What theorem states that black holes are fully described by only three parameters?

The no-hair theorem states that a black hole is fully described by only three observable parameters: mass, electric charge, and angular momentum. The phrase “no hair” means that other details about the matter that formed the black hole cannot be determined from outside it.

For an isolated, stationary black hole in classical general relativity, these three quantities determine the external spacetime geometry. The corresponding mathematical solution is the Kerr–Newman solution. A non-rotating, uncharged black hole is described by the Schwarzschild solution, while a rotating uncharged black hole is described by the Kerr solution.

The idea developed through work by Werner Israel, Brandon Carter, Stephen Hawking, and others during the 1960s and 1970s. Strictly speaking, the broadest version is often called the no-hair conjecture because its proof requires restrictive assumptions and exceptions exist in some alternative theories or higher-dimensional models. Nevertheless, “no-hair theorem” is the standard and canonical answer to this quiz question. Penrose’s theorem, the Bekenstein bound, and Hawking radiation address different aspects of black-hole physics.