[jasonkolb]

Isn't the speed at which photons travel, by definition, the speed of light?

And as for the "c" in e=mc^2, doesn't this suddenly make "c" an unknown constant? Doesn't the fact that "c" changes suddenly change the values of the other variables in that equation as well? That seems pretty fundamental to me...

[zipdog]

As I understand it, Einstein's work rests on there being a fundamental maximum 'speed' and it seemed to him as though the speed of photons was that limit, so 'speed of light' became synonymous with this maximum. But it doesn't necessarily have to be so.

So if there's something faster, it changes our understanding of photons but not the existence of this fundamental maximum speed.

As you note, our efforts to measure c may have been off due to measuring the wrong thing, but I don't know the ramifications of a small % change in c.

(I'm not a physicist)

[kaybe]

Yes, it is -the- basic assumptions for special relativity. And no, it doesn't just change our understanding of photons, it changes everything, since it's all connected. All theories I've looked at so far have (at least in higher versions) incorporate relativitity.

(Another basic assumption, this time for general relativity, is the equality of inertial and gravitational mass, which is not a self-evident thing. However, so far no difference has been found. (see http://en.wikipedia.org/wiki/E%C3%B6tv%C3%B6s_experiment)

[kevinpet]

I think what he's getting at is that there's this value "c" that's really important to physics appearing in equations like e=mc^2 and determining the absolute upper bound on speed, and by the way, we used to assume that photons traveled at c, rather than their actual rate of 99.9975% of c.

I don't know whether changing c by this amount would break many experimental results. Adding a rest mass to photos sounds potentially revolutionary.

[iand]

Einstein based his theory on the maximum speed at which information can propogate. That's always been assumed to be the speed of light (photons). It may be possible that there is something else that can propogate information faster (e.g. neutrinos). There would still be an upper speed limit, but it wouldn't be the one we thought it was :)

[dfranke]

Isn't the speed at which photons travel, by definition, the speed of light?

Photons speed up and slow down routinely, depending on what medium they're traveling through. c, as it is used in the equations of relativity, is currently believed to be equal to the speed of light in a vacuum. But, with my limited knowledge of GR, my understanding is that gaika is correct and that the rest of the theory can still stand if this equality is broken.

[doctoboggan]

This isn't technically correct. Photons always travel the same speed but in certain materials they are absorbed and emitted by atoms, causing their apparent speed to slow down.

A photon's instantaneous speed is always the speed of light.

[felipemnoa]

>>Photons speed up and slow down routinely, depending on what medium they're traveling through

Do they really? As far as I know their speed is always constant in any medium. They just seem to slow down because they get absorbed and re-transmited. That is where the lost of velocity comes from. When traveling between one atom and another, which is a vacuum, they are always traveling at the speed of light.