together so that the cloud of quantum fluctuations in between the plates is less dense than the cloud surrounding the plates. As a result, the plates attract each other. It would seem that this effect extracts energy from nothing. In reality, the energy that comes out of the system in the form of moving plates comes from the particles in the plates. As their cloud of quantum fluctuations changes, they lose mass. Even quantum fluctuations obey the law of conservation of mass-energy. Historically, free energy machines were called "perpetual motion" machines. This name is confusing because perpetual motion is possible, you just can't extract free energy from an object in perpetual motion. The earth is in perpetual motion as it repeatedly orbits the sun. If we were to build a giant generator and extract a large portion of the energy contained in the earth's orbital motion, it would destroy the orbit and the earth would spiral into the sun. From a socioeconomic perspective, it should also be obvious that free-energy machines don't work. If a free-energy machine actually worked, it would make its inventor instantly rich. If such free-energy machines were possible, high-tech corporations such as Intel or Apple would pursue them because they would literally yield an infinite return on investment. And yet, no big-name technology company sells free-energy machines, or is even researching their possibility. Corporations know that reading up on, researching, or developing free energy machines is a futile waste of time and energy that is better diverted to more productive channels.

context: tag/conservation-of-energy/ question: How do levers create energy if the conservation of energy does not allow energy to be created?

Levers do not create energy. Levers convert a small force applied over a long distance to a large force applied over a small distance. Work is the force times the distance, W = Fd, so the total work done is the same with or without the lever. Look closely at a lever as you use it. The end that lifts the fridge moves a very small distance d as the job is carried out, but the end of the lever that your hand is pushing on moves a large distance as the job is carried out. Because the total work done must be constant, and work is force times distance, the force must go up if the distance goes down. The lever converts the little force of your hand at one end to a large force at the other end; large enough to do a big job. But it does this at the cost of a larger distance. You must therefore push on the one end of the lever for a longer time than you would have to without the lever. According to the work energy theorem, the amount of work you do on a system becomes the energy contained in the system. Because the work is constant with or without the lever, the energy is also constant. A lever therefore does not create energy. The energy inputted to do a certain job is exactly the same with or without the lever. The lever just maximizes efficiency. Human muscles are built for endurance, and not short-term, high-intensity performance. It is easier for humans to carry 10 pounds up 100 steps than to carry 100 pounds up 10 steps, even though the total mechanical energy required for both jobs is exactly the same. Even though the total mechanical energy required is the same, one job is easier because human muscles operate more efficiently at low intensities for a long time. If a person tries to carry 100 pounds up 10 steps, he would waste a lot of energy that would end up as heat because of the inefficiency of human muscle for such a task. The person in this case would have to expend more total energy to get the job done: the energy to lift the 100 pounds up the 10 steps + all the energy that gets wasted to heat because of the muscle's inefficiency in this performance mode. All simple machines – levers, pulleys, ramps, screws, gears, etc – operate on this same principle. They increase efficiency by allowing a high-force/small-distance job to be accomplished by humans who operate best in low-force/large-distance mode. But the total work and total energy expended for a given job is always they same, no matter what machine you use.

context: tag/conservation-of-energy/ question: How does quantum theory allow a rock to turn suddenly into a duck?

Quantum theory does not allow a rock to turn suddenly into a duck. It does not allow any other bizarre transformation to happen either. This idea is a myth perpetuated by people who misunderstand quantum theory. Foundational to quantum theory is the concept of particle uncertainty. It is impossible to know a particle's exact location and exact momentum at the same time. Furthermore, a quantum particle's exact location and exact momentum does not even exist at the same time. The uncertainty in quantum theory is not a result of measurement limitations but is inherent to each quantum object itself. The more you try to trap a single electron and therefore force it to have a more defined position, the less defined becomes its momentum. This uncertainty means that quantum particles can end up in unexpected places. The more unexpected its destination, the less likely it is to end up there, but the probability is never zero. The line or reasoning goes that because a rock is made out of quantum particles (electrons, protons, and neutrons), and because quantum particles can do unexpected things on account of the uncertainty principle, then a rock can do unexpected things like turn suddenly into a duck. According to this line of reasoning, the probability of a rock turning into a duck is very low, so that it almost never happens, but it is physically possible and so it will happen if we wait long enough. This line of reasoning is wrong. It goes wrong in applying the uncertainty principle to large-scale objects. The uncertainty principle applies only to individual quantum objects and small collections of quantum objects. When trillions of quantum objects are involved, the uncertainty goes completely away. The behavior of a trillion atoms is not just the behavior of one atom times a trillion. The reason for this is that the atoms are interacting with each other. As a loose analogy, the behavior of 10 kids at one birthday party is very different from the summed behavior of 10 kids, each alone at 10 different birthday parties. Even the smallest objects visible to human eyes, such as a grain of sand, have trillions upon trillions of atoms. Human-sized objects therefore have no uncertainty and will never do anything bizarre no matter how long you wait. Quantum theory is fundamentally statistical. This means that there is always uncertainty for a single particle, but for a statistically-significant collection of particles, the system predictably obeys the equations of quantum theory. Quantum uncertainty is not a magic ticket that allows you to break all other laws of physics. The conservation laws are fundamental, universal laws of physics that hold everywhere and cannot be broken. The conservation of mass/energy states that matter/energy cannot be created or destroyed but only transformed from one state into another. The conservation of momentum states that the total momentum of a system is constant. Momentum can not be created or destroyed. If a tiny pebble suddenly transformed into a large duck, it would definitely break the law of conservation of mass/energy as matter would have been created out of nothing. If a rock sitting motionless suddenly shot in the air with no external force applied, it would break the law of conservation of momentum as momentum would have been created out of nothing. The conservation laws of physics and the statistical nature of quantum theory keep the universe ticking forward in a very predictable way despite the uncertainty present in single particles. A rock will never turn into a duck no matter how long you wait.

context: tag/conservation-of-energy/ question: Why is mass conserved in chemical reactions?

Mass is not conserved in chemical reactions. The fundamental conservation law of the universe is the conservation of mass-energy. This means that the total mass and energy before a reaction in a closed system equals the total mass and energy after the reaction. According to Einstein's famous equation, E = mc2, mass can be transformed into energy and energy can be transformed into mass. This is not some exotic process, but in fact happens every time there is a reaction. Mass is therefore never conserved because a little of it turns into energy (or a little energy turns into mass) in every reaction. But mass+energy is always conserved. Energy cannot be created out of nothing. It can only be created by destroying the appropriate amount of mass according to E = mc2. Between mass and energy, energy is the more fundamental property. In fact, modern physicists just consider mass an alternate form of energy. For this reason, they don't usually call it the "Law of Conservation of Mass/Energy" but rather call it the "Law of Conservation of Energy" with the implication that this statement includes mass. In nuclear reactions (changes to the nucleus of atoms), there is enough energy released or absorbed that the change in mass is significant and must be accounted for. In contrast, chemical reactions (changes to only the electrons in atoms) release or absorb very little energy compared to nuclear reactions, so the change in mass of the system is often so small that it can be ignored. As a reasonable approximation only, therefore, chemists often speak of the conservation of mass and use it to balance equations. But strictly speaking, the change in mass of the system during a chemical reaction, though small, is never zero. If the change in mass were exactly zero, there would be no where for the energy to come from. Chemists like to speak of "chemical potential energy" and talk as if the energy released in a reaction comes from the potential energy. But "chemical potential energy" is just an old-fashioned term for what we now know is mass. Fundamentally, when chemists say "potential energy" they mean "mass". There is not some bucket of potential energy in an atom from which a reaction can draw. There is just mass. The loss (or gain) of mass during all reactions, whether chemical or nuclear, is very well established and has been confirmed experimentally. There are four general types of basic reactions: Note that when a chemical reaction absorbs energy, and therefore gains mass, it's not like electrons are created. The extra mass is not caused by the appearance of new particles. Rather, the extra mass is held in the system as a whole. Depending on the position and state of particles in a system relative to each other, the system gains or loses mass in addition to the individual masses of the particles. This concept is very similar to the classical concept of potential energy, but we now know that the energy is actually stored as mass. If you measure with very sensitive equipment the sum of the mass of two million hydrogen atoms and one million oxygen atoms that are separated from each other, then measure the mass of one million water molecules, you will find theses masses to be different.

context: tag/conservation-of-momentum/ question: How do tractor beams work?

Up until recently, tractor beams (beams of light that tow objects) existed only in the world of science fiction. While large-scale tractor beams that can tow space ships are still machines of the future, microscopic tractor beams are here today. The idea at first seams to defy physics. Shoot light at an object and the light's momentum should push the object away from the light source according to the law of conservation of momentum; not pull it closer like a tractor dragging its cargo. But what if you managed to create a situation where upon striking the object, the light gains forward momentum instead of losing it. In that case, the law of conservation of momentum tells us that the object will lose momentum in the forward direction, even to the point of going backwards, towards the light source. So there is a way to make a tractor beam without violating any physics, if you can get the light to gain forward momentum upon interacting with the object. You need to the light to slingshot past the object instead of bouncing off of it. The most effective method to accomplish this feat at this point in time is to use solenoidal light beams. A solenoidal shape is like a wire wrapped in circles into a helix that forms a hollow cylinder. But in this case, there are no wires. The light itself, traveling through the air, is formed into this hollow, spiraling shape. San-Hyuk Lee of UC Berkeley and his collaborators have experimentally verified that the solenoidal light shape indeed draws objects – in their case, microscopic glass spheres – towards the light source. They have therefore successfully created a real tractor beam. You can think of the spiral-shaped light beam as a screw that twists the object up the beam. Only microscopic objects have been tractor-beamed so far because light carries so little momentum. In order to tow large objects, you would need very strong light beams. So strong, in fact, that damage of the object may become a problem. Research is ongoing.

context: tag/conservation-of-momentum/ question: If I'm on an elevator that breaks loose and plummets down the shaft, can I avoid harm by jumping at the last second?

First of all, elevators never plummet down their shafts. For the past century, elevators have had a backup break that automatically engages when an elevator starts to fall. If all the cables snapped (highly unlikely), the elevator would only fall a few feet before the safety breaks would activate. The safety breaks are mechanical so that they work even if power is out or no one is around. Secondly, even if a terrorist managed to saw off the huge metal breaks and cut all the cables, the plummeting elevator would land on a cushion of air at the bottom. The elevator shaft traps air much like a giant air bag, which would soften the blow. But the essence of the original question is still interesting. Imagine that you are standing on a high, heavy, outdoor platform (with holes in it so that air resistance is negligible). Suppose the platform's supports break so that you and the platform fall together. Could you minimize the damage of hitting the ground by jumping off the platform at the last second? If we neglect air resistance, the answer is a weak yes. But because you are both in free fall, you would not really be “jumping” off the platform, but would be pushing it away. By the law of conservation of momentum, pushing the platform downwards will give it some of your momentum and slow you down. (Think of a speeding ice skater pushing another stationary ice skater. The speeding skater slows down.) The platform has to be heavy in order to be able to give it much of your momentum. Pushing against a penny won't do much unless you shoot the penny off at high speed using a gun. But pushing away a heavy platform will give it more momentum. The first question to approach is: when is the best time to push off the platform? If you go through the kinematic equations (neglecting air resistance), you can indeed show that your ground impact velocity is minimized by jumping close to the last second. If you jump exactly at the last moment, the platform will not have time to receive your momentum, and hit the ground. So it is best to jump close to the last moment. Simple mathematics show that near impact your impact velocity is just your free-fall-acquired final velocity minus your jump velocity. Your jump velocity is how much upward velocity you gain by pushing away the platform. Assuming the maximum human jump velocity to be 7 mph, if your platform fell from 33 feet (about three floors), you would reduce your impact velocity from 31 mph to 25 mph by pushing the platform down with all your might near the last moment possible. If you fell from 165 feet (about 15 floors), pushing at the right moment would reduce your impact velocity from 72 mph to 65 mph. The bottom line is that “jumping” at the last moment would help, but not much. Of course, the actual damage depend not only on your impact velocity, but also on the solidity of the ground. Cement will do more damage than haystacks. These calculations neglect air resistance, which would also help slow you down. Air resistance would be another reason to stick close to the platform until late in the fall. In practical situations where the ground is soft and the platform is hard, it may be more important to jump clear of the platform early on to avoid its impact shrapnel than to try to give it your momentum.

context: tag/conservation-of-momentum/ question: Why are truck drivers rude?

Truck drivers are professional operators and are therefore less rude on the road in general than your typical driver. Truck drivers are often perceived to be rude for one simple reason: inertia. Inertia (also called "momentum" or "Newton's First Law") means that an object in motion tends to stay in motion and resists change to its motion. Momentum is the product of two things: mass and speed. The higher the mass of an object, the more momentum it has, and the harder it is to stop or turn. Similarly, the faster an object is traveling, the more momentum it has and the harder it is to stop or turn. A bullet shot from a gun can do so much damage because it is traveling very fast despite being so light. Also, a road roller can do so much damage to your foot because it is very massive despite going very slowly. Large trucks are very massive compared to normals cars. Fully loaded, a semi-trailer truck has a mass 20 to 50 times that of a four-wheeled automobile. This fact means that it is 20 to 50 times harder to stop or turn a truck than a car simply because of its momentum. While a truck has a stronger engine, better brakes, and more wheels to help compensate for more inertia, these components are not magic. The bottom line is that a truck takes much longer to get up to speed and to slow down than a car because it is so massive. Also, trucks can not turn or change lanes as quickly as cars, as their high inertia would tip them over. It is simple physics at work and has nothing to do with driver attitudes or styles. When a truck does not suddenly slow down to give your car room to merge onto a highway, it is not because the driver is rude. It is because the truck is so massive that it physically can not suddenly slow down. When a truck behind your car seems to be bearing down and you think he should back off, most likely you slowed down faster than the truck was able. His inertia carried him right onto your tail without the driver being able to do anything about it. If a truck seems to be going exceptionally slow around a sharp curve and is therefore in your way, realize that if he went faster he would likely tip over from his huge inertia. When changing lanes and expecting a truck to yield to you or make space for you, and he doesn't, he most likely can't. The heavier the load in the truck, the more inertia, and the more the truck has to drive like a lumbering beast. According to Report 400 of the National Cooperative Highway Research Program, cars require a minimum of between 450 and 900 feet in order to brake to a stop from a speed of 70 mph on a wet road. In contrast, trucks require a minimum of between 700 and 1400 feet to stop from the same speed under the same conditions. These spreads in values depend mostly on the state of the tires. Therefore, a truck with bad tires needs three times the distance of a car with good tires in order to screech to a stop from highway speed, even with the brakes completely engaged. The best thing to do around trucks is to give them plenty of space, so they can safely turn, speed up, and slow down. Also, when merging to the same lane at the same time, the lighter vehicle should always yield to the more massive vehicle, as the more massive vehicle has more inertia and will have a harder time yielding.

context: tag/conservation-of-momentum/ question: Why doesn't light carry momentum?

Light does carry momentum. Momentum can be thought of as an object's ability to push another object due to its motion. Classically, momentum is defined as the mass of the object times the velocity of the object, p = mv. Since light has no mass, you may be tempted to say that light has no momentum. Additionally, everyday experiences may seem to confirm that light has no momentum (sunlight does not knock over soda bottles like baseballs do). However, light does indeed carry momentum in the form of energy. In fact, for photons (the smallest bits of light), the energy E and momentum p are related by the simple equation E = pc, where c is the speed of light. The momentum that light carries is so small that we don't notice it in everyday life. We do not get knocked over when we turn on light bulbs, and the light from candles does not make the curtains sway. But the momentum of light is large enough to be measurable, and can in fact be used in certain applications. For instance, laser cooling machines shoot a sample from all directions with laser light in order to use the laser light's momentum to slow down the atoms in the sample, thereby cooling it. In optical traps, also called optical tweezers, the momentum of the light is used to trap and manipulate small objects. Solar sails on space probes catch sunlight and use its momentum to propel the probe forward. Interestingly, light always travels at the same speed in vacuum, and can't go any slower in vacuum. Unlike a baseball, light loses momentum by lowering its frequency rather than by lowering its speed. The fact that light carries momentum has profound effects on particle interactions because of the law of conservation of momentum. For instance, if an electron and a positron fly at each other from opposite directions with equal speeds, their total momentum is zero. After the particles annihilate each other and convert their mass totally to energy, they must turn into something that has zero total momentum. A single photon would not do, as it carries momentum. But two photons traveling in opposite directions would add up to zero total momentum (because they are traveling in opposite directions). For this reason, electron-positron annihilation events always create two photons traveling in opposite directions, and not just one. This fact is used in medical positron emission tomography (PET) scans to image human tissue. The patient is given a radioactive drink. When the radioactive chemicals decay in the body, they emit positrons. Each positron annihilates an electron in the patient's body, creating two photons traveling in opposite directions. The machine detects these two photons, and can use their direction and timing to pinpoint where they were created and where the drink is therefore congregating.

context: tag/electricity/ question: Could electronic devices charge themselves without being plugged into an electricity source?

Yes, electronic devices can charge their batteries through various methods without being plugged into a source of electricity. What all the different methods have in common is that they absorb energy that is in some other form (heat, light, vibrations, radio waves, etc.) from the external environment and then convert the energy into electrochemical energy that is stored in the device's batteries. While such methods are scientifically sound and have already been successfully demonstrated, the energy captured from the outside environment is often not enough to be practically useful. Intense research is currently underway to make energy capture more efficient, and breakthroughs are beginning to be achieved in this area. Many phones are already available that offer wireless charging. Let us look at the main types of energy a device can use to charge itself without being plugged in. Solar Energy The oldest energy capture method used on electronic devices is most likely solar energy capture using a solar cell. Small calculators that use solar cells to help power them have been around for decades. Solar cells (photovoltaics) absorb ordinary light and convert it directly to electricity using layers of semiconductors. While photovoltaics are well understood and commercially established at this point, there are several drawbacks in using photovoltaics to recharge a handheld electronic device. The main drawback is that the light-to-electricity conversion process of photovoltaics is inefficient. Recent advances have however been made that boost the efficiency, and intense research is ongoing to continue to increase the efficiency. The other drawback is that for typical lighting levels, light does not contain that much energy to begin with. For traditional solar cells to provide reasonable output power, they have to be large in size, out in the direct sunlight for a long time, and oriented towards the sun. None of these conditions is very practical for a cell phone that you keep in your pocket or your hand most of the day. Vibrational/Kinetic Energy Whenever an object moves or vibrates, it contains kinetic energy. If the object's motion is stopped in the right way, this kinetic energy can be converted into electricity instead of into the usual waste heat. Hybrid cars use this concept to put electricity back into the batteries whenever you step on the brakes. The kinetic energy of the car moving forward gets converted into electrical energy using a generator on the wheels, instead of the energy ending up as waste heat in the brake pads. For a handheld device, the most significant kinetic energy it has access to is the regular bumps and jostles the device experiences as you walk around and carry the device in your pocket. Research is ongoing to make kinetic energy capture practical and efficient. For instance, Zhong Lin Wang and Jinhui Song demonstrated the conversion of vibrational energy to electrical energy using piezoelectric nanowire arrays. Piezoelectric crystals have the interesting property that when they are squeezed, they produce a small amount of electricity. Typically, the amount of energy captured through the piezoelectric effect is too small to power a device, but recent advances in nanoscale structures are boosting their efficiency. Heat Energy The ambient heat in the natural environment can be captured and converted to electricity. There are many ways to do this, but the basic concept is to funnel the random thermal motion of ions or electrons into a more ordered motion of charge, which constitutes an electrical current. This funneling is often accomplished by layering various materials with different thermal and electrical properties. For instance, the researchers Guoan Tai, Zihan Xu, and Jinsong Liu have recently demonstrated the conversion of heat to electricity using the ion layer that forms between silicon and a copper(II) chloride solution. Radio Waves and Electromagnetic Induction All electromagnetic waves carry energy. Typically, the radio waves surrounding us are strong enough to carry a signal (such as a cell phone signal), but too weak to provide significant power to a device. By using more intense radio waves, energy can be beamed wirelessly at significant power levels to a device. Nikola Tesla is famous for pioneering research in wireless power transmission in the 1890's. In such an approach, the ambient radio waves from the rocks, trees, stars, and so forth are not strong enough to provide power. Instead, a dedicated power transmitter is needed to create the intense radio waves, which could be considered a drawback. Furthermore, if a simple tabletop radio transmitter is used, the device to be charged would have to be in the same building as the transmitter in order to efficiently capture the electromagnetic energy. This may not be a severe limitation as wireless signal transmitters such as WIFI routers and cell phone towers are already becoming common enough to provide internet connectivity with few gaps. The wireless power transmission equipment could be built into the existing wireless signal transmission infrastructure. Because wireless power transmitters just need to be connected to a power source and do not need to be connected to the internet, they could even be installed in automobiles, boats, and in remote areas. Note that there is a fundamental difference between radio waves and electromagnetic induction. Radio waves are self-propagating traveling waves in the electromagnetic field. In contrast, induction effects are more localized electromagnetic disturbances that do not wave, but still carry energy. From a technology standpoint, radio wave power transmission and induction power transmission are virtually the same thing. Radio wave/inductive charging methods are already being used on several commercial products, such as Google's Nexus 4 phone and Nokia's Lumia 920 phone.

context: tag/electricity/ question: Does a source of electricity ever run out of electrons?

The answer to this question depends on the situation. We can roughly classify all electrical systems into two categories: static electricity systems and circuit electricity systems. Note that all electrical effects are actually part of one unified set of physical laws. This classification is therefore ultimately arbitrary and over-simplified. However, this classification is sufficient for our current purpose of understanding electric current. A static electricity system involves the flow of electric current as a result of a buildup of electric charge somewhere. Such a system does not involve a closed electrical circuit. Examples of this type of system include lightning and the sparks you get when you rub your feet on a carpet. Electrons naturally repel each other. When a lot of electrons get piled up in one place, they can push on each other so strongly that some of the electrons get pushed right off of the object. They end up getting pushed out through the air, the water, or whatever surrounds the object. We call a collection of moving electrons an electric current, therefore a buildup of charge can drive a current. The electrons simply flow away from the pile and ultimately end up attached to atoms in the environment. In this way, we can have an electric current even if we don't have a complete electrical circuit. In air, an electrical current takes the form of dark discharge, corona discharge, or sparks (depending on if the current is weak, medium strength, or strong, respectively). Note that the name "static electricity" is a poor name since the electric charge is not always stationary in this type of system. More accurate names would be "non-circuit electricity" or "charge buildup electricity." Since charge buildup is the cause of the electric current in static electricity systems, the current will stop flowing once the buildup goes away. As the electrons flow away from the pile, the pile gets smaller. Eventually, the pile of excess electrons is gone (the electrons that are needed to keep the molecules neutral still remain, but they don't do much). Quite literally, electricity stops flowing because the source runs out of excess electrons. This is why lightning bolts and the sparks between statically-charged socks go away quickly. It's not that electrons are destroyed. Rather, they are leaked away to distant points until none remain. In contrast, circuit electricity systems involve the flow of electric current through a closed loop. This current is the result of a charge pump operating somewhere in the loop. This pump is also called a voltage source and can take the form of a battery, a solar cell, a generator, or the cord from a power grid. The pump creates a voltage difference along the circuit which drives charges like electrons through the circuit. The pump can either constantly pump electrons in one direction, which leads to a direct current (DC), or it can periodically switch off the direction in which it is pumping electrons, which leads to an alternating current (AC). For simplicity, let's focus on direct current. As the electrons flow through the circuit, they flow down the potential energy slope that is created by the voltage. Once they reach the pump at the end of the circuit, the low-energy electrons are boosted back up to a high potential energy so that they can start flowing through the circuit again. The situation is a bit like an artificial waterfall in your backyard. Water flows down the waterfall and into a pool because of the natural pull of gravity, just like how electrons flow through the circuit because of the pull of the applied voltage. A water pump then pushes the water in the pool back up to a high energy state at the top of the waterfall, just like how a battery pushes electrons back up to a higher energy state at the beginning of the circuit. The cycle then repeats. Since the pumping of charge is the cause of the electric current in a circuit electricity system, the current will never stop flowing as long as the pump remains on and the circuit remains uninterrupted. Circuits don't create, destroy, use up, or lose electrons. They just carry the electrons around in circles. For this reason, circuit electrical systems can't really run out of electrons. The energy delivered through a circuit is not the result of electrons existing in the circuit. Electrons always exist in the circuit as part of the atoms and molecules that make up the circuit. The electrical energy that is delivered is the result of the electrons moving through the circuit. Turn off the pump (i.e. disconnect the battery), and the electrons stop moving through the circuit. But the electrons don't go away. They are still there as a natural part of the materials in the circuit. As I said before, the categorization of systems into static and circuit systems is somewhat arbitrary and oversimplified. Real electrical systems contain a combination of both effects. For instance, a circuit often contains a capacitor. While the circuit acts overall like a circuit electricity system, the capacitor acts more like a static electricity system. As a result, a capacitor can indeed run out of electrons. As soon as one side of the capacitor is depleted of electrons, the electric current stops flowing through the capacitor. At that moment, the part of the circuit containing the capacitor switches from acting like a circuit electricity system to a static electricity system. This happens in the sense that current is now being stopped by a lack of electrons, and not by the lack of an electron pump or the lack of a complete circuit.

context: tag/electricity/ question: How much money do I save by recharging my cell phone at work?

By consistently recharging your cell phone at work (where your employer pays for electricity) instead of at home (where you pay for electricity), you save less than 50 cents a year. According to Wikipedia, the iPhone 5 has a battery that can hold 0.00545 kiloWatt-hours of energy (about 20 kilojoules). Therefore, if you completely drain this phone's battery every day and completely recharge it everyday, it will consume: 0.00545 kiloWatt-hours x 365 days = 2.0 kiloWatt-hours According to the US Energy Information Administration, residential electrical energy costs about 12 cents per kiloWatt-hour in the United States on average. Therefore, you would pay 24 cents a year to recharge your iPhone 5 if you drained it every day and recharged it every day at home. If you diligently recharged your iPhone 5 at work every single day rather than at home, the most you could ever save is 24 cents a year. Other cell phones have batteries with different capacities, but the difference is not large. No matter what type of cell phone you use, it costs less than 50 cents a year to recharge your phone. Public Domain Image, source: Christopher S. Baird. As should be obvious, this amount of money is so small that forcing yourself to only recharge your phone at work is a waste of time and effort. Feel free to recharge your phone where ever it is the most convenient since it is effectively free. Also, this amount is so small, that you don't need to feel guilty that you are wasting your employer's money by recharging your phone at work. Your employer spends far more money on providing you with well-lit halls than with electricity to charge your phone. If this low amount surprises you, perhaps you had little conception of the small amount of electricity that handheld devices actually use. In comparison to the 2 kWh used by your cell phone every year, a new refrigerator uses about 500 kWh a year. Older, less-efficient refrigerators use two to three times this amount of electricity. At 12 cents per kWh, it costs you about $80 a year to run a new refrigerator. Similarly, an electric clothes dryer uses about 900 kWh per year. A big household appliance tends to use about a hundred to a thousand times more electricity per year than a handheld electronic device. According to data complied by Efficiency Vermont, household items consume the amount of electricity per year shown below (the numbers are averages over typical products and users). Household Appliance Electricity Usage Per Year (kWh) Incandescent Light, 100W, 30 bulbs 3600 Portable Electric Heater 2160 Electric Baseboard Heater, 10 ft 1500 Swimming Pool Pump, 1 HP 1096 Electric Clothes Dryer 900 Freezer, 17 CF, auto-defrost, new 684 Television, 42-inch plasma 588 Refrigerator, 18 CF, new 492 CFL Light, 25W, 30 bulbs 360 Window Air Conditioner 270 Electric Oven 252 Dishwasher, Heat Dry 156 Microwave Oven 132 Clothes Washer 108 Desktop Computer 96 Dishwasher, Air Dry 96 Laptop Computer 12 Cell Phone 2 Looking at this chart, you see that if you really want to save money on your electricity bill, you should turn off unused lights, get energy-efficient light bulbs, avoid portable electric heaters, get rid of the swimming pool, hang your clothes to dry, and stop watching so much television. In contrast, you can use portable electronic devices as much as you want, since the electricity that they consume is a drop in the bucket compared to everything else in your house. Topics: battery, cell phone, charging, efficiency, electricity, energy, kWh, phone, power According to the US Energy Information Administration, residential electrical energy costs about 12 cents per kiloWatt-hour in the United States on average. Therefore, you would pay 24 cents a year to recharge your iPhone 5 if you drained it every day and recharged it every day at home. If you diligently recharged your iPhone 5 at work every single day rather than at home, the most you could ever save is 24 cents a year. Other cell phones have batteries with different capacities, but the difference is not large. No matter what type of cell phone you use, it costs less than 50 cents a year to recharge your phone. As should be obvious, this amount of money is so small that forcing yourself to only recharge your phone at work is a waste of time and effort. Feel free to recharge your phone where ever it is the most convenient since it is effectively free. Also, this amount is so small, that you don't need to feel guilty that you are wasting your employer's money by recharging your phone at work. Your employer spends far more money on providing you with well-lit halls than with electricity to charge your phone. If this low amount surprises you, perhaps you had little conception of the small amount of electricity that handheld devices actually use. In comparison to the 2 kWh used by your cell phone every year, a new refrigerator uses about 500 kWh a year. Older, less-efficient refrigerators use two to three times this amount of electricity. At 12 cents per kWh, it costs you about $80 a year to run a new refrigerator. Similarly, an electric clothes dryer uses about 900 kWh per year. A big household appliance tends to use about a hundred to a thousand times more electricity per year than a handheld electronic device. According to data complied by Efficiency Vermont, household items consume the amount of electricity per year shown below (the numbers are averages over typical products and users). Looking at this chart, you see that if you really want to save money on your electricity bill, you should turn off unused lights, get energy-efficient light bulbs, avoid portable electric heaters, get rid of the swimming pool, hang your clothes to dry, and stop watching so much television. In contrast, you can use portable electronic devices as much as you want, since the electricity that they consume is a drop in the bucket compared to everything else in your house.

context: tag/electricity/ question: What is the speed of electricity?

The speed of electricity really depends on what you mean by the word "electricity". This word is very general and basically means, "all things relating to electric charge". I will assume we are referring to a current of electrical charge traveling through a metal wire, such as through the power cord of a lamp. In the case of electrical currents traveling through metal wires, there are three different velocities present, all of them physically meaningful: In order to understand each of these speeds and why they are all different and yet physically meaningful, we need to understand the basics of electric currents. Electric currents in metal wires are formed by free electrons that are moving. In the context of typical electric currents in metal wires, free electrons can be thought of as little balls bouncing around in the grid of fixed, heavy atoms that make up the metal wire. Electrons are really quantum entities, but the more accurate quantum picture is not necessary in this explanation. (When you add in quantum effects, the individual electron velocity becomes the "Fermi velocity".) The non-free electrons, or valence electrons, are bound too tightly to atoms to contribute to the electric current and so can be ignored in this picture. Each free electron in the metal wire is constantly flying in a straight line under its own momentum, colliding with an atom, changing direction because of the collision, and continuing on in a straight line again until the next collision. If a metal wire is left to itself, the free electrons inside constantly fly about and collide into atoms in a random fashion. Macroscopically, we call the random motion of small particles "heat". The actual speed of an individual electron is the amount of nanometers per second that an electron travels while going in a straight line between collisions. A wire left to itself carries no electric signal, so the individual electron velocity of the randomly moving electrons is just a description of the heat in the wire and not the electric current. Now, if you connect the wire to a battery, you have applied an external electric field to the wire. The electric field points in one direction down the length of the wire. The free electrons in the wire feel a force from this electric field and speed up in the direction of the field (in the opposite direction, actually, because electrons are negatively charged). The electrons continue to collide with atoms, which still causes them to bounce all around in different directions. But on top of this random thermal motion, they now have a net ordered movement in the direction opposite of the electric field. The electric current in the wire consists of the ordered portion of the electrons' motion, whereas the random portion of the motion still just constitutes the heat in the wire. An applied electric field (such as from connecting a battery) therefore causes an electric current to flow down the wire. The average speed at which the electrons move down a wire is what we call the "drift velocity". Even though the electrons are, on average, drifting down the wire at the drift velocity, this does not mean that the effects of the electrons' motion travels at this velocity. Electrons are not really solid balls. They do not interact with each other by literally knocking into each other's surfaces. Rather, electrons interact through the electromagnetic field. The closer two electrons get to each other, the stronger they repel each other through their electromagnetic fields. The interesting thing is that when an electron moves, its field moves with it, so that the electron can push another electron farther down the wire through its field long before physically reaching the same location in space as this electron. As a result, the electromagnetic effects can travel down a metal wire much faster than any individual electron can. These "effects" are fluctuations in the electromagnetic field as it couples to the electrons and propagates down the wire. Since energy and information are carried by fluctuations in the electromagnetic field, energy and information also travel much faster down an electrical wire than any individual electron. The speed at which electromagnetic effects travel down a wire is called the "signal velocity", "the wave velocity", or "the group velocity". Note that some books insinuate that the signal velocity describes a purely electromagnetic wave effect. This insinuation can be misleading. If the signal traveling down an electric cable was an isolated electromagnetic wave, then the signal would travel at the speed of light in vacuum c. But it does not. Rather, the signal traveling down an electric cable involves an interaction of both the electromagnetic field fluctuations (the wave) and the electrons. For this reason, the signal velocity is much faster than the electron drift velocity but is slower than the speed of light in vacuum. Generally, the signal velocity is somewhat close to the speed of light in vacuum. Note that the "signal velocity" discussed here describes the physical speed of electromagnetic effects traveling down a wire. In contrast, engineers often use the phrase "signal speed" in a non-scientific way when they really mean "bit rate". While the bit rate of a digital signal traveling through a network does depend on the physical signal velocity in the wires, it also depends on how well the computers in the network can route the signals through the network. Consider this analogy. A long line of people is waiting to enter a restaurant. Each person fidgets nervously about in their spot in line. The person at the end of the line grows impatient and shoves the person in front of him. In turn, when each person in the line receives a shove from the person behind him, he shoves the person in front of him. The shove will therefore be passed along from person to person, forwards through the line. The shove will reach the restaurant doors long before the last person in line personally makes it to the doors. In this analogy, the people represent the electrons, their arms represent the electromagnetic field, and the shove represents a fluctuation or wave in the electromagnetic field. The speed at which each person fidgets represents the individual electron velocity, the speed at which each person individually progresses through the line represents the electron drift velocity, and the speed at which the shove travels through the line represents the signal velocity. Based on this simple analogy, we would expect the signal velocity to be very fast, the individual velocity to be somewhat fast, and the drift velocity to be slow. (Note that in physics there is also another relevant speed in this context called the "phase velocity". The phase velocity is more of a mathematical tool than a physical reality, so I do not think it is worth discussing here). The individual electron velocity in a metal wire is typically millions of kilometers per hour. In contrast, the drift velocity is typically only a few meters per hour while the signal velocity is a hundred million to a billion kilometers per hour. In general, the signal velocity is somewhat close to the speed of light in vacuum, the individual electron speed is about 100 times slower than the signal velocity, and the electron drift speed is as slow as a snail.

context: tag/electricity/ question: Why can't lightning strike the same place twice?

Lightning does strike the same place twice. Lighting tends to strike the highest and pointiest object, because it is an electrical current being attracted to the easiest path. If your church steeple is on a hill, it is going to be struck many times. The Empire State Building in New York City gets struck by lightning 100 times a year according to the National Weather Service. One spot on the Catatumbo River in Venezuela receives thousands of lightning strikes a night (many of them cloud to cloud) as researched by Nelson Falcon of the University of Carabobo.

context: tag/electricity/ question: Why do car tires protect you from lightning strikes?

Car tires do not protect you from lightning strikes. Although the rubber in a tire acts as an insulator at low voltages, the voltage in a lighting bolt is far too high to be stopped by tires or air. No matter how thick your tires are, they don't stop lightning according to physicist Martin Uman in his book "All About Lightning". Dr. Uman states that inside a car can be a safe place to wait out a lighting storm, but it's not because any materials are blocking the lightning. Rather, if the car is struck by lightning, its metal frame redirects the electrical current around the sides of the car and into the ground without touching the interior contents. The ability of a hollow conducting object to protect its interior from electrical fields and currents is one of the fundamental principles of electromagnetics. Such an object is called a Faraday cage. For this reason, riding around in a convertible, on a motorbike or on a bicycle during a lightning storm is a bad idea, no matter what kind of tires it has. If you are in a fully-enclosed metal vehicle, you should be protected from the lighting by the Faraday-cage effect. However, you should still park the vehicle and wait out the storm since a lightning strike can blow out your tires or blow out your vehicle's electronic control circuits, potentially causing you to crash if you are driving. If you are riding in a convertible or roofless vehicle, on a motorbike, or on a bicycle and are caught in a lightning storm, you should quickly seek out the nearest shelter. If a building, tunnel, or other large sheltering structure is not readily available, seek out a low point in the terrain away from water, away from isolated trees, and away from other tall structures (e.g. windmills, power-line towers).

context: tag/electricity/ question: Why does lightning push electricity through air, but common batteries do not?

Actually, a common low-voltage battery does push a small electrical current through the air. But this current is so small that in most cases it can be ignored. Nevertheless, an unconnected battery does slowly leak electricity through the air and will eventually end up uncharged if left sitting long enough (internal chemical effects also contribute to this loss of charge). People often say that air is an insulator until you reach the breakdown voltage, at which point it becomes a conductor. This statement is over-simplified. Air is a good insulator at low voltages, but not a perfect insulator. In fact, air is a lot more complex than most people realize. In high school you may have learned that the electrical current I through an object and the electrical resistance R of an object are related through Ohm's law, I = V/R. This equation means that for a given applied voltage V, an object with high resistance will only allow a very small electrical current through. Naively applying Ohm's law to air, you may think that since air does not seem to carry any electrical current, it must have infinite resistance. But... the problem is that air does not obey Ohm's law. In fact, nothing obeys Ohm's law perfectly. Ohm's law is not a fundamental law of nature. Rather, it is a quick and dirty approximate equation that only works for certain materials within certain ranges of voltage. In science, we describe a material that roughly obeys Ohm's law at the voltages of interest as "ohmic" and all other materials and voltages as "non-ohmic". If you plot the current-voltage curve of a material, the ohmic range (if present at all) is the range where the curve is a straight line. Some materials are ohmic over a broad range of voltages. Other materials are ohmic over only a narrow range of voltages. Still other materials are not ohmic at all. Air is not ohmic at all. This means that there is no linear relationship between the electrical voltage applied to air and the resulting electrical current that travels through it. The behavior of electricity in air is far more complex. In order to create an electrical current, electrons must be ripped off of the air molecules so that they are free to move and form a current. Also, the ions left behind by the electrons are positively charged and can form part of the current. Ripping electrons off of air molecules is hard to do, but there are different mechanisms that can do this, leading to the complex electrical nature of air. As the diagram above shows, there are three main types of electrical currents that flow through air: 1. Dark discharge is the electrical current that flows through air before reaching the first breakdown point of air. As its name suggests, dark discharge currents do not emit light and are invisible to the human eye (except at the tips of sharp objects where a dark discharge can form a visible corona discharge). In dark discharge, the voltage is not high enough to allow any mechanism to rip electrons off of air molecules. As a result, dark discharge relies on background radiation or normal thermal collisions to rip the electrons free. Once free, the electrons accelerate in the electric field of the applied voltage and form a current. At the low-voltage end of the dark discharge curve, all that is happening is that the electrons freed by background ionization are moving in the electric field. At the high-voltage end of the dark discharge curve, the recently-freed electrons accelerate enough to collide into other air molecules and rip more electrons off in the process. This process repeats, leading to avalanche ionization and a swift increase in electrical current at these higher voltages. Even with avalanche ionization, there is not enough energy present in the dark discharge setting to keep the ionization completely self-sustaining. Low-voltage dark discharge is the type of electrical current created by common batteries sitting in air unattached to anything. Note that the electrical current created in air by common batteries is about a trillion times weaker than lightning. The textbook Industrial Plasma Engineering by J. Reece Roth states, "The dark discharge receives its name from the fact that, with the exception of the more energetic corona discharges, it does not emit enough light to be seen by a human observer. The number density of excited species is so small in this regime that what little excitation light is emitted is not visible...radiation, from cosmic rays, radioactive minerals in the surroundings, or other sources, is capable of producing a constant and measurable degree of ionization in air at atmospheric pressure." 2. Glow discharge is the electrical current that flows through air after reaching the first breakdown point of air. Glow discharge is the type of current that makes a Neon sign glow and makes the gas inside a fluorescent light bulb emit UV rays (which are converted to visible light by the fluorescent coating). Once the first breakdown point of air is reached, the electrical current through the air becomes self-sustaining and does not rely on background ionization. A glow discharge is self-sustaining because the ions left behind by the freed electrons have enough energy when they hit the cathode (the negative end of the voltage source) to knock free new electrons, which then start the avalanche ionization process anew. 3. Arc discharge is the electrical current that flows in air after reaching the second breakdown point of air. Arc discharge is the type of current found in lightning bolts and is typically loud, bright, and hot. At the second breakdown point of air, the cathode becomes hot enough to directly eject electrons into the air, which then rip off more electrons in the normal avalanche pattern. The ions left behind are accelerated by the electric field of the applied voltage until they smash into the cathode, heat it up even more, and cause it to emit even more electrons. Arcing is a run-away process where the high electrical current causes high heat, which causes even more electrical current in a feedback loop. The run-away process continues and the current gets stronger until the applied voltage source is depleted of its charge. In summary, common low-voltage batteries do drive electrical currents through air, but these currents are very weak and dark, they rely on background ionization, and they behave very differently from lightning.

context: tag/electricity/ question: Why is a 12-volt household battery harmless, but the shock from a 12-volt car battery will kill you?

The shock from a car battery will not kill you. In fact, under normal conditions, a 12-volt car battery will usually not even shock you. Car batteries are not harmless, though. There are many ways you can be injured by car batteries: There are enough dangers present that it is a good idea to be cautious around car batteries and follow the maintenance instructions in the car's manual, even though electrocution by car battery is not going to happen. Also, this question tacitly contains a common misconception about high voltage being independently dangerous. The ability of electricity to damage biological tissue is dependent on both current and voltage. A very high voltage source providing a very low current does not carry enough energy to harm you. For example, a tabletop Van de Graaff generator (those charged metal balls you see at the science museum) can generate voltages up to 100,000 volts. And yet, children regularly enjoy shocks and hair-raising experiences from these generators without being harmed. In contrast, a high current (even at relativity low voltage) contains enough energy to hurt you. A better indicator of the danger of an electricity source is therefore how much current it gets running through your body, which depends partly on voltage but also depends on resistance and the amount of current the source can provide. Voltage is a measure of the electrical potential difference between two points and is similar to the amount a river drops as it flows from point A to point B. In contrast to voltage, current measures the total charge flowing past a point on its path per second. Current is similar to how much water in a river is flowing past a point along the river per second. A few drops of water running down a steep hill carries far less energy than a mighty river flowing down a gentle slope. In reality, both voltage and current play a role. A mighty river plummeting over a cliff carries more energy than a mighty river coasting down a gentle slope. Let us now apply these concepts to the car battery, which is a bit more complicated than it first appears. Car batteries can provide high currents. And yet they won't electrocute you. The key to this curiosity is that it is current running through your body that does the damage, and not the maximum current that a battery can provide. They are different. How much current actually ends up running through an object depends on three things: 1) the electrical resistance of the object, 2) the voltage applied, and 3) the amount of current the source can provide. For a human touching a car battery, the skin has a very high resistance, leading to low current; and the battery has a low voltage, leading to low current. Even though a car battery can provide high current if connected properly, your body does not draw this high current. Voltage does play a role in that it helps limit the total current in your body (along with your body's resistance). The handbook Auto Electricity, Electronics, Computers states that the "Battery or charging system voltage will normally not produce enough current flow to cause a severe electric shock."

context: tag/electricity/ question: Why were electrons chosen to be negatively charged? Wouldn't it make more sense to call electrons positively charged because when they move they make electricity?

First of all, "electricity" does not mean "moving electric charge". If "electricity" meant "moving electric charge" then "static electricity" would mean "stationary moving electric charge," which is nonsense. "Electricity" is a general term describing all effects connected to electric charges. When people use the word "electricity" to describe what is going on in an electric wire, they usually mean "electric current". Secondly, electric current is not just a bunch of moving electric charges. Electric current is the net movement of electric charges and the movement of electric field disturbances connected to the charges. That is why electrical signals in a wire, such as telephone calls, travel on the order of the speed of light while the electrons themselves travel much slower. If electric currents were nothing more than moving electrons, then it would take 6 months for your voice on the telephone call to reach the other side of town. The objects in a material that are contributing the most to an electric current are called the charge carriers. Third, electric charge carriers aren't always electrons and they aren't always negative. In fact, in the natural world, the charge carriers are usually not just electrons. In animals, the electric charge carriers are primarily sodium, potassium, calcium, and magnesium ions, all positively charged. They are the things that are moving when a nerve passes an electric signal. In the ionosphere, the charge carriers are oxygen, hydrogen, and helium ions along with electrons. In a gas discharge sign, the electrical current is due to ions and electrons. In lightning, it's both ionized air molecules and electrons that are moving. The solar wind is in fact a blast of electrical current from the sun comprised of protons and electrons. In semiconductors, like those used in computer chips, the charge carriers are holes and electrons. In the ocean, it's the movement of salt ions, and not electrons, that sustains an electrical current. In metals, like in household electrical wire, the charge carriers are indeed just electrons. Renaming electrons as positively charged would require renaming almost all the other charge carriers as negatively charged. Such an action would probably make things less simple, not more. Benjamin Franklin was the one who first chose to call electrons negative and protons positive. According to the textbook "Physics for Scientists and Engineers" by Raymond A. Serway, Franklin identified electric charge carriers after a series of rubbing experiments. Without much knowledge of the underlying physics, he simply made a choice that made sense to him by calling electrons negative.

context: tag/electromagnetism/ question: How can it be so hard to drag rubber across smooth glass if friction is caused by surface roughness?

Friction is not caused mainly by surface roughness. According to the book "Sliding Friction" by Bo N. J. Persson, friction is caused by weak electromagnetic forces between molecules, such as experienced in hydrogen bonds and Van der Waal bonds. If two electrically neutral, non-polar atoms are brought close enough together, they induce electric dipole moments in each other which then attract. These weak bonds cause the two objects with surfaces touching to be attracted, making it hard to slide one object along the other. Additionally, the sliding motion can generate vibrations (phonons) in the objects touching, which takes energy away from the sliding motion and therefore slows down the objects. Because friction is mainly caused by electrical attraction and not surface roughness, smooth objects can still have relatively high friction coefficients. In fact, nanoscopically smooth objects experience very high friction (as long as their chemical structure promotes significant electromagnetic forces) because they have much more of their surface in contact. Surface roughness can become significant if the roughness is very high. The best way to lower the friction of a surface is to combat both. Smooth down the surface to lower surface roughness, and then sprinkle on a lubricant which gets in the way of the chemical bonding, such as oil or graphite.

context: tag/electromagnetism/ question: How do magnets heal?

Magnets have no healing properties. Magnetic Resonance Imaging (MRI) employs very strong magnetic fields, far stronger than a household magnet can produce, and yet MRI's have no direct effect on the health of the patient (an MRI may have an indirect effect as a diagnostic tool). The iron in our blood is in far too low of a concentration to be affected by the weak magnetic fields of household magnets. Furthermore, the iron atoms in our blood are bound in heme molecules. The chemical bonds that hold the iron atom in the heme molecule interfere with the state of its electrons, causing the iron atom to lose its usual ferromagnetic properties. The lack of healing properties for magnets has been established multiple times through controlled experiments. For instance, M.S. Cepeda and colleagues found that static magnetic fields had no effect on pain levels. A study performed by M.H. Pittler reviewing multiple experiments also confirmed the inability of magnets to heal. It's true that all materials (including oxygen and frogs) have some kind of magnetic response. The problem is that it takes very intense magnetic fields for this response to be significant. Household magnets, and even MRI's, are too weak to evoke any lasting effect in humans. The intense magnetic fields of an MRI are used to temporarily reorient the magnetic dipoles of protons inside the body for imaging purposes. After alignment, the protons quickly become unaligned due to natural thermal and biological motion, and the rate that they become unaligned can be used to image different tissues. If a human were exposed to static magnetic fields that were strong enough to have a significant effect, the result would be harmful, not healing. Blood would build up in places where it is not supposed to, or even explode out of its vessels. Note that we are talking about static fields here, such as produced by permanent magnets. Changing magnetic fields can have significant effects, but once the field starts changing it's not really just a magnetic field anymore. It becomes an electromagnetic field. Electromagnetic fields only have an effect on biological tissue if they are extremely intense (such as sunlight giving you a sunburn or a laser cutting your eye), or if they are high frequency (such as X-rays giving you cancer). In any case, handheld permanent magnets create only static magnetic fields and not electromagnetic fields. The FDA considers any magnet that is sold with the claim that it has healing properties to be fraud, and litigates against such products.