General relativity is a profoundly complex mathematical theory, but its description of black holes is amazingly simple. A stable black hole can be described by just three properties: its mass, its electric charge and its rotation or spin.

Since black holes aren't likely to have much charge, it really takes just two properties. If you know a black hole's mass and spin, you know all there is to know about the black hole.

Mass is mass as far as general relativity is concerned. In every case, the event horizon of a black hole is perfectly smooth, with no extra features. As Jacob Bekenstein said, "black holes have no hair."

But with all its predictive power, general relativity has a problem with quantum theory. This is particularly true with black holes. If the no-hair theorem is correct, the information within an object is destroyed when it crosses the event horizon.

Quantum theory says that information can never be destroyed. So the valid theory of gravity is contradicted by the valid theory of the quanta. This leads to problems such as the firewall paradox, which can't decide whether an event horizon should be hot or cold.

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Since black holes aren't likely to have much charge, it really takes just two properties. If you know a black hole's mass and spin, you know all there is to know about the black hole.

Mass is mass as far as general relativity is concerned. In every case, the event horizon of a black hole is perfectly smooth, with no extra features. As Jacob Bekenstein said, "black holes have no hair."

But with all its predictive power, general relativity has a problem with quantum theory. This is particularly true with black holes. If the no-hair theorem is correct, the information within an object is destroyed when it crosses the event horizon.

Quantum theory says that information can never be destroyed. So the valid theory of gravity is contradicted by the valid theory of the quanta. This leads to problems such as the firewall paradox, which can't decide whether an event horizon should be hot or cold.

Continued...

Source