On Some Alleged Paradoxes of Time Travel
Horwich (Paul)
Source: Journal of Philosophy, Vol. 72, No. 14, Time, Cause, and Evidence (Aug. 14, 1975), pp. 432-444
Paper - Abstract

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  1. It has been claimed by Kurt Gödel that time travel1 is physically possible. Not just that travel into the future is possible – this is a well-known consequence of the Special Theory of Relativity, and of fairly little philosophical interest. But also, more controversially, that travel backwards in time is possible. Gödel’s ground for this provocative view is his discovery of certain solutions of the field equations of General Relativity that permit the existence of closed causal chains. A journey back in time would be nothing more than the "backwards" part of such a chain. Thus Gödel is led to the following startling result: "by making a round trip on a rocket ship it is possible in these worlds [i.e., worlds in which his field equation solutions describe the structure of space-time] to travel into any region of the past, present, and future and back again, exactly as it is possible in other worlds to travel to distant parts of space" (560).
  2. Many people would like to deny this claim. Some would quarrel with the physics involved. They might argue that Gödel’s derivation of his solutions was invalid or that those solutions are physically incompatible with various known facts about the universe. Others take the position that time travel2, especially into the past, is conceptually absurd, and a fortiori physically impossible. They might rule out Gödel’s solutions in the way that we often reject unacceptable mathematical solutions to physical problems.
  3. My purpose in this paper is to defend Gödel’s claim against the objection that time travel3 as he envisions it is impossible since it would engender absurd consequences. I will consider five alleged paradoxes that have been held to refute the possibility of time travel4 into the past. All but one of them can, I think, be dealt with fairly quickly. So I shall devote most of the paper to a consideration of the fifth5, which is due to John Earman.

In-Page Footnotes

Footnote 5: [C]onsider a rocket ship which at some space-time point x can fire a probe which will travel into the past lobe of the null cone at x. Suppose that the rocket is programmed to fire the probe unless a safety switch is on and that the safety switch is turned on if and only if the 'return' of the probe is detected by a sensing device with which the rocket is equipped. Is the probe fired? We find that the answer is that it is fired if and only if it is not fired, which is a contradiction if standard logic holds.

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