| Light–matter interaction |
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| Low-energy phenomena: |
| Photoelectric effect |
| Mid-energy phenomena: |
| Compton scattering |
| High-energy phenomena: |
| Pair production |
Pair production refers to the creation of an elementary particle and its antiparticle, usually from a photon (or another neutral boson). For example an electron and its antiparticle, the positron, may be created. This is allowed, provided there is enough energy available to create the pair – at least the total rest mass energy of the two particles – and that the situation allows both energy and momentum to be conserved. Other pairs produced could be a muon and anti-muon or a tau and anti-tau. However all other conserved quantum numbers (angular momentum, electric charge, lepton number) of the produced particles must sum to zero – thus the created particles shall have opposite values of each. For instance, if one particle has electric charge of +1 the other must have electric charge −1, or if one particle has strangeness +1 then another one must have strangeness −1.
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In nuclear physics, this occurs when a high-energy photon interacts with a nucleus. The energy of this photon can be converted into mass through Einstein's equation E = m c2 where E is energy, m is mass and c is the speed of light. The photon must have enough energy to create the mass of an electron plus a positron. The mass of an electron is 9.11 × 10−31 kg, the same as a positron.
If there is more energy in the photon than this bare minimum, the electron and positron will have some kinetic energy – meaning they will be moving. The electron and positron can move in opposite directions (at an angle of 180 degrees) meaning they have a total momentum of zero or they can move at an angle of less than 180 degrees resulting in a net combined momentum. However, if the photon had only just enough energy to create the mass of the electron-positron pair then the electron and positron will be at rest. This could violate the conservation of momentum since the photon has momentum and the two resulting particles have none if they are stationary (since momentum = mass × velocity). This means that the pair production must take place near another photon or the nucleus of an atom since they will be able to absorb the momentum of the original photon. In other words, since the momentum of the initial photon must be absorbed by something, pair production by a single photon cannot occur in empty space; the nucleus (or another particle) is needed to conserve both momentum and energy.[1]
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 as its antiparticle partner is captured by the black hole.
Pair production is also the hypothesized mechanism behind the pair instability supernova type of stellar explosions, where pair production suddenly lowers 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.[2]