[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.