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...
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)
(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)
I don't know whether changing c by this amount would break many experimental results. Adding a rest mass to photos sounds potentially revolutionary.
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.
A photon's instantaneous speed is always the speed of light.
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.