Because GR only applies to objects with rest mass. Photons have no rest mass. They only impart momentum when striking an object; they have have no real mass. By definition a photon cannot have mass. If it did, you could slow it down below its c velocity in a vacuum (a big no-no). Also, there is no time that a photon is accelerating (a GR requirement). It's at c from its creation until you cause it to interact with some medium. That's not to say that you can't modify light in a vacuum. Gravitational blueshifting is an example of wavelength shifting. But it still continues to travel at c.Originally posted by Jack
OK, now for my next stupid question-
If time slows down to a stop as you approach the speed of light then how can light travel?