The general theory of relativity states that black holes have only three observable properties; additional ones, or “hair,” do not exist. Or do they?
Identical twins have nothing on black holes. Twins may grow from the same genetic blueprints, but they can differ in a thousand ways—from temperament to hairstyle. Black holes, according to Albert Einstein’s theory of gravity, can have just three characteristics—mass, spin and charge. If those values are the same for any two black holes, it is impossible to discern one twin from the other. Black holes, they say, have no hair.
“In classical general relativity, they would be exactly identical,” said Paul Chesler,
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