# Thread: The Speed of Light and the Laws of Physics

1. If the speed of light were to change would the laws of physics (like in general relativity or QED) change as well? If so, why? I understand that that E=mc^2 (for stationary objects) and this works for fusion, fission and presumably anti-matter reactions, but why c^2 and not some other constant?

2.

3. Originally Posted by Golkarian
If the speed of light were to change would the laws of physics (like in general relativity or QED) change as well? If so, why? I understand that that E=mc^2 (for stationary objects) and this works for fusion, fission and presumably anti-matter reactions, but why c^2 and not some other constant?
c is not just the speed of light.

If you look closely at the foundations of special relativity, you will find that what is actually shown in the usual approach to the derivation of the Lorentz transformations is that if there is any phenomena that is propagated at some fixed speed, say X, in all inertial reference frames then one arrives at the Lorentz transformations with X in the role of c. What this shows is there can be only speed that is the same in all inertial reference frames. One then simply notes the experimental fact that light propagates at the same speed in all inertial reference frames (or notes that this is implied by Maxwell's equations of classical electrodynamics) to arrive at special relativity in its usual form. That is, at least in part, why any particle of zero rest mass propagates at c. Since photons have zero rest mass, light propagates at c.

E-mc^2 is an expression of the fact that mass and energy are really the same thing. This is true no matter what form of energy one considers. As to why it is c, the only answer is that science does not attempt to explayin "why" but only "how", and it appears that nature obeys E-mc^2.

http://en.wikipedia.org/wiki/Mass%E2...gy_equivalence

4. And what about QED? From the limited reading I've done (by Feynman) it seems that electrons are held inside the atom by the exchange of photons. Am I correct in this? And would the speed of light affect this process?

Also are you saying that c could not possibly be different?

5. Originally Posted by Golkarian
And what about QED? From the limited reading I've done (by Feynman) it seems that electrons are held inside the atom by the exchange of photons. Am I correct in this? And would the speed of light affect this process?

Also are you saying that c could not possibly be different?
Electrons are bound to the nucleus by the electromagnetic force, which, in QED, is the result of the exchange of virtual photons. In QED photons, on average, travel at c. The speed of light is part of QED, and thus it is not clear what you mean when you as whether the speed of light affects the process.

QED is the result of the need for a quantum theory that is macroscopically compatible wth special relativity. The result is a theory with a bit of weirdness, as is common with quantum theories and that means that photons need not always travel at c, but they do on the average and they do so in a manner that is consistent with special relativity at macroscopic scales. So the simple answer is that photons travel at c, but in reality QED allows all sorts of weird possibilities in potential "histories", but all that goes away in Feynman's integration over histories.

I said that in order to be consistent there can be only one speed at which some phenomena can propagate in all inertial reference frames. It happens that light propagates at the same speed in all reference frames and therefore the only such speed is the speed of light. Anything else that propagates at a fixed speed in all reference frames therefore must also propagate at the speed of light. This is required by the general principles of special relativity -- those principle being that light propagates at c in all inertial reference frames and that the laws of physics are the same in all inertial reference frames.

Now, the value of c, about 3x10^8 m/s, is an experimentally determined value and the theory says nothing about that particular value. It could be anything. But it isn't. It is what it is.

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