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. To appear in: The future of theoretical physics and cosmology; Proceedings of the conference held in honour of Stephen Hawking on the occasion of his 60’th birthday. (Cambridge, 7–10 January 2002.) E-mail: [email protected] Homepage: http://www.physics.wustl.edu/ ̃visser Archive: gr-qc/0204022 Permanent address after 1 July 2002: School of Mathematics and Computer Science, Victoria University, PO Box 600, Wellington, New Zealand. [email protected]