Energy
Photon-nucleus pair production can only occur if the photons have an energy exceeding twice the rest energy (me c2) of an electron (1.022 MeV). These interactions were first observed in Patrick Blackett's counter-controlled cloud chamber, leading to the 1948 Nobel Prize in Physics. The same conservation laws apply for the generation of other higher energy particles such as the muon and tau.
In semiclassical general relativity, pair production is also invoked to explain the Hawking radiation effect. According to quantum mechanics, particle pairs are constantly appearing and disappearing as a quantum foam. In a region of strong gravitational tidal forces, the two particles in a pair may sometimes be wrenched apart before they have a chance to mutually annihilate. When this happens in the region around a black hole, one particle may escape while its antiparticle partner is captured by the black hole.
Pair production is also the hypothesized mechanism behind the pair instability supernova type of stellar explosion, where pair production suddenly lowers the pressure inside a supergiant star, leading to a partial implosion, and then explosive thermonuclear burning. Supernova SN 2006gy is hypothesized to have been a pair production type supernova.
In 2008 the Titan laser aimed at a 1-millimeter-thick gold target was used to generate positron–electron pairs in large numbers.
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