- What is time travel1? Intuitively, the idea is simple enough to have dominated science fiction books, movies and TV throughout the twentieth century. But defining time travel2 is not simple. According to Paul Davies time travel3 is travel to other times, just as we travel spatially to other places4. No doubt this is right, but spelling out the analogy is not as straightforward as it may at first seem (try defining space travel, then exchanging 'time' for 'place' in the definition.)
- However, one way to define time travel5 is via the notion of a causal process. The simplest case of a causal process is the trajectory of a particle through time, where we think of the earlier stages of the particle as the cause of later stages. The same can be said for the history of a person – one's earlier stages are (partly) responsible for one's later stages. A full stomach causes nourishment, earlier perceptions are the cause of later memories, and so on. Time travel6 is where a causal process connects two times in a special way. Dr Who's body is a causal process, and when the Tardis takes him from 1976 to 1876 that causal process connects two times in a special way. I say 'special way' to distinguish time travel7 from the normal causal process, such as John's life, a causal process, connecting 1965 and 2005, the dates of his birth and death.
- Following David Lewis, we can say that a causal process connects two times in a special way when the 'personal' time defined by the causal process is at odds with 'external' time8. As measured by his watch, or digestive processes, Dr Who's trip takes 2 hours, let's say, which is at odds with the external time of minus 100 years. In the case of normal causal processes such as John's life, the personal time and external time coincide, so that is not time travel9. So we can say that time travel10 is where a causal process connects two times in a way that involves a temporal discrepancy11.
- This idea of time travel12 has long given philosophers difficulties. Most recently, in his paper 'Troubles with Time Travel13' William Grey presents a number of objections to time travel14, some well-known in the philosophical literature, others quite novel15. In particular Grey's 'no destinations' and 'double occupation' objections I take to be original, while what I will call the 'times paradox' and the 'possibility restriction argument' are versions of well-known objections. I show how each of these can be answered, thereby defending the plausibility of time travel16.
Footnote 4: See "Davies (Paul C.W.) - About Time: Einstein's Unfinished Revolution", p. 234.
Footnote 8: See pp. "Lewis (David) - The Paradoxes of Time Travel", pp. 67-68 (of "Lewis (David) - Philosophical Papers Volume II").
Footnote 15: See "Grey (William) - Troubles with Time Travel".
- Lewis' notion of a temporal discrepancy, by itself, without the notion a causal process, will not suffice as a definition of time travel because it will count cases of closed timelike curves, discussed below, as time travel even if nothing travels.
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