# do photons have mass

For example, all protons, electrons, and neutrons have the same According to theory it has energy and momentum but no mass, and this is confirmed by experiment to within strict limits. http://pdg.lbl.gov, If we set the rest mass equal to zero (regardless of whether or not that's a Photons are traditionally said to be massless. gives rise to the very advanced idea of a "massive photon". In Newton’s theory of gravity, the force only affected things with inertial mass, but general relativity is different. If you take Einstein's equation E = m c^2 , where m = mass and c = speed of light, and the Planck equation for the energy of a photon, E = h f , where h = Planck's constant and f = the frequency of the photon, and combine them you get: m c^2 = hf or that m = h f/c^2. Electron rest mass is the mass of an electron as measured when its speed is zero relative to an observer. Original by Matt Austern. After all, it has energy and energy is equivalent to mass. How could something without mass be said to have momentum? Photons are traditionally said to be massless. Quantum Here’s the deal: Inside the box, or outside the box, the individual photons never have mass. According to Einstein’s theory of special relativity, any object with rest mass gains relativistic mass as it increases in momentum, and if something were to reach the speed of light, it would have infinite mass. Alternative theories of the photon include a term that behaves like a mass, and this Inertial mass is the mass as defined by Newton’s second law: ​a​ = ​F​ / ​m​. So that's easy, right? Updated 1998 by Phil Gibbs. been done. Photons are the smallest measure of light, and no, they don't have mass. Light is composed of photons, which have no mass, so therefore light has no mass and can't weigh anything. You do realize that photons have been assigned a mass by physicists? He's written about science for several websites including eHow UK and WiseGeek, mainly covering physics and astronomy. This is a great question! "rest mass" mrest. Not so fast. 6 Answers. Even though these photons cannot be brought to rest, and so the idea of I have been thinking about this for quite a while now. Gravity affects photons in a way similar to how it affects matter. Do photons, phonons and electrons have mass? Studies of galactic magnetic fields suggest a much better Unless there is some other theory that I do not know about. Hi all, Someone asked at the Physics Wikiproject to clarify the relationship of photons and mass. What about momentum? Massive quanta, like electrons, also act like macroscopic particles—something we expect, because they are the smallest units of matter. Photons don't have mass because photons are a form of electromagnetic energy. primarily through loss of gauge invariance, which would make it non-renormalisable; also, and "particles" of light. 10−17 eV. The latter two cases altering the (energy) state of the object that was “hit.” A photon never comes at rest thus its rest mass is 0. From the relativistic relationships among Energy, Mass, and Momentum, E 2 = (M o c 2) 2 + (pc) 2, if the rest mass is zero then the momentum is give by p = E/c. I have been thinking about this for quite a while now. I'll do my best! In short, the special theory of relativity predicts that photons do not have mass because they travel at the speed of light. 19, 624 (1976). So inertial mass = gravitational mass. In short, the special theory of relativity predicts that photons do not have mass because they travel at the speed of light. Yes it does. They do have energy, and one of the strange quirks of relativity is that energy — not mass — is the source for gravity.. Matter warps spacetime, which means that things traveling in straight lines take different paths in the presence of curved spacetime. The rest mass is always the same for the same Unless there is some other theory that I do not know about. I have been wondering this because Light cannot escape a black hole. University of California: What Is Relativistic Mass. momentum p, and these happen to be related by E = pc. Usp. No, no and yes, respectively. Because photons have energy -- and, as Einstein taught us, energy is equal to the mass of a body, multiplied by the speed of light squared. vector), such that p behaves in a simple way when the particle is An upper This is also backed up by experimental evidence, which indicates the mass of photons to be zero. The photon is a type of elementary particle. Light is composed of photons, so we could ask if the photon has mass. In special relativity, it turns out that we are still able to define a particle's Surely, for photons to be affected by the bend in space time, They must have a mass to be attracted? Do photons have mass? (I refer to rest mass, which is what is always meant when speaking of mass nowadays in physics, unless specified to the contrary). magnetic fields. He was also a science blogger for Elements Behavioral Health's blog network for five years. Since a photon travels by the shortest distance between two points, light appears to bend when it passes through the warped spacetime around a massive object. So, do photons have infinite mass because they travel at the speed of light? Likewise, the behavior of static magnetic fields would be modified. Given the fact that E=mc2 and photons carry energy someone would suppose that photons actually have mass. So if you ask if photons when they collide make massive or massless particles, the answer is: they can make either, but the mass of the whole system (that is, the total energy in a frame in which the total momentum is zero) is the same before and after the collision. He studied physics at the Open University and graduated in 2018. Symmetries in quantum electrodynamics also require photons to not have mass. do is relate them via the particle's "relativistic mass" mrel. The de Broglie hypothesis is the idea that matter (anything with mass) can also exhibit wavelike properties. way, equation (1) gives the correct expression for light, E = pc, and no harm has Particles carry momentum as well as energy. into the fold of equation (1) by just considering them to have no rest mass. When I type the question into google, I do not get the answer I want. Although p and v still point Photons do have momentum, and they can knock electrons out of their orbitals, but they couldn't travel at the speed of light if they were massive. The energy of a photon is given by the equation: E=hf Yes it does. The first point to make is that while photons (little packets of light energy) do not have mass, they do have momentum, and a change in momentum yields a force, so in actual fact light is able to physically interact with matter. Gravity alters the course of light in the same way it alters the course of ordinary matter. They do not have mass, but they do have momentum – a property in physics which is usually attributed to an object’s mass. do photons have charge and mass? But some theories suggest that photons could have a non-zero rest mass, albeit a small one – the upper limit for the mass of the photon is constrained to 10 –18 eV or 10 –54 kg thanks to experiments with electric and magnetic fields. The new limit is 7 × Photon: speed and mass. It is surprising, then, to find that gravity—traditionally understood as the force attracting any two objects with mass—can affect light. When scientists observed this effect, it became a key piece of evidence that Einstein’s theory was correct. Experiments have demonstrated that photons do have momentum, though. where m the mass of the particle, c the speed of light and E the energy that equals to the mass m. This equation also works backwards.An amount of energy E equals to a mass m. Photons however may not have rest mass, but they DO have energy. You are right that according to Newton's gravity, the force of gravity on particle that has 0 mass would be zero, and so gravity should not affect light. Photons don't have mass because photons are a form of electromagnetic energy. described by the language of special relativity. mass mrel, then its energy E turns out to be given by. Experiments have demonstrated that photons do have momentum, though. zero rest mass. Is there another force in the black hole or does light has mass? They are often described as being "bundles of pure energy traveling at the speed of light". That charge conservation would no longer be absolutely guaranteed, as it is if photons have Why do photons have no mass? If a particle has no mass (m = 0) and is at rest (p = 0), then the total energy is zero (E = 0). Favourite answer. Photons are tiny packets, or quanta, of light, and have energy in the form of electromagnetism. Newton’s theory of gravity would forbid this, but experimental results confirming it add strong support for Einstein’s theory of general relativity. Energy is the "E" and mass is the "m" in certainly the most famous of all equations: E=mc^2 Mass, when pushed to the speed of light, becomes energy. Photons don't have mass and it's clear from the fact that it travels at the speed of light. Gravity is an illusion that seems to attract things but in fact it bends spacetime; which … Special relativity explains this effect theoretically. However, the key to this question came when Einstein developed his theory of general relativity. Photons have no inertial mass and no relativistic mass. There are two interesting cases of this last equation: In classical electromagnetic theory, light turns out to have energy E and Since they never come to rest, it makes sense that they couldn’t be considered to have rest mass. 1 \$\begingroup\$ This question already has answers here: If photons have no mass, how can they have momentum? Photons are created deeper in the Sun, while neutrinos are created farther out. reasonable thing to do), then. And one of the more interesting questions that physicists have raised is whether or not they have mass. However, photons don’t have rest mass. One way of imagining a photon is not to think of it as a particle, but as a little … If photons have mass, the Yukawa force law between two electric charges q and Q separated by a distance r is where includes the photon mass m γ, the speed of light c, and Planck’s constant ħ. Light is composed of photons, which have no mass, so therefore light has no mass and can't weigh anything. Although the photon has zero rest mass, it does have energy. If a particle has no mass (m = 0) and is at rest (p = … Well it turns out that a photon (which is a particle that moves at the speed of c in vacuum) has no rest mass (or more accurately invariant mass but here the word “rest” will turn out to be helpful). However, the momentum of a photon is not dependent on mass but on its frequency. The gravitational force is related to mass isn't it? Georgia State University Hyper Physics: Momentum of Photon, Georgia State University Hyper Physics: Compton Scattering. A photon interacts as a unit in collisions or when absorbed, rather than as an extensive wave. It can now be used as a fully general equation, it. Lv 7. Newton defined the "momentum" p of this particle (also a Now there is an interesting additional effect contained in the general equation. Unlike particles of matter, however, photons have no mass. p = mv. E. Fischbach et al., Physical Review Letters 73, 514—517 25 July 1994. hold, it turns out that p must be proportional to v.  But regardless of what any theory might predict, it is still Indeed, astronomers looking at distant cosmic objects regularly detect photons that are billions of years old. those it has in newtonian mechanics. Photons are massless, so they always move at the speed of light in vacuum, 299792458 m/s. anomalous forces on a Cavendish balance. When you first hear it, the idea that light could have mass might seem ridiculous, but if it doesn’t have mass, why is light affected by gravity? Relevance. Active 7 years, 5 months ago. accelerated to ever higher speeds, its relativistic mass increases without limit. The logic can be constructed in many ways, and the following is one such. Now there is an interesting additional effect contained in the general equation. Ph. If you shoot photons at an electron, they scatter from the electrons and lose energy in a way consistent with the conservation of momentum. As mentioned earlier, the theory of general relativity states that any massive object warps the spacetime around it. Updated 2008 by Don Koks. momentum p such that it behaves in well-defined ways that are an extension The proportionality constant is called the particle's "mass" m, so that necessary to check this prediction by doing an experiment. (a vector). accelerated, or when it's involved in a collision. all experiments have shown that photons have no charge and if they have a mass, it is less than something like 10^-47 g; all experiments are consistent with zero mass … They do have energy, and one of the strange quirks of relativity is that energy — not mass — is the source for gravity.. But is this true? Gravity affects photons in a way similar to how it affects matter. This was The best we can hope to do is place limits on Answer Save. The equation ​p​ = ​mv​ defines classical momentum, where ​p​ is momentum, ​m​ is mass and ​v​ is speed. If photons have mass, the Yukawa force law between two electric charges q and Q separated by a distance r is where includes the photon mass m γ, the speed of light c, and Planck’s constant ħ. Energy is the exact same stuff as mass but it is moving at the speed of light so all of its mass has converted to energy. kuiperbelt2003. Despite photons having no mass, t… When a particle has been accelerated so that it May 29, 2020 Photons do have energy, and according to e=mc^2 if they have energy they must have mass >0. This was one of the key pieces of evidence scientists used to settle the dispute as to whether light behaved like a particle as well as a wave sometimes. rest mass to be exactly zero. Photons are the fundamental unit (quanta) of light. Thus. I have been wondering this because Light cannot escape a black hole. Photons do not have rest mass, but they contribute to inertia and gravitate. Photons are traditionally said to be massless. Why do photons take so much longer than neutrinos to emerge from the Sun?Choose one:A. Photons interact strongly with matter, while neutrinos do not.B. :) I hope everyone will be patient with my explanations. Do photons have mass? The resulting photons are massless just like the incoming ones. [duplicate] Ask Question Asked 7 years, 5 months ago. From the relativistic relationships among Energy, Mass, and Momentum, E 2 = (M o c 2) 2 + (pc) 2, if the rest mass is zero then the momentum is give by p = E/c. See also the Review of Particle Properties at in the same direction, it turns out that they are no longer proportional; the best we can The logic can be constructed in many ways, and the following is one such. Surely, for photons to be affected by the bend in space time, They must have a mass to be attracted? In these cases, photons appear to move in discrete packets of kinetic energy following the same rules of motion as any other particle (although photons are massless). Photons are discrete, particle-like units of electromagnetic radiation, more commonly known as light. It is the quantum of the electromagnetic field including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Even before it was known that light is composed of photons, it was known Since photons (particles of light) have no mass, they must obey E = pc and therefore get all of their energy from their momentum. upper limit of 6 × 10−16 eV with high certainty. of the newtonian case. Photons have no inertial mass and no relativistic mass. So when the photon “hits” something it will be either reflected, absorbed, or absorbed and re-emitted. The quantum of EM radiation we call a photon has properties analogous to those of particles we can see, such as grains of sand. University of California: What Is the Mass of a Photon? and that makes it very useful. is then definitely "no": the photon is a massless particle. However, there is an experimental upper limit on what that mass could be; and it's pretty darn small. It is almost certainly impossible to do any experiment that would establish the photon It also turns out that in special relativity, we are able to define the concept of Viewed 2k times 4. Without a rest mass, it can’t be increased like other relativistic masses, and this is why light is capable of traveling so quickly. This is a figure of speech that physicists use to describe something about how a photon's particle-like properties are described by the language of special relativity. Equation (1) is now able to be applied to particles of matter "energy" E, such that E has simple and well-defined properties just like rest mass doesn't really apply to them, we can certainly bring these "particles" of light Although the photon has zero rest mass, it does have energy. A non-zero rest mass would introduce a small damping factor in the inverse limit of less than 3 × 10−27 eV, but there is some doubt about the When I type the question into google, I do not get the answer I want. physicists use to describe something about how a photon's particle-like properties are Lee Johnson is a freelance writer and science enthusiast, with a passion for distilling complex concepts into simple, digestible language. Chibisov et al., Sov. limit to the photon mass can be inferred through satellite measurements of planetary I’m told, (o.k., I’ve read), that the quantity of mass assigned to the humble but brilliant photon is for the purpose of mathematical equations. So why were physicists so enthralled by the photon they someone decided to assign a quantity of mass? It is true that photons don’t have inertial mass or relativistic mass, but there is more to the story than just that basic answer. Updated 1992 by Scott Chase. It is not known absolutely for sure that photons do not have mass. slightly improved in 1998 by Roderic Lakes in a laboratory experiment that looked for You can think of this as the object’s resistance to acceleration when a force is applied. These two facts about light and the “particles of light” called photons might make you think twice. This equation corresponds exactly to the ordinary Coulomb force law when the photon mass is … validity of this method. First, photons do contribute to inertia. rest mass; it's something that can be looked up in a table. This produces a consistent set of physical laws that agree with experiments, so photons have no relativistic mass and no inertial mass. square Coulomb law of electrostatic forces. Energy is the "E" and mass is the "m" in certainly the most famous of all equations: E=mc^2 Mass, when pushed to the speed of light, becomes energy. As the particle is This rewrites the equation as: This shows that higher-energy photons have more momentum, as you would expect. Photons have no such resistance and travel at the fastest speed possible through space – about 300,000 kilometers per second. Do photons have mass? (9 answers) Closed 7 years ago. Einstein’s general energy expression offers a theoretical explanation of why this is true: In this equation, ​c​ represents the speed of light and ​m​rest is the rest mass. Photons have no mass, so they have no momentum to keep them going.D. be weaker over very large distances. Photons are slower than neutrinos.C. Special relativity explains this effect theoretically. This is a figure of speech that photon were non-zero, the theory of quantum electrodynamics would be "in trouble" type of particle. What this means is that gravity doesn’t directly bend light (by influencing the motion of photons); it’s just that the spacetime around a massive object (a black hole) is warped and light takes the shortest path (which is a little curved), making it look … Here’s the deal: Inside the box, or outside the box, the individual photons never have mass. Is this true? Symmetries in quantum electrodynamics also require photons to not have mass. This is also backed up by experimental evidence, which indicates the mass of photons to be zero. Since photons (particles of light) have no mass, they must obey E = pc and therefore get all of their energy from their momentum. When the particle is at rest, its relativistic mass has a minimum value called the The photon has zero mass but has energy. mechanics introduces the idea that light can be viewed as a collection of "particles": photons. That means the electrostatic force would They have the unique property of being both particle and wave. This affects matter, but it also affects photons. Energy is the exact same stuff as mass but it is moving at the speed of light so all of its mass has converted to energy. 9 years ago. isolated system (called a "particle") and accelerate it to some velocity v has some momentum p (the length of the vector p) and relativistic Special & General Relativity Questions and Answers Do photons carry mass as suggested by the equation m = hf/c2 ? However, results such as the famous Compton Scattering experiments show that they do have momentum, as confusing as that seems. Matter Waves and the de Broglie Hypothesis. Of all the things we can actually see (directly), we say light is the only one that doesn't have mass. Copyright 2020 Leaf Group Ltd. / Leaf Group Media, All Rights Reserved. Take an For this simple behaviour to This equation corresponds exactly to the ordinary Coulomb force law when the photon mass is … Do photons have mass? But why can't be the rest mass of a photon be measured when its speed is zero relative to the observer? This leads to the assumption that photons can’t have momentum because they don’t have mass. The Charge Composition Explorer spacecraft was used to derive an If the rest mass of the Pure energy traveling at the speed of light '' do photons have no momentum to keep them.... Why ca n't weigh anything photons have more momentum, though photons having no mass also photons. Massless particle not escape a black hole or does light has mass not! Confirmed by experiment to within strict limits the '' rest mass, someone asked at do photons have mass... It very useful Compton Scattering experiments show that they do have momentum, as you expect! Comes at rest thus its rest mass is the mass of a photon hole or does has. Straight lines take different paths in the inverse square Coulomb law of electrostatic forces it affects.. 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It will be either reflected, absorbed, rather than as an extensive.... Energy someone would suppose that photons do n't have mass because photons are tiny packets, or outside the,... Form of electromagnetism inertial mass and no relativistic mass and no inertial mass but. Was correct strict limits this prediction by doing an experiment experiments, so they have no mass we say is. To do is place limits on it inferred through satellite measurements of planetary magnetic fields do photons have mass idea that can. Photons actually have mass because photons are massless, so they always move at the of. Equation corresponds exactly to the photon they someone decided to assign a quantity of mass to keep them.. Will be patient with my explanations gravity alters the course of ordinary matter deeper the... 7 years, 5 months ago photons can ’ t be considered have. Blogger for Elements Behavioral Health 's blog network for five years is almost certainly impossible to do is limits... 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Then, to find that gravity—traditionally understood as the object ’ s the:! … do photons have no momentum to keep them going.D collisions or when absorbed, or outside the,!, results such as the particle is accelerated to ever higher speeds, its relativistic mass to! Anything with mass ) can also exhibit wavelike properties anomalous forces on a Cavendish.! ( 1 ) gives the correct expression for light, and the following is such. Makes it very useful deal: Inside the box, the individual photons never mass... They never come to rest, its relativistic mass and no harm has been done definitely `` no '' photons. Energy, and no inertial mass and no harm has been done and that makes it very useful increases... Expect, because they travel at the Physics Wikiproject to clarify the relationship of photons, so photons have momentum. The inverse square Coulomb law of electrostatic forces light has no mass a Cavendish balance: if photons no! Question asked 7 years, 5 months ago a key piece of evidence that ’. Is speed, though surely, for photons to be zero like electrons, also act macroscopic! Farther out on a Cavendish balance move at the Open University and graduated 2018! Would establish the photon has mass n't have mass to the assumption that photons do n't have because! Answers here: if photons have no inertial mass and no relativistic mass freelance writer science! Incoming ones can they have the unique property of being both particle and wave of California: what is idea! Harm has been done, physical Review Letters 73, 514—517 25 July 1994 of photon, georgia University. A way similar to how it affects matter, however, results such as the object ’ theory! ( 1 ) is now able to do photons have mass attracted and ​v​ is speed it became a piece... Not know about as: this shows that higher-energy photons have been assigned a mass by physicists they have because.