The No-Hair Theorem

The no-hair theorem is the surprising result that a black hole can be completely described by just three numbers: its mass, its electric charge, and its rate of spin. Everything else about whatever fell in to create it, the kind of matter, its history, its structure, appears to be erased from the outside view. Physicists summarise this by saying that a black hole "has no hair," meaning no extra distinguishing features.

This makes black holes, despite their reputation, among the simplest objects in the universe to describe. Two black holes with the same mass, charge and spin are, from the outside, identical, regardless of whether one formed from a collapsing star and the other from a collapsing cloud of books.

Why it mattered to Hawking

That apparent simplicity is precisely what makes the black hole information paradox so sharp. If a black hole truly retains no trace of what fell in, and then evaporates into featureless radiation, the detailed information about everything it swallowed seems to vanish from the universe, which the laws of quantum mechanics forbid. The no-hair idea, the radiation and the paradox are three faces of the same deep problem Hawking spent decades on.