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Each particle is defined by a set of values. When two particles overlap, their values add together and they re-evaluate their possible identities based on the new total. An anti-particle is just a particle whose values, when added together with the corresponding particle, produce the same values of a photon. When a particle-antiparticle pair collides, it "forgets" whether it was a pair, or if it was a photon.
The reason why photons do not spontaneously split back into particle-antiparticle pairs is because of some shit with conservation of momentum and the fact that light cannot slow down while traveling through empty space. You can create positron-electron pairs by shooting a powerful laser at a sheet of gold foil.
