One of the most exciting predictions of Einstein's theory of gravitationisthat there may exist 'black holes': putative objects whose gravitational fields are so strong that no physical bodies and signals can break free of their pull and escape. Even though a completely reliable discovery of a black hole has not yet been made, several objects among those scrutinized by astrophysicists will very likely be conformed as black holes. The proof that they do exist, and an analysis of their properties, would have a significance going far beyond astrophysics. Indeed, what is involved is not just the...