Hi, hopefully this isn't a dumb question. I've read essentially that in the center of mass/momentum frame an object has invariant mass, and that the system's total mass will be composed of the constituent particles' masses and any other kinetic and potential energies within the system. I also think I know that these various forms of rest mass are interconvertible with kinetic photon energies that can be radiated away from the system should the particles be annihilated. I guess what I'm trying to ask is, what's the mechanism that determines a particle having rest mass and, hence, occupying space and time sublight. Anyway, I hope this is phrased correctly and is factually meaningful. Thanks for any answers! C.