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RICHARD HANLEYNO END IN SIGHT: CAUSAL LOOPS IN PHILOSOPHY, PHYSICS
AND FICTION ABSTRACT. There have been many objections to the possibility of time travel. But all the
truly interesting ones concern the possibility of reverse causation. What is objectionable
about reverse causation? I diagnose that the truly interesting objections are to a further
possibility: that of causal loops. I raise doubts about whether there must be causal loops if
reverse causation obtains; but devote the majority of the paper to describing, and dispelling
concerns about, various kinds of causal loop. In short, I argue that they are neither logically
nor physically impossible. The only possibly objectionable feature that all causal loops
share is that coincidence is required to explain them. Just how coincidental a loop will be
varies: some are really quite ordinary, and some are incredibly unlikely. I end by speculating that the tendency amongst physicists to avoid discussion of causal loops involving
intentional action may have been unfortunate, since intentional action is an excellent way
to non-mysteriously bring about what otherwise would have been an unlikely coincidence.
Hence causal loops may be more likely in a world with beings like us, than in one without.1. INTRODUCTIONTime travel, I maintain, is logically, physically, and epistemically
possible.1 But I am not concerned to argue directly for these claims here.
It seems to me that, despite the widespread failure to notice it (amongst
philosophers and non-philosophers alike), several issues concerning time
travel have effectively been laid to rest.2 Bugbears remain, however. Reverse causation is a causal relationship between events in which effect
precedes cause. And in David Lewiss characterization, causal loops are
closed causal chains in which some of the causal links are normal in dir- Thanks to John Bigelow, Stephen Coleman, Michael Rea, and Aubrey Townsend, for
valuable discussion; and to Tim Jones for the graphics.Synthese 141: 123152, 2004.
2004 Kluwer Academic Publishers. Printed in the Netherlands.124 RICHARD HANLEYection and others are reversed (1986, 74). Given these characterizations,
here is the No Loops Argument against the possibility of time travel:P1. If time travel is possible, then time travel to the past is
possible.P2. Time travel to the past requires reverse causation.P3. Reverse causation requires causal loops.P4. Causal loops are not possible.
C. Time travel...