Time Machines
Earman (John) & Wuthrich (Christian)
Source: Stanford Encyclopaedia of Philosophy, 2004-10
Paper - Abstract

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Author’s Abstract

    Recent years have seen a growing consensus in the philosophical community that the grandfather paradox and similar logical puzzles do not preclude the possibility of time travel1 scenarios that utilize spacetimes containing closed timelike curves. At the same time, physicists, who for half a century acknowledged that the general theory of relativity is compatible with such spacetimes, have intensely studied the question whether the operation of a time machine2 would be admissible in the context of the same theory and of its quantum cousins. A time machine3 is a device which brings about closed timelike curves—and thus enables time travel4 — where none would have existed otherwise. The physics literature contains various no-go theorems for time machines5, i.e., theorems which purport to establish that, under physically plausible assumptions, the operation of a time machine6 is impossible. We conclude that for the time being there exists no conclusive no-go theorem against time machines7. The character of the material covered in this article makes it inevitable that its content is of a rather technical nature. We contend, however, that philosophers should nevertheless be interested in this literature for at least two reasons. First, the topic of time machines8 leads to a number of interesting foundations issues in classical and quantum theories of gravity; and second, philosophers can contribute to the topic by clarifying what it means for a device to count as a time machine9, by relating the debate to other concerns such as Penrose's cosmic censorship conjecture and the fate of determinism in general relativity theory, and by eliminating a number of confusions regarding the status of the paradoxes of time travel10. The present article addresses these ambitions in as non-technical a manner as possible, and the reader is referred to the relevant physics literature for details.

  1. Introduction: time travel11 vs. time machines12
  2. What is a (Thornian) time machine13? Preliminaries
  3. When can a would-be time machine14 be held responsible for the emergence of CTCs?
  4. No-go results for (Thornian) time machines15 in classical general relativity theory
  5. No-go results in quantum gravity
  6. Conclusion
  7. Bibliography
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Recommended by "Richmond (Alasdair) - Time Travel and Philosophy". First published Thu Nov 25, 2004; substantive revision Thu Oct 7, 2010. For the full text, see Link.

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