Black hole energy, conceptual image. Computer artwork illustrating Stephen Hawking's theory of the generation of energy (coloured swirls) from a black hole (centre). Energy fluctuations in the vacuum cause the spontaneous generation of particles and anti-particles, which would normally self-annihilate. At the event horizon, however, one particle could escape and the other be captured by the black hole's gravity. This emitted particle forms Hawking Radiation, the process by which black holes can slowly evaporate.