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Are black holes bald or hairy? On that strange and esoteric question may hang the future of the universe’s past. The present, the past and the future are all connected by physical laws, a phenomenon called “causal determinism”. With complete information about a system’s present, it ought therefore be possible to determine all its past and future states. In theory, that applies to any system, up to and including the entire universe.

In 1976, however, an up-and-coming Cambridge-based cosmologist called Stephen Hawking challenged this idea by showing that black holes (which are part of the universe, albeit a rather odd part) should evaporate over the course of time, and eventually vanish. That would cause information about anything they had swallowed (and thus a part of the universe’s past) to be lost, meaning the future could not be determined, even in principle. The naive might think this information loss had happened already, as a consequence of the swallowing. But even though it is inside the hole the information continues to exist for as long as the hole does. Causal determinism is not violated.

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