On New Year’s Eve, my brother and I observed an annual tradition: we watched The Time Machine (1960). If you haven’t seen it, I heartily recommend it. I like everything about it.
First of all, it has a solid design aesthetic. It was steam punk before steam punk was a thing. The time machine itself is a thing of beauty.
Younger readers may recognize it from its appearance on The Big Bang Theory.
It’s a hugely entertaining film, but it also provides the occasion for philosophical reflection on the nature of time and time travel. Disclaimer: I’m not an expert in this area, and certainly an amateur with respect to the relevant science (quantum mechanics, etc.), so take what I say with a grain of salt.
According to the film, time is the fourth dimension. It’s analogous to space. Again, based on my layman’s knowledge of arcane concepts in physics like M-Theory, scientists now believe that there are many dimensions of space (at least 11?). I’ll come back to that, but given the concept of time presented in the film, I think the plot is coherent. Spoilers ahead!
My brother and I discussed whether George’s (the time traveler’s) multiple trips through time would change the previous timeline(s). For example, when he travels back to the 19th century, having rescued Weena and the Eloi from the Morlocks, does he thereby erase that timeline? Does he have to do everything over again? The way the movie sets it up, I’d say ‘no.’ But, again, based on my layman’s understanding of time, time travel would create multiple timelines. So there would be one timeline for his initial trip, in which he stops along the way and meets his friend Filby’s son, for example, and one timeline in which he doesn’t. In either case, the plot is at least coherent; in neither timeline does George ever return to the 19th century for good.
For my purposes here, I’d like to ask, “What would have to be true metaphysically for time travel to be possible?” A better way of asking the question might be “What are the minimum metaphysical requirements that would allow one to travel through time, thereby changing the past or future?” I think there are three minimum requirements: a multiverse, a B-Theory of time, and a perdurantist theory of identity over time.
Why a multiverse? Well, without multiple universes, it seems to be impossible to change the past (let’s bracket the future for the … um … present). Let’s take the famous grandfather paradox, designed to show that you can’t change the past. If I go back in time, and kill my grandfather, then I presumably wouldn’t exist. But if I didn’t exist, I couldn’t go back in time to kill my grandfather. Thus, it seems impossible to change the past. Some would say that I’m destined to fail in my attempt. However, let’s assume they’re wrong. If I succeed in killing my grandfather, clearly I’m not altering my native timeline. After all, in that timeline, my grandfather survived long enough to reproduce. So, I’m creating a new timeline, even a new universe. The Everett-Wheeler interpretation of quantum mechanics posits a branching universe. Every time a quantum ‘choice’ is made, the universe branches. On this interpretation of QM, time travel and changing the past is possible, with the caveat that the change doesn’t erase the ‘old’ timeline but creates a ‘new’ one. We might call this soft change as opposed to hard change. Multiverse theories are used to explain time travel in many fictional universes like the Star Trek universe and the DC comics universe. (As if I haven’t demonstrated enough geek cred already.)
Another metaphysical condition for time travel is a B-Theory of time. There are two major realist theories of time: presentism and eternalism, which philosophers have creatively labeled A-Theory and B-Theory respectively. A-Theory holds that only the present is real; the past is gone and the future isn’t yet real. On A-Theory, time travel doesn’t make any sense because there’s nowhere to go. B-Theory, by contrast, holds that both the past and future are as real as the present. On B-Theory, time travel at least makes sense. I’m not competent to defend the relative merits of each theory; all I’m saying is that the truth of B-Theory would be a prerequisite for the possibility of time travel and the possibility of changing either the past or future.
The final metaphysical condition is a perdurantist theory of identity over time. Again, there are two major theories with respect to the problem of identity over time (roughly, how does a thing remain the same despite changing its properties over time): endurantism and perdurantism. On endurantism, a thing (for example, you) endures over time and there is a strict identity between the you now and the you in the past. There is just one thing persisting over time. On perdurantism, by contrast, there are many things, many distinct temporal parts, over time. In other words, you are a space-time worm. Why is this relevant to time travel? Well, upon endurantism, it would be impossible for you to travel to the past or future and meet your past or future self. Remember, there’s just one entity called ‘you.’ However, upon perdurantism, it is possible to travel forward or backward in time and meet a space-time segment of yourself. So any time travel story in which the protagonist meets his or her younger or older self, presupposes perdurantism as its metaphysical backdrop. (This isn’t relevant to The Time Machine since George doesn’t meet himself, but it’s relevant to other sci-fi stories.)
So there you have it. Those are the three metaphysical conditions that would make time travel possible. Oh, and watch The Time Machine if you get the chance.