In essence, what Hawking, who died six years ago today, found is that black holes should not be truly black, because they constantly radiate a tiny amount of heat. That conclusion came from basic principles of quantum physics, which imply that even empty space is a far-from-uneventful place. Instead, space is filled with roiling quantum fields in which pairs of ‘virtual’ particles incessantly pop out of nowhere and, under normal conditions, annihilate each other almost instantaneously.

However, at an event horizon, the spherical surface that defines the boundary of a black hole, something different happens. An event horizon represents a gravitational point of no return that can be crossed only inward, and Hawking realized that there two virtual particles can become separated. One of them falls into the black hole, while the other radiates away, carrying some of the energy with it. As a result, the black hole loses a tiny bit of mass and shrinks — and shines.

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