X‐ray astronomy has already provided us with about a dozen rather convincing stellar black hole candidates. (See PHYSICS TODAY, November 1995, page 58.) But until now the evidence was simply that an object too compact to be an ordinary star was too massive to be just another neutron star. So, by elimination, what else could it be but a black hole? Convincing as that kind of evidence is, it does not address the defining exotic properties that general relativity attributes to black holes: the remorseless event horizon and the strongly curved spacetime leading down to it.

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