Thanks, Peter. So am I right then in the following thought: If a massive photon's antiparticle WERE distinct that would actually be an argument against massive photons, using the following reasoning. Massless photons don't interact since they are pure waves and pass through each other so we see no interactions. If photons had distinct antiparticles, given the number of photons zipping around there WOULD be interactions (annihilations) but we never see any such thing.Not necessarily. I didn't mean to imply that a massive photon's antiparticle must be distinct, just that the antiparticle's mass has to be the same as the particle's mass even if the antiparticle is distinct.