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Pair production creates a particle and its antiparticle while conserving energy, momentum and relevant charges. For an electron–Positron pair, the available invariant energy must at least supply their combined rest mass, with additional kinetic energy and other final-state requirements included.
A single free photon in empty flat spacetime cannot produce just that pair: its four-momentum has zero invariant mass, while the massive pair has a positive invariant mass. A second photon or another participant, such as a nucleus taking recoil, can satisfy the full kinematics. The pair's charges cancel globally, but each outgoing particle can leave its own detector track. A laboratory-frame photon energy threshold therefore depends on the complete reaction, not only on writing twice the electron rest energy.
