Does the uncertainty principle apply to the speed of light? Do photons merely average the speed of light on large scales, but fluctuate on small scales? Or if you know the exact speed of a photon, does it mean that you can't ever know its position? It seems to me that if a photon did, for an instant, travel at slower than the speed of light, then it would need a mass, but then again I guess there would also be uncertainty in the mass of a photon within a certain time range? Could a photon, for an instant, travel faster than light? Would it then have negative mass?