Hawking radiation

In classical general relativity, a black hole's event horizon marks a boundary from which light cannot escape. Hawking's 1974 calculation considered quantum fields in the curved spacetime around a black hole and predicted that it emits particles with a thermal spectrum. This is not ordinary light escaping from inside the horizon: it is an effect of quantum fields in the black-hole spacetime. The predicted temperature is proportional to the black hole's surface gravity; for a non-rotating, uncharged black hole, it is inversely proportional to its mass.

The temperature-mass relationship has an unusual consequence: a smaller black hole is hotter than a larger one. As radiation carries energy away, an isolated black hole loses mass. In the standard theoretical picture, that mass loss raises its temperature and increases its emission, a process often called black-hole evaporation. Hawking's original paper estimated that a black hole with the Sun's mass would have a lifetime far longer than the current age of the universe; its predicted effect is therefore negligible for ordinary stellar-mass black holes on present cosmic timescales.

Hawking radiation gives black holes a temperature, complementing earlier work associating black-hole horizons with entropy. Together, these ideas form black-hole thermodynamics and connect gravitational physics with quantum theory and statistical physics. The subject also raises deep questions about how information is accounted for when a black hole radiates, which remain an active area of theoretical research.

Hawking radiation is a theoretical prediction, not a signal established by direct observation of an astrophysical black hole. Its expected weakness for large black holes makes detection difficult. The prediction nevertheless transformed the study of black holes: they are not simply objects described by classical gravity, but systems whose quantum properties can be discussed in terms of temperature, energy and entropy.