Pair Production questions: 1) When a gamma ray photon pair produces an electron and a positron, do the two particles always have the opposite spin? That is, one always has +1/2(h-bar) spin and the other has -1/2(h-bar) spin? 2) Other than charge and spin, what are some other notable properties of the two particles that "cleave" in a symmetric way in order to preserve conservation laws? 3) Is it true that pair production cannot happen in "empty space" but rather has to happen in a solid or at least near a nucleus in order to maintain the conservation of momentum? What does this mean for "vacuum" models of empty space that posit pair production events going on constantly in universe everywhere and all the time?