Is it possible for a photon to be at rest? I know it doesn't really make sense if looking at the photon from the wave point of view, but is there any law of physics which prohibits a photon from stopping completely? Thanks.
 A: The photon is a massless particle. Massless particles move at the speed of light. The special theory of relativity postulates that the speed of light is independent of the frame of reference and the maximal possible speed at which a particle can travel. If there existed a reference in which the photon  rested, it would be possible to take a lorentz boost into that frame and so the speed of light at which that massless particle travels would depend on the choice of the frame of reference, which is a contradiction to the Einstein postulates of the special relativity. Therefore massless particles like the photon do not have a rest frame.
A: Those laws of physics would be Maxwell's Equations. I won't go into too much detail but from those equations you can get a wave equation for light. The speed of the wave is determined by two fundamental constants, $\epsilon_{0}$ and $\mu_{0}$. If the speed of light was variable in anyway then those constants would also be variable. No experiment has ever shown these constants to be different, therefore the speed of light must of constant. Since we know that light is composed of photons then photons must move at light speed.
