No-Hair Theorem

The no-hair theorem states that stationary black holes in general relativity are completely characterized by only three externally observable parameters: mass, electric charge, and angular momentum. All other information — the hair — about the matter that collapsed to form the black hole is lost to external observers, hidden behind the event horizon. This remarkable uniqueness result, proven through a series of black hole uniqueness theorems by Israel, Carter, Hawking, and others, makes black holes the simplest macroscopic objects in physics despite being formed by the most complex gravitational collapses.

The theorem implies that a black hole formed from a star with a complicated magnetic field, irregular shape, and internal composition is indistinguishable from one formed from completely different initial conditions, provided only the final mass, charge, and spin match. This has profound consequences for black hole thermodynamics and the information paradox, since the no-hair property seems to destroy information irreversibly — a process incompatible with the unitary evolution required by quantum mechanics.

Whether quantum gravity preserves some form of quantum