No-Hair Theorem

The No-Hair Theorem, also called the black hole uniqueness theorem, states that every stationary black hole solution of the combined Einstein and Maxwell equations of gravitation and electromagnetism can be completely described by only three externally observable quantities: its mass, its angular momentum, and its electric charge. Every other detail about the matter that collapsed to form the black hole, or that later fell into it, becomes permanently inaccessible to outside observers once the black hole settles down, a loss of information popularly summarized by the physicist John Archibald Wheeler's phrase that black holes have no hair. The simplified case for a non-rotating, uncharged black hole was proved by Werner Israel in 1967, and while the result has been extended to charged and spinning black holes under further assumptions, no fully general mathematical proof exists, so mathematicians still refer to it as the no-hair conjecture.