Over the last few years leading scientific journals have been publishing articles dealing with time travel and time machines. (An unsystematic survey produced the following count for 1990–1992. Physical Review D: 11; Physical Review Letters: 5; Classical and Quantum Gravity. 3; Annals of the New York Academy of Sciences: 2; Journal of Mathematical Physics: 1. A total of 22 articles involving 22 authors.) Why? Have physicists decided to set up in competition with science fiction writers and Hollywood producers? More seriously, does this research cast any light on the sorts of problems and puzzles that have featured in the philosophical literature on time travel?
The last question is not easy to answer. The philosophical literature on time travel is full of sound and fury, but the significance remains opaque. Most of the literature focuses on two matters, backward causation and the paradoxes of time travel. Properly understood, the first is irrelevant to the type of time travel most deserving of serious attention; and the latter, while always good for a chuckle, are a crude and unilluminating means of approaching some delicate and deep issues about the nature of physical possibility. The overarching goal of this chapter is to refocus attention on what I take to be the important unresolved problems about time travel and to use the recent work in physics to sharpen the formulation of these issues.
The plan of the chapter is as follows. Section 1 distinguishes two main types of time travel – Wellsian and Godelian. The Wellsian type is inextricably bound up with backward causation.
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