While a photon's rest mass = 0, it's mass at c must be proportional to it's energy level. A gamma ray photon of sufficient energy can split into an electron and positron, both of which have defined masses and,of course, the process is reversable. What's really intriguing is to think about the size/length of a single medium wave photon in the broadcast band of say 180 meters wavelength. Remember ALL electromagnetic radiation is made up of photons, not just those frequencies in the visible light spectrum. While these light photons are conveniently tiny especially for the one by one two slit interference pattern displays, no physicist I've ever talked to has thought seriously about conceptualizing low energy, long wavelength photons. Is a single broadcast band photon 180 meters long and if not why?
