Where the light comes from
In physics, empty space is not empty: pairs of particles are constantly appearing and disappearing. Usually, they annihilate each other instantly, so nothing gets out.
At a black hole's boundary, this balance is disrupted. One particle from the pair can fall inside, while the other remains outside and flies away. From the outside, this looks like a faint glow from the boundary itself.
The energy for the escaping particle comes from the black hole itself, so the hole gradually loses mass. This picture is simplified, but it conveys the essence: the radiation takes energy away from the black hole.
How slow is it
The first tile explains the key point. The larger the black hole, the colder it is; stellar-mass and especially supermassive black holes are colder than the surrounding cosmos, so they absorb more than they emit.
They will be able to evaporate only in the distant future, when the Universe has cooled more than they have. Small black holes would evaporate quickly and brightly, but none have been detected so far.
This prediction gave rise to an unresolved question about the fate of information. If a black hole evaporates, while the radiation that comes out is identical no matter what fell into it, then the information about what fell in disappears, and physics does not allow that. The debate has been going on for half a century.
The prediction is tested indirectly. Researchers build laboratory systems with similar mathematics, including fluid flows, sound waves, and optical media, and observe similar radiation in them; it is not the same thing, but it supports the calculation.
The result matters for another reason. It brought together three fields that had previously barely overlapped: gravity, quantum physics, and thermodynamics, and it is still considered one of the main clues toward unifying them.
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Sources
- The thermal radiation from a black hole's horizon, predicted in 1974, is inversely proportional to its mass
- for stellar-mass and supermassive black holes, the temperature is below that of cosmic background radiation. The radiation has not been observed directly
- the information paradox associated with it remains unresolved