Where do the charge comes in electron-positron pair production
As far as I know, they don't come from anywhere. Charge is simply conserved. Processes which create charged particles must conserve overall charge. This means that a neutral photon can create an electron-positron pair because the total charge before and after the creation process is zero.
two photons (or a photon not in vaccuum)... otherwise charge may be conserved but not the energy/momentum.
Indeed. Thanks for the correction.
Just to add, the fact that two charged particles can be created from particles that lack electric charge is a pretty amazing and profound result, even though it flows trivially from conservation of charge. Other conservation laws (e.g. baryon number and lepton number) have some equally profound implications.
Generally speaking, this favors a view of the universe as made of "all purpose stuff" that can be differentiated in various ways according to various laws, as opposed to an atomic/lego like preon kind of conception of the fundamental nature of the universe.
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