- Might we some day be in a position to move about in time, just as we can already move about in space? Today, few would question that deliberate change in temporal location is logically possible. There is no contradiction in the thought that Tim could step into a time machine and travel backwards to visit his grandfather, or forwards to visit his grandchildren. That is, there is no contradiction provided that we take time travel to involve influencing the course of history rather than changing it (no event can both happen and not happen).
- But is time travel metaphysically possible? Is there a genuinely possible world — as opposed to a merely coherent scenario — in which one can freely change one's location in time?
- Here is where puzzlement and bewilderment lead to philosophical controversy. For the answer depends on one's views concerning a wide range of other matters, and such views are themselves the subject of major philosophical controversy.
This is the foreword to Monist, July 2005, Vol. 88 Issue 3. Other articles in that issue were:-
- "Sider (Ted) - Traveling In A- And B- Time",
- "LePoidevin (Robin) - The Cheshire Cat Problem And Other Spatial Obstacles To Backwards Time Travel",
- "Simon (Jonathan) - Is Time Travel A Problem For The Three-Dimensionalist?",
- "Slater (Matthew H.) - The Necessity Of Time Travel (On Pain Of Indeterminacy)",
- "Vranas (Peter B.M.) - Do Cry Over Spilt Milk: Possibly You Can Change The Past",
- "Smith (Nicholas J.J.) - Why Would Time Travelers Try To Kill Their Younger Selves?",
- "Stevenson (Gordon Park) - Time Travel, Agency, And Nomic Constraint",
- "Savitt (Steven) - Time Travel And Becoming", and
- "Horacek (David) - Time Travel In Indeterministic Worlds".
Varzi considers the following (alleged) “issues” with time-travel:-
- Incompatibility with Presentism: if only the present exists, there’s “nowhen” to travel to.
- Incompatibility with Endurantism: if x visits his former self, he cannot be wholly present at two places at the same time.
- Incompatibility with Chronological Monism: the view that there is only one time series, with our psychological time arrow coinciding with that of the external world.
- Incompatibility with Free Will: when X goes into the past, he has already been there and done whatever he did; he cannot do what he has not done.
- Entails Brute Necessities: there’s no obvious difference between going back in time and killing my grandfather (before my father’s conception) and doing likewise to his identical twin; yet the first is impossible, while the second isn’t.
- Requires Reverse Causation: Does stepping into a time machine cause the stepping out at an earlier time?
- Entails Fatalism: Not only are the backward time-traveller’s actions constrained, but he is constrained to be a time-traveller.
- Entails Ex Nihilo Proceedings: arise from causal loops. Varzi gives three examples of varying cogency:-
- Buying a best seller, and then going back in time and publishing it.
- Self-creation - impregnating the person who turns out to be your own mother1.
- Dying of a heart attack on reading your own obituary in tomorrow’s paper.
- Requires Spatial Coincidence: are the time-machine persisting forward in time and the time machine travelling back in time spatially coincident2?
Varzi lists some potentially interesting Review Articles and Books, but I’ve not followed them up.
- Ie. You are your own father.
- Genetically this doesn’t seem possible (given that an offspring shares the genes of its parents, rather than taking all the genes from one parent), but this is only a nomological impossibility.
- ie. it might have been the case (as was believed until the rise of modern genetics) that all the genes come from the father.
- I’ve not tried very hard to understand this example.
- As Varzi describes it, there’s no time machine present after the “reverse” button is pressed, so it sounds as though the time machine disappears, as does the Tardis.
- But otherwise the machine is described as staying put, so that – it might be assumed – the time machine is a mere portal, and its only its contents that goes back in time.
- This differs from the usual Tardis situation, where the time machine goes forward in time without leaving any spacetime path. It may be that such notions are physically impossible, so tacitly discarded.
- In the case as described, the machine just stays in one place. Someone gets out of it in the past, and then gets into it again in the future, and loops round. Is this the only possible method? There’s only backward time-travel in this example.
- Maybe the situation is covered in one of the Papers?
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