= Pair production
<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>.
Back to article page
