Radiation seen by static observers due to quantum effects in close vicinity to a black hole, leading to the potential for eventual evaporation of black holes. Pictorially, a particle-antiparticle pair is created, with one falling through the horizon, becoming causally lost to the rest of the universe, while its neighbor escapes the gravitational potential. Originally predicted by S. Hawking in the 1970s using quantum field theory in curved spacetime.
Brief Summary
Hawking radiation, or Hawking effect, is a prediction of qft-in-curved-spacetime. Within the framework of QFTCS, particles are not fundamental entities of the theory, but rather a concept that emerges in certain situations as excitations of a quantum field. As a consequence, the notion of particle is observer-dependent, and it highly depends on symmetry considerations, such as the existence of a notion of time-translation symmetry. In the 1970s, S. W. Hawking showed that due to these effects, a black hole that was formed after gravitational collapse (for example, at the death of a star) will be perceived by static observers as emitting a thermal spectrum of particles, with temperature being given, up to a multiplicative constant, by the black hole's surface gravity.
Black Hole Thermodynamics
Before Hawking's discovery, it was already known that general-relativity predicted that black-holes obeyed mechanical laws extremely similar to the ordinary laws of thermodynamics, but the analogy between these results was handled with care. In the analogy, the temperature would be analogous to the surface gravity, the entropy would be analogous to the black hole's event-horizon surface area, the energy would be analogous to the mass, and so on. The analogy seemed interesting due to the fact that the equations were quite similar, and some of the quantities would be the exact relativistic counterpart of the thermodynamical quantities (for example, a black hole's mass is literally its energy within the framework of GR). However, in the classical theory, it makes no sense to speak of a black hole with any non-vanishing temperature since nothing can escape out of a black hole and, as a consequence, it would never be possible for it to reach thermal equilibrium with an external bath, for example.
However, the prediction of Hawking radiation has shown that the surface gravity, which before was seen as only analogous to temperature, was literally the black hole's temperature once quantum effects were taken into account. This conclusion made the analogy between the Laws of Black Hole Mechanics and the Laws of Thermodynamics much more serious since it suggests they might be different expressions of the very same physical principles. More specifically, it suggests that the Mechanical Laws are, simply, the ordinary laws of Thermodynamics applied to a black hole system. This gave birth to what is today known as Black Hole Thermodynamics.
Unruh Effect
A related but different prediction of QFTCS is the unruh-effect. Suppose the quantum field is perceived by an inertial observer as being in the vacuum state. Accelerated observers on Minkowski spacetime perceive the same physical state as being a thermal state at a temperature proportional to their acceleration. Similarly to the Hawking effect, one notices the observer-dependence of the notion of particles.
However, it should also be pointed out that the effects are different: in the Hawking effect, one assumes a static observer on a spacetime which is undergoing gravitational collapse. Before the collapse, the observer sees no particles. After the collapse, the observer sees the black hole emitting particles on a thermal spectrum. On the Unruh effect, on the other hand, one considers an observer undergoing constant acceleration. They have always seen a thermal spectrum of particles and will always see it. Hence, the Hawking effect predicts that the observer will see particles being created, while the Unruh effect predicts the observer will see as if the particles have always been there. Furthermore, while the Hawking effect predicts the particles will be seen as coming from the black hole, the Unruh effect predicts the particles will be seen as coming from all around the observer.
Introductory Resources
Most textbooks that deal with Quantum Field Theory in Curved Spacetimes will discuss the Hawking effect. Some standard references are
- R. M. Wald's Quantum Field Theory in Curved Spacetime and Black Hole Thermodynamics;
- N. D. Birrell & P. C. W. Davies' Quantum Fields in Curved Space.
