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One of the axioms of special relativity is that the speed of a massless "particle" such as light is c in all reference frames. In this comment from another question, The reference frame of $c$, someone stated that there is no reference frame that moves at speed $c$ due to this axiom. This is confusing. Is there no reference frame for massless particles such as light? Are reference frames and mass linked?

Would time freeze if you could travel at the speed of light? kind of answers the question.

Qmechanic
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Varun Pattalachinti
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  • The Lorentz factor $\gamma=\left(1-\frac{v^2}{c^2}\right)^{-1/2}$ is undefined when $v=c$. – Charlie Mar 18 '20 at 23:22
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    Why is it confusing? How could light possibly move at $c$ in its own reference frame? – G. Smith Mar 18 '20 at 23:23
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    Does this answer your question? Does a photon in vacuum have a rest frame? – sammy gerbil Mar 18 '20 at 23:36
  • Once there is a second photon it is possible to have a reference frame as the speed of the photon is less than c? That's interesting! Would that mean nothing is moving at the speed of light as there are certainly two photons that are moving non co-linear in our universe? – Varun Pattalachinti Mar 18 '20 at 23:43
  • Nobody said anything about photons’ speed being less than $c$. – G. Smith Mar 18 '20 at 23:46
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    Once there is a second photon it is possible to have a reference frame as the speed of the photon is less than c? No. – G. Smith Mar 18 '20 at 23:47
  • What is your candidate for the reference frame of a light beam? – WillO Mar 18 '20 at 23:50
  • I mean if what safesphere is saying is true, I can have a reference frame when I have two non co linear photons. If I can have a reference frame that means that the photons are moving less than c. Also they have a rest mass once there are two of them? So if that is true they can't be moving at the speed of light. I mean this is all axiom stuff so .... – Varun Pattalachinti Mar 18 '20 at 23:52
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    If I can have a reference frame that means that the photons are moving less than c. NO! – G. Smith Mar 18 '20 at 23:54
  • Also they have a rest mass once there are two of them? Neither has a rest mass. The system of two photons has a rest mass. Rest masses are not additive. – G. Smith Mar 18 '20 at 23:55
  • You have lost track of the axiom that you started with and keep contradicting it. Photons have speed $c$ in all inertial reference frames. And it isn’t just an axiom, it is an observational fact. – G. Smith Mar 18 '20 at 23:57
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    Safesphere’s comment was true but vague and confusing. In any reference frame two nonparallel photons both travel at $c$ and both are massless, but their system has mass. They do not have just one reference frame. However, there is a particular one where their total momentum is zero, and he may have been referring to that one. – G. Smith Mar 19 '20 at 00:16

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