The speed of light and the mass of a photon

juanrga

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.

Mr Boom

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.

D H

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.

pervect

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.

JollyOlly

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.

Passionflower

When you say travel at c do you mean locally or in the large?

What do you mean by 'not as good', I thought there was no evidence whatsoever.

atyy

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.

Integral

Einstein's postulation of a constant speed of light was based on Maxwell's 1860's derivation of the speed of light as :
[tex] c= \sqrt {\frac 1 {\mu_0 \epsilon_0}} [/tex]

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.