This paper argues that the most famous objection to backward time travel can carry no weight. In its classic form, the objection is that backward time travel entails the occurrence of impossible things, such as auto-infanticide—and hence is itself impossible. David Lewis has rebutted the classic version of the objection: auto-infanticide is prevented by coincidences, such as time travellers slipping on banana peels as they attempt to murder their younger selves. I focus on Paul Horwich‘s more recent version of the (...) objection, according to which backward time travel entails not impossible things, but improbable ones—such as the string of slips on banana peels that would be required to stop a determined auto-infanticidal maniac from murdering her younger self—and hence is itself highly improbable. I argue that backward time travel does not entail unusual numbers of coincidences; and that, even if it did, that would not render its occurrence unlikely. (shrink)

The founder of Psychical Research, though he has not yet received the honour due to him, seems to have been King Croesus of Lydia, who reigned from 560 to 546 B.C. He carried out an interesting experiment, recorded in detail by Herodotus,2 to test the clairvoyant powers of a number of oracles. He sent embassies to seven oracles, six Greek and one Egyptian. They all started on the same day. On the hundredth day each embassy was instructed to ask its (...) oracle, “What is King Croesus, the son of Alyattes, now doing?” The answer was to be written down and brought back. The oracle of Delphi replied as follows, in hexameters, as its custom was: “I know the number of the sands and the measures of the sea. I understand the dumb, and I hear him who does not speak.” Then it went on: “There comes to my mind the smell of a strong-shelled tortoise, which is being cooked along with lamb's flesh in a brazen vessel; brass is spread beneath it, and with brass it is covered.” As a matter of fact, this answer was perfectly correct. Herodotus tells us that “having considered what would be the most difficult thing to discover and to imagine,” Croesus “cut up a tortoise and a lamb, and himself boiled them together in a brazen pot.” What happened afterwards illustrates the difficulties of this sort of investigation, difficulties which still perplex us to this day. Croesus argued, reasonably enough, that if the Delphic priestess had clairvoyant powers, she probably also had the precognitive powers, she probably also had the precognitive powers which she claimed to have. But when he consulted the oracle later about his forthcoming expedition against the Persians, he received two answers, one of which was ambiguous—“When Croesus crosses the Halys he shall destroy a great empire”—and the other correct, but too obscure to be easily interpreted. (shrink)

Robert Casati and Achille C. Varzi, argue that time machines would be useless or have no practical applications on the grounds that travelling to the past would involve doing what has already been done. I argue that the sense in which travelling to the past involves doing what has already been done fails to support the claim that time machines would have no practical applications.

This paper demonstrates that John Wheeler and Richard Feynman's strategy for avoiding causal paradoxes threatened by backward causation and time-travel can be defeated by designing self-interacting mechanisms with a non-simple topological structure. Time-travel therefore requires constraints on the allowable data on space-like hypersurfaces. The nature and significance of these constraints is discussed.