## Relativity and Speed of Light

So relativity is defined about the speed of light. The speed of light is what it is because the photon is massless. What if, sometime in the future, it's discovered that the photon has a small mass so that it travels a little less than the speed of light. What does that do to relativity?
 Recognitions: Gold Member Do you hear yourself? You just asked what happens if light travels at less than the speed of light. Actually, light travels at the speed light travels at. As to what happens if photons have mass, that's irrelevant since they don't.
 Recognitions: Science Advisor One important piece of evidence that light has no mass is that the speed is constant irrespective of inertial frame (starting with Michelson-Morley). Particles with mass have different speeds when measured in different frames.

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## Relativity and Speed of Light

 Quote by PlanckShift So relativity is defined about the speed of light.
Better: relativity is about a universal speed limit. Particles with mass can never reach that speed, and particles without mass must always travel at that speed.

 What if, sometime in the future, it's discovered that the photon has a small mass so that it travels a little less than the speed of light. What does that do to relativity?
Nothing. The universal speed limit remains unchanged. If photons have a small mass, then they must travel at slightly less than that speed. We went through something similar with neutrinos about ten years ago, when they were discovered to have mass instead of being massless.
 One of the germinating ideas behind de Broglie's theory is that photons might have a very small rest mass. Obviously his theory accommodates that possibility. Another eminent physicist, Proca, developed a slight modification of the Maxwell equations or even a Lagrangian which bears his name. The homogeneous Maxwell equations are not changed. More detail on that can be found in Jackson's "Classical Electrodynamics". On page 5, Jackson even considers how a massive photon would affect the Inverse Square Law of electrostatics.
 Photons have a rest mass in superconductors; about 10^-11 of the proton. See Wiki's Photon page, et al. I can't find any mention of it, but this would seem to mean the photon must go
 On the wiki page, there is one line stating that photons in superconductor have rest mass. However, there are no citations supporting that statment, and as far as I know, there are no transparent superconductors. All the wiki page can give is an upper limit for any mass, meaning no mass has been detected to the present limit of measurement.
 The Anderson-Englert-Brout-Higgs-Guralnik-Hagen-Kibble mechanism, isn't it?

 Tags photon, relativity, speed of light