I was in doubt, so I went to wikipedia. There it says "the photon has zero rest mass", but on the side description it says the mass is $<1.10^{-18} \:\mathrm{eV}/c^2$. So is the mass of the photon really zero or do we just consider it to be zero because it's negligible? More generally, are there massless particles at all?

  • $\begingroup$ arxiv.org/abs/gr-qc/0509110 $\endgroup$ – Count Iblis Jul 9 '16 at 19:58
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    $\begingroup$ Photon has zero rest mass, but actually it's never at rest (relative with any frame). $\endgroup$ – ABcDexter Jul 11 '16 at 9:20
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    $\begingroup$ Essentially a duplicate of Does a gluon have mass or not? $\endgroup$ – ACuriousMind Jul 18 '16 at 11:35
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    $\begingroup$ @ACuriousMind Only if you explain why are these the same. It is probably trivial for you, but not for the OP. This explanation doesn't exist in the related question, and not in its answers, too. This explanation would be off-topic in the related question. $\endgroup$ – peterh - Reinstate Monica Jul 20 '16 at 0:34
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    $\begingroup$ @peterh: Both questions ask whether a massless particle is "truly" massless based on the experimental upper bound on Wikipedia. Since the answer does not actually depend on whether the particle in question is a gluon or a photon, the questions are essentially duplicate, since that is the main difference between them. $\endgroup$ – ACuriousMind Jul 20 '16 at 13:18

Here is a quick & simple answer until the professionals arrive. In the Standard Model, it is zero. This $< 1\cdot 10^{-18} \frac{\mathrm{eV}}{c^2}$ is an experimental upper limit (i.e. if it has a rest mass, because of physics beyond the Standard Model, it must be smaller than this value).

This value is very small, compared to the estimated rest mass of the neutrinos (which is of the order of some tenths of an $\mathrm{eV}$).


We can't measure to infinite precision; so even if a particle had in fact zero mass we couldn't experimentally measure it to the infinite precision needed to justify this; which is why certain amount of judgement is called for, and that judgement is made in the context of a theoretical framework.

The second point to make is that all particles with zero rest mass travel at the speed of light and they have momentum due to this motion.

As one answer has pointed out already such particles are gauge bosons which mediate the weak, strong & EM forces. For the EM force, this is the photon.

  • $\begingroup$ It would be nice to have an explanation for the down-vote; not enough detail, too little detail or wrong detail... $\endgroup$ – Mozibur Ullah Jul 10 '16 at 4:01
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    $\begingroup$ or bad english, silly fonts or just plain bad spelling? $\endgroup$ – Mozibur Ullah Jul 10 '16 at 4:21
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    $\begingroup$ Perhaps you got the downvote for the 2nd paragraph. Relativistic mass is generally deprecated in modern treatments. It can be a useful concept, if you know what you're doing, as the Usenet Physics FAQ article What is Relativistic Mass? explains, but it has lead to so many misconceptions and errors that most physics educators of the last several decades prefer to avoid it. $\endgroup$ – PM 2Ring Jul 10 '16 at 7:19
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    $\begingroup$ OTOH, stating that a photon has non-zero momentum is not controversial. IMHO, the notion of zero rest mass is slightly paradoxical, since a particle with zero rest mass does not have a rest frame in which we can directly measure this zero mass. :) But of course, we can "measure" its rest mass indirectly via the energy–momentum relation E² = (pc)² + (m₀c²)². $\endgroup$ – PM 2Ring Jul 10 '16 at 7:45
  • $\begingroup$ @PM2Ring: good point - thanks, I was thinking of momentum but I wrote mass; thats what time does to you, I mean me... $\endgroup$ – Mozibur Ullah Jul 10 '16 at 7:48

There are indeed massless particles.

As of 2015 there were two known massless particles (both gauge bosons): the photon (carrier of electromagnetism) and the gluon (carrier of the strong force). It should be noted, however, that gluons are never observed as free particles, since they are confined within hadrons.

Gravitons (if discovered) would be another massless particle.

Of course, it must be kept in mind that nothing can be measured to infinite precision. Because of this, we will never measure a photon's rest mass and find it to be zero. As our measurements get better and better, it will get closer to zero, but it will never quite get there.

Interestingly, according to this website, scientists are able to look at Coulomb's Law and other experimental results and place upper bounds on what the photon's rest mass can be measured as. The best upper bound to date is $1.07×10^{−27}$ atomic mass units. The equivalent of this is what you saw on the Wikipedia sidebar.

There's more information at this website about massless particles.

  • $\begingroup$ It is arguable that a photon does not have mass since it has some properties of wavelengths and can be sucked in by black holes $\endgroup$ – nelomad Jul 13 '16 at 19:12
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    $\begingroup$ @Adamawesome4, I assume you mean it is arguable that photons do have mass. If photons had mass they would not travel at the speed of light since no object with mass can reach the speed of light according to general relativity. $\endgroup$ – heather Jul 13 '16 at 19:30
  • $\begingroup$ Wouldn't they be considered wavelengths then? $\endgroup$ – nelomad Jul 13 '16 at 19:40
  • $\begingroup$ I'm not quite sure I understand what you mean. Light has a dual nature: it is both wave and particle. If you could elaborate on your objection, I'd appreciate it. $\endgroup$ – heather Jul 13 '16 at 19:42
  • $\begingroup$ I actually know very little about the tendencies of light, when I was trying to answer the question I came across this And just wondering, where did you start learning about physics? I want to learn more, but I don't know where to begin. $\endgroup$ – nelomad Jul 13 '16 at 19:51

The upper limit you mention reflects the hypothesis that photons in vacuum could have some tiny rest mass.

But it seems to be more important that c is the velocity of massless particles such as photons in vacuum. However, there is no real vacuum in the universe: Not only that even in outer space you will always find some interstellar atoms. But also, the gravity field of each single mass particle is interacting within the whole universe, even if this happens in extremely small dimensions.

By consequence, strictly speaking, massless photons are a concept which does not exist in the universe.

  • $\begingroup$ But how could massive particles travel at the speed of light ? $\endgroup$ – Yves Daoust Jul 11 '16 at 9:52
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    $\begingroup$ @Yves Daoust: As only massless particles travel at speed of light, photons with mass do not. The massless photon traveling at speed of light is an idealized concept, even if reality is not far from it. $\endgroup$ – Moonraker Jul 11 '16 at 9:59

The least you may require of something to be able to call it a particle is that it can be established where it is when, that it interacts, however weakly, with the objects in its environment. Since according to relativity theory a massless particle must move at the speed of light and at that speed there passes no time at all, the particle -its state- is completely frozen in time so it cannot interact, express its existence, so no, there are no massless particles. Photons and gravitons, however useful in our models and equations are fictitious particles.

  • $\begingroup$ On my opinion, it is not so simple. For example, the speed of the electromagnetic waves is also $c$. Maybe they also don't interact? $\endgroup$ – peterh - Reinstate Monica Jul 18 '16 at 12:55
  • $\begingroup$ There is a contradiction in (the present formulation / interpretation of) SR: while on the one hand it states that the photon transmission is instantaneous -from the point of view of the photon; on the other hand it favors the observer's point of view according to which the transmission takes time -a contradiction which originates in the idea that the universe grows older, as a whole, in time, as the photon travels: in the idea that it lives in a time realm not of its own making. $\endgroup$ – Anton Jul 20 '16 at 2:26
  • $\begingroup$ You are using a terminology which is known in details only for you. $\endgroup$ – peterh - Reinstate Monica Jul 20 '16 at 7:19

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