 Finally, although mass and energy seem to “disappear” and “appear” respectively when we focus on the individual constituents of the physical system containing the incoming electron-positron pair and the outgoing photons, the mass and energy of the entire system remains the same throughout the interaction. Before the collision, the rest-mass of the system is simply the sum of the rest-masses of the electron and positron plus the mass-equivalent of the total kinetic energy of the particles. Consequently, the entire system (if we draw the boundary of the system around the reactants and products—which is, of course, a spatial and temporal boundary), has a non-zero rest-mass prior to the collision. However, after the collision, the system, which now consists of two photons moving in non-parallel directions, also has a non-zero rest-mass (for a detailed discussion concerning the rest-mass of systems of photons, see Taylor and Wheeler, 1992, p. 232). 