Metaphysics is not something that I gladly spend time on, and for two main reasons: i) I’m skeptical that we humans, with out limited cognitive capacities, can ever really carve nature at its joints, to use the Platonic expression (and even if we did, we wouldn’t be able to recognize it as such). ii) Metaphysical questions give me a headache – they make my head spin.
But anyway, I was asked to deliver a short lecture to introduce the film Primer for an event organized by the philosophy students’ association in Groningen. So it had to be about time travel, and thus about time, and thus about metaphysics… (The reason why I was ‘singled out’ was my course on paradoxes that one of the students organizing the event had attended. But in said course I do my best to steer away from metaphysics and deal mostly with logical paradoxes, though we do discuss the Ship of Theseus and metaphysical vagueness.) As is often the case, I was saved by this very informative entry on time over at the Stanford Encyclopedia; I also read Lewis’ classic ‘The paradoxes of time travel’, and a few other related papers, by e.g. Sider and Keller and Nelson.
As people are hoping for a fun night out, I figured I shouldn’t make my lecture too difficult (and thus aim for being the only one with a headache at the end of it). But the point I will argue for is that time travel as depicted in movies such as the Back to the Future trilogy (ah, the golden days of my childhood!) and Primer is inherently paradoxical, as it involves two aspects that are difficult to reconcile: time travel as such, and the possibility of altering the causal course of events by means of time travel.
Here is why. If time travel is like ‘regular’ travel at all, it presupposes the (almost) simultaneous existence of at least two distinct ‘places’: the point of departure and the point of arrival (modulo the time it takes for the trip itself). This is what eternalism/the block universe view gives you: different times are like ‘places’ you can visit. But this view has a hard time accounting for causal connections between different time-slices, and thus for the idea that, if I travel back in time and kill my grandfather, the time of my entering the time travel machine is thereby modified (in particular, by making me never coming into existence, and thus not being able to enter the time machine, travel back in time, and kill my grandfather – paradox!). If eternalism is true, then all these ‘places’ already exist exactly as they are, and thus cannot be modified.
Presentism, on the other hand, at least seems to offer more leeway for the possibility of causally altering different time-slices by means of time travel, given that past and future do not exist for the presentist. However, time travel as such is hard to make sense of from a presentist point of view (though many a philosopher have tried), since past and future times are not ‘places’ one can visit. So there you go: either you have the cake but not the knife, or you have the knife but not the cake – in both cases, you don’t get to eat the cake. I’m sure somebody must have made this rather trivial point before, but it seems like a nice way to warn the students that the movie they are about to watch, Primer, is philosophically incoherent – so they may as well enjoy the ride for what it is, hopefully with no headaches.
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