Emission of particles by a black hole as a result of quantum-mechanical effects. It was discovered by Stephen Hawking in 1974. The gravitational field of the black hole causes production of particle–antiparticle pairs in the vicinity of the event horizon (the process is analogous to that of pair production). One member of each pair (either the particle or the antiparticle) falls into the black hole, while the other escapes. To an external observer, it appears that the black hole is emitting radiation (Hawking radiation). Furthermore, it turns out that the energy of the particles that fall in is negative and exactly balances the (positive) energy of the escaping particles. This negative energy reduces the mass of the black hole and the net result of the process is that the emitted particle flux appears to carry off the black-hole mass. It can be shown that the black hole radiates like a black body, with the energy distribution of the particles obeying Planck’s radiation law for a temperature that is inversely proportional to the mass of the hole. For a black hole of the mass of the sun, this temperature turns out to be only about 10−7 K, so the process is negligible. However, for a ‘mini’ black hole, such as might be formed in the early universe, with a mass of order 1012 kg (and a radius of order 10−15 m), the temperature would be of order 1011 K and the hole would radiate copiously (at a rate of about 6 × 109 W) a flux of gamma rays, neutrinos, and electron–positron pairs. (The observed levels of cosmic gamma rays put strong constraints on the number of such ‘mini’ black holes, suggesting that there are too few of them to solve the missing-mass problem.) Further insight into Hawking radiation has been obtained using superstring theory.