It seems kind of weird to me that particles convert between each other. As I understand it, if a positron and an electron "collide", two gamma ray photons are created, and the positron and electron are gone. 1st - What is meant by a collision? The positron and electron are each described completely by a wavefunction -- what mathematical condition must these wavefunctions meet to constitute a "collision"? The classical view would be two marbles that have overlapping trajectories and roll into each other. The probability of a collision is partly a function of the marbles' cross-sectional area. Obviously, when the "marbles" are wavefunctions, this analogy doesn't translate well at all. 2nd - Does the "conversion process" take place within Planck time? If so, then we can't know whether the conversion occupies finite time or is instantaneous. However, if the conversion has a duration longer than Planck time, we should be able to model this process, right? 3rd - How is it that rest mass just disappears? As a thought experiment, if we had a highly sensitive "test particle" near the collision site that measured the gravitational force vector due to the two particles' masses, at the time of the particles' collision, the gravity probe would not register any instantaneous change in the magnitude of the force vector since total energy/mass is conserved, right? But d[force magnitude reading]/dt would be different now since the energy/mass of the photons now moves at c, correct? (If this 3rd point isn't clear, I can type up some equations trying to explain what I mean).