Summary Quantization of fields in a black hole background. Vacuum choice. Hawking radiation and black hole evaporation. Thermodynamics of black holes.

Hawking radiation

Black holes are massive objects which have such a strong gravitational field that even light cannot escape from them. According to the classical General Relativity, a black hole can only absorb matter and its size never decreases. In 1974 Hawking considered quantum fields in a classical black hole background and discovered that the black hole emits thermal particles and thus evaporates. This theoretical result came to a certain extent as a surprise. In fact, at that time one thought that particles can be produced only by a nonstatic gravitational field. For example, for a rotating black hole there exist negative-energy states outside its horizon, and therefore the gravitational field can convert a virtual particle-antiparticle pair into a pair of real particles with zero total energy. The positive-energy particle can then escape to infinity, while the negative-energy particle falls into the black hole. In this case the black hole can emit energy. This effect is known as superradiance. On the contrary, a nonrotating black hole has no negative energy states outside its horizon. Therefore, at first glance its mass cannot decrease and hence no particles can be produced from the quantum fluctuations outside the black hole horizon.