““When we cause two of one kind of particle- say, an electron and a positron-to collide, the final products might be totally different particles; they might be a proton, an antiproton, two photons, plus whatever additional particles the available energy and the conservation laws permit. That is what we mean when we say that particles lose their identity in a collision; as far as the other conservation laws permit, they become pure energy, and the same particles or other particles may result from that energy in the final count.””
