The Philosophy of Time Travel

Like many children of the Eighties, I consumed plenty of pop culture takes on time travel. But one thing that always frustrated me was the storytellers’ apparent indifference to the question how time travel was possible at all. Examples ranged from Terminator’s complete silence to Back to the Future’s “flux capacitor,” an admission that something physics-y was happening, even if no one knew what it was. Perhaps the 2004 film Primer came closest to tackling the underlying mechanics, but its relative obscurity and reputation for difficulty testify to the fact that, when it comes to time travel, audiences have always been a lot less interested in the how than the what-ifs. Time travel is a fun plot device, but that’s about it.

Fast forward twenty years or so. I’m now a stay-at-home father of three with a lot of time to ruminate on philosophical matters as I vacuum carpets, sweep floors, wash dishes, and fold laundry. And maybe it was the nature of so many of these domestic duties, duties that basically involve battling the forces of entropy, that led me to realize that time travel has nothing to do with going anywhere, and everything to do with returning the universe to some previous configuration. The phrase “time travel” suggests the past is a destination, but it’s a lot more helpful to think of it as a particular arrangement of matter and energy.

The idea that “time” is really just another word for the effects of entropy is not new. In A Brief History of Time, Stephen Hawking described the “arrow of time” as the evolution of the universe from less to more probable configurations of matter and energy. But if you really commit to this understanding of time, it does have some interesting implications for the concept of time travel.

For one thing, while the Second Law of Thermodynamics cannot be violated for the universe as a whole, localized entropy reversal is possible. Creationists have tried to point to the emergence and evolution of life as proof that God must have played a role, since otherwise, they claim, there is no way to account for such improbable configurations of matter and energy. Their error lay in taking the Earth to be a closed system, when in fact it receives a great deal of energy from the sun. That’s the key point: inject enough energy, and you can reverse entropy, at least at small scales.

Yet recreating the past isn’t just a question of making a local area of the universe evolve towards a less probable state, but towards a specific state. And for that you wouldn’t just need gargantuan amounts of energy, you’d also need unfathomable quantities of data about the precise location of all the matter and energy at that particular state. You would need a machine capable of crunching all that data and focusing all that energy on all those points.

And your machine would have to be huge. How huge? Well, the light that is just now hitting my eyes had to travel 186,000 miles in the last second. That means that the more states you have to pass through to get to the goal state (i.e. the farther back in time you want to go), the more data you will need from an ever-expanding volume of space. To return the Earth to the configuration that obtained one day ago, you’re already looking at incorporating data from a volume of space stretching out billions of miles in every direction.

So as a practical matter, bringing back the past is impossible. The more details you omit from your calculations — so as to need less energy and data — the less your “past” resembles the actual past. And even if you could construct a machine capable of accounting for the fall of every sparrow, as it were, the universe in which your recreated past existed would differ in an important respect from the universe of the actual past. Consider your refrigerator. If you’ve ever felt behind it, you’ve noticed that it gives off heat. It takes a lot of energy to keep the temperature inside steady and cold. And as anyone who has been through a prolonged power outage knows, when you cut the power, the temperatures inside and outside the refrigerator eventually reach equilibrium. Similarly, since the Second Law of Thermodynamics cannot be violated for the universe as a whole, the surface area outside our expanding volume of localized entropy reversal would have to be radiating heat. And so, however much the universe inside the zone of entropy reversal resembled prior configurations of matter and energy, the existence of this heat halo would testify to the fact that the universe was still evolving toward a more probable state. Our artificial past would be just that — artificial — embedded in a universe still hurtling into the future.

But time travel may be impossible even in principle. Even if we could marshal the levels of energy necessary to reverse entropy on so large a scale, the data being fed into our hypothetical machine would still reflect the ontological assumptions of the machine’s makers. The universe will unfold into the future on its own, according to its own physical laws. We understand those laws to an amazing degree, considering how little time we’ve devoted to their discovery. But our understanding is not perfect. Nor could it ever be, given that all theories are necessarily the products of finite minds drawing inferences to the best explanation of their finite observations. We cannot hope for a final, irrefutable Theory of Everything for the simple reason that we can never occupy an observational posture that encompasses, not just all actual experience, but all possible experience as well.

And the thing is, to truly recreate the past, our machine would have to incorporate just such an impossible Theory of Everything. Remember, when our machine recreates past configurations, the fidelity of that recreation to the actual past can only be as good as our understanding of the present and the laws that govern the processes that got us here. The more states we have to pass through to get to our target configuration, the more opportunities there will be for discrepancies between the actual laws of nature and our understanding of those laws to creep in. Those discrepancies will in turn cause our artificial past to deviate more and more from the actual past. It will be impossible for us to say how and to what degree they diverge, but we cannot deny the divergence will happen.

So time travel is impossible not only in practice but also in theory. Is that the end of the story, then? Not at all. Since you’re here, we might as well see whether this account of time travel can provide a new spin on some old questions the genre has always traded in. For example, can you change the future? If, as I’ve argued, the past we recreate will diverge in unknowable ways from the actual past, then the answer to that question is tricky. On the one hand, since you cannot recreate the past perfectly, any future that evolves from the recreated past will almost certainly deviate from the future that evolved from the actual past. So yes, you’ve changed the future, but not because you made different choices in the same past. If “changing the future” assumes being able cause a past A to evolve, not into the future B, but into an alternative future B’, then the answer is simply “no,” because the first part of the hypothetical is impossible. We don’t even have to wade into the thorny philosophical question whether hard determinism is true.

A more interesting question is who “you” refers to in these hypotheticals. Of course, if you recreate a past configuration that obtained before you existed, you’d simply be annihilated, Benjamin Button-style. But if you recreated a past in which you existed, that recreation would, as we’ve seen, deviate in some unknowable way from the actual past. The “you” in that scenario would not be the you from the actual past but a facsimile. The closest analogy I can think of in the philosophical literature is the “you” that emerges from the teletransporter in Derek Parfit’s famous thought experiment.

The genius of that thought experiment was the way it laid bare the inadequacy of our intuitive concept of identity. Recall that Parfit gives two versions of the experiment. In one version the teletransporter breaks up your body, records the nature and position of all the atoms, and then reconstitutes them in another place. You emerge with the feeling that you have fallen asleep and woken up somewhere else. In the second version the teletransporter simply records the position and nature of all the atoms that make you up, then copies them, but without breaking them up. Now there are two of you, and each feels that it is the real you, though our intuition tells us that the copy is just that — a copy. But if the “you” that emerges from the other end of the teletransporter in the second version is not really you, then why should it be you in the first version?

To resolve the apparent contradiction between our intuitive judgments, consider Robert Nozick’s “closest continuer” argument, which provides a nice practical test for resolving thorny metaphysical questions of identity. Basically, an object x at time t’ is the same x that existed at time t if it is the closest continuer of x at t. Since the undeconstructed “you” in the second version of the teletransporter story not only has the same appearance and memories as the “you” that emerges from the other end of the teletransporter, but is also physically located exactly where you were before entering the teletransporter, it is the closest continuer. Applied to our time travel hypothetical, since there is no rival “you”, the “you” that exists in the recreated past is just you. Easy-peasy.

So in conclusion, time travel is not possible. Not in practice. Not in theory. Even if it were, it would shed no new light on questions of free will, determinism, or the nature of identity. It’s almost as if thinking about time travel is a complete waste of time. But before you start wishing you could travel back in time and not read this article, remember this: now that you know there’s no point in thinking about time travel, you won’t ever have to waste any more time worrying about it. See! I’ve actually saved you more time than a time machine ever could!

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