How does it make sense for a particle to have zero mass? For example, the photon has exactly zero mass. If it has no mass, then it’s a particle of what? If you demand that mass is equal to stuff, then you have to conclude that a massless particle literally has nothing to it. Instead of thinking about a particle’s mass as how much stuff is crammed into a supertiny little ball, just think of it as a label that we apply to an infinitesimal quantum object.
