I'm having a bit of trouble understanding why a nucleus is needed for a gamma ray to interact with in particle pair production. I know that all properties are conserved - spin, charge, etc. -, and I know that the photon must have at least the energy of twice the standing energy of the particle it creates (for electron-positron pairs this is 1.022 MeV, correct?), but why does a nuetral, massles particle (photon) create two charged, massive particles (i.e. an electron and positron), and why is a nucleus necessary? I'm asking simply out of curiosity, and any answers would be highly appreciated. Thanks!