From my reading i am seeing the answer But according to special relativity, light ALWAYS travels with the light speed c, and is NEVER at rest and from this statement i am supposed to gather there is no rest mass. But logic tells me just because light is never at rest, doesn't necessarily mean it would have no mass if it was at rest. to get its regular mass i used m=hf/c^2 its seems impossible to solve for m0 by using because its undefined. So am i missing something? or is a more correct description, the rest mass of a photon simply unknowable at this time?