The quantum physics of chronology protection

This is a brief survey of the current status of Stephen Hawking’s chronology protection conjecture''. That is: Why does nature abhor a time machine?‘’ I’ll discuss a few examples of spacetimes containing time machines'' (closed causal curves), the sorts of peculiarities that arise, and the reactions of the physics community. While pointing out other possibilities, this article concentrates on the possibility of chronology protection’‘. As Stephen puts it: ``It seems that there is a Chronology Protection Agency which prevents the appearance of closed timelike curves and so makes the universe safe for historians.’’