Particles and anti-particles are constantly appearing and vanishing in the physical vacuum. This can be indirectly observed in numerous lab experiments. The separation between the virtual particles is governed by Heisenberg's uncertainty principle so that vacuum fluctuations that carry large amounts of energy survive for much shorter times than low energy ones, and the particles travel less far before annihilation back into the vacuum state.

According to Steven Hawking's theory, near a black hole, the gravitational field changes in strength due to its tidal component. When this tidal scale equals the scale of a virtual particle pair at a particular 'threshold' energy, the pair can be tidally ripped apart. This causes one of the particles to escape to infinity and the other to fall through the horizon and be lost. The gravitational field, meanwhile, has lost energy in doing the work to separate the particles and to confer 'positive energy' to the particles that escapes to infinity as the Hawking Radiation. The net effect is that the black hole has lost mass equal to the mass of the escaping particle.

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