What is the mass of the photon?
). It is certainly too small to have any practical significance for the definition of the metre in the foreseeable future but it cannot be shown to be exactly zero even though currently accepted theories indicate that it is. If it wasn’t zero, the speed of light would not be constant but from a theoretical point of view we would then take c to be the upper limit of the speed of light in vacuum so that we can continue to ask if c is constant.
