## The speed of light and the mass of a photon

Quote by juanrga
c is not measured, its value in the SI is defined exactly.
 Quote by D H The standards organization didn't make c a defined value until 1983, almost 100 years after the Michelson Morley experiment. A century of ever refined measurements of c (and they were measuring c back then) demonstrated an ever improving agreement between theory and reality.
Right and the SI was established only 50 years ago. I would have been more accurate and emphasize that c is not measured today in the SI and that its value is taken to be, by definition, c = 299 792 458 ms−1.

 Quote by D H The title alone of Einstein's 1905 paper says otherwise: "On the Electrodynamics of Moving Bodies." Look inside and you will find that the paper is indeed very much about EM waves. The second half of the paper is a relativistic formulation of Maxwell's equations.
Well, of course Einstein was talking about light. He says that pretty plainly and it's obviously the most straightforward analogy. However, the universe does not just preserve c as the speed of light, but rather information in general, correct? That's why I gave the example of a group of blind train riders that can only detect gravity. If relativity really defines the true nature of spacetime, than surely the type of energy wouldn't matter, no?

I guess it would be easier if I just rephrase this into a question: Is there anything in Einstein's paper or subsequent interpretations that says the universal c comes from the fact that EM radiation has a speed limit rather than information in general has a speed limit? It seems like all of the arguments could be made by postulating another type of energy transfer medium with no mass.

Mentor
 Quote by Mr Boom However, the universe does not just preserve c as the speed of light, but rather information in general, correct?
Some physicists prefer to hold that a constant speed of light (better said: speed of any massless particle) as axiomatic, others prefer a geometric set of axioms, yet others that information has a speed limit. There is no one axiomatization of physics. (There isn't even a consistent, unified description of all of physics.)

Which interpretation is "best" is getting into metaphysics rather than physics. They are either indistinguishable (interpretations of quantum mechanics) or incommensurate (quantum vs. relativistic physics). I'd rather not have this thread become yet another discussion of which is the right way to go. Such discussions on interpretations of physics have a marked tendency to devolve into a shouting match and then get locked.

 Recognitions: Science Advisor Staff Emeritus Re the question about blind men and gravity waves: It is predicted by GR that gravity waves travel at 'c' in a vacuum, just as light waves do. Of course the experimental evidence for this is not as good as the experimental evidence that light waves travel at 'c', because we have yet to detect our first gravity wave. While it is not about gravity waves, experiments such as the detection of muons at the Earth's surface shows that relativity isn't just an electromagnetic phenomenon. If muons didn't respect the laws of special relativity, they would decay before they reached the Earth's surface.

 Why use a gut feeling when you can calculate it? Use mc^2 = 10^{-17} eV and E = 1 eV and see what you get.
My thanks to jtbell for this timely rebuke. What you get is the amazing result that v differs from c by only 1 part in 10-34. Even over a distance of 700km this would slow a photon down by totally negliible amount. Not nearly enough to explain the OPERA results.

This link gives some interesting information about experiments to measure the mass of a photon. Apparently the most sensitive experiments involve measuring the deviations from Coulombs inverse square law. It also transpires that if photons had significant mass, charge would not be conserved either. (Thanks again to Simon for pointing me in the right direction)

On balance, I think we had better stick with a zero rest mass for now and look for another explanation of the OPERA results.

 Quote by pervect It is predicted by GR that gravity waves travel at 'c' in a vacuum, just as light waves do.
When you say travel at c do you mean locally or in the large?

 Quote by pervect Of course the experimental evidence for this is not as good as the experimental evidence that light waves travel at 'c', because we have yet to detect our first gravity wave.
What do you mean by 'not as good', I thought there was no evidence whatsoever.

Recognitions:
 Quote by JollyOlly Of course, if c was slightly larger than vl, photons would have a small mass. Does this have consequences? It would make sense of the recent suggestion that neutrinos can travel faster than light.
Photon mass limits are in http://arxiv.org/abs/0809.1003

Photon mass is a quantum concept. The corresponding classical concept is "wavelength dispersion" in which different wavelengths or frequencies of light travel at different speeds.

Vanadium 50 says that the photon mass limits are tight enough that if the OPERA neutrino result were true, then Lorentz invariance would probably be violated.

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 Quote by JollyOlly I agree that one of Einsteins postulates is that the speed of light is the same for all observers but what is the evidence for this? A postulate is not evidence. If c (the constant in Einstein's equations) was just a fraction of a % larger than the speed of light - would it make any significant difference to our GPS devices or predictions of the orbit of Mercury for example? I think not. So I repeat, what experiments have been carried out to check this important equivalence between c and the speed of light?
Einstein's postulation of a constant speed of light was based on Maxwell's 1860's derivation of the speed of light as :
$$c= \sqrt {\frac 1 {\mu_0 \epsilon_0}}$$

This was known, in the day, as Maxwell's conundrum and was the root of the great schism in physics which lasted from publication of Maxwell's work to publication of Einstein's Theory of Relativtiy.

 Tags light, mass, photon, speed