Pair production

Pair production refers to the creation of an elementary particle and its antiparticle. This is allowed, provided there is enough energy and momentum available to create their mass and motion, because they have opposite quantum numbers (which are therefore conserved in the process).

In nuclear physics, this occurs when a high-energy photon interacts with an atomic nucleus, allowing it to decay into an electron and a positron without violating conservation of momentum. Since the momentum of the initial photon must be absorbed by something, pair production cannot occur in empty space. One could argue that the created particles take the momentum, but this cannot be true in the case where the initial photon only carries exactly twice the electron rest mass. At this limit, pair production can take place, and an electron and its counterpart (positron) are created - but they are at rest and thus do not carry any momentum. Therefore, a nucleus is needed, for it can absorb the photon's momentum without "stealing" too much energy (due to its enormous mass). One can imagine that the transferred momentum is carried by a virtual photon in the strong coulomb field of the nucleus. Pair production is the chief method by which energy from gamma rays is observed in condensed matter. The photon need only have a total energy of twice the electron mass (i.e. 1.02 MeV) for this to occur as described above; if it is much more energetic, heavier particles may also be produced. These interactions were first observed in Patrick Maynard Stuart Blackett's counter-controlled bubble chamber, leading to the 1948 Nobel prize for physics.

In semiclassical general relativity, pair production is also invoked to explain the Hawking radiation effect. According to quantum mechanics, at short scales short-lived particle-pairs are constantly appearing and disappearing (see 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, with its antiparticle being captured by the hole.