no-hair theorem

Two people can look utterly different — different faces, voices, histories. But imagine if, once they walked through a certain door, every personal detail vanished and you could only ever tell them apart by their weight, their spin, and their electric charge. That is the strange truth about black holes. No matter what fell in — a star, a library, an elephant — the resulting black hole remembers only three numbers. This is the no-hair theorem: a black hole 'has no hair', meaning no extra distinguishing features.

Those three numbers are mass, spin (rotation), and electric charge. Everything else about whatever formed or fell into the black hole — its chemistry, its shape, its colour, its memories — is erased from the outside view, hidden forever behind the event horizon. Two black holes with the same mass, spin, and charge are identical in every measurable way, even if one was made of antimatter stars and the other of fallen bicycles. In practice astrophysical black holes have essentially no charge, so real ones are described by just two numbers: mass and spin.

The no-hair idea is profound because it means black holes are the simplest macroscopic objects in nature — far simpler than a single atom. This simplicity is what made it possible to predict the exact shape of gravitational waves from merging black holes, since the final merged black hole is fully specified by just mass and spin. A caveat worth stating: it is strictly a result of classical general relativity. Whether information truly vanishes, or is somehow encoded on the horizon, is the heart of the famous black-hole information puzzle, still unresolved.