It’s common to think of time as though it were a medium one travels through in the same way one travels through space–as in, you can travel one meter through space, or you can travel one second through time. Popular science education reinforces this idea by telling us that time is just another dimension, like the three spatial dimensions of up/down, left/right, and front/back. Yet there seems to be a weird exception in that we can only travel through time in one direction at a constant speed, whereas we can travel through space in any direction and at any speed we like. What if that weren’t the case? What if time’s seeming constancy were simply a technical limitation that someday we could solve? What if we could travel in either direction through time, just as we can now travel through any direction in space? This is the “science fiction” conception of time travel, and although it’s an immensely fun idea that has enjoyed huge popularity for over a century, it rests on a false analogy: time “travel” is a complete misnomer. The seemingly one-way nature of time, and its interconnectedness with space, are foundational to our conception of what it means to “travel” somewhere at all–without them, the term becomes meaningless.
The first problem with the science fiction conception of time travel is that time and space are not, in fact, two different media that we can travel through independently: they are the same medium. Any travel through one implies travel through the other; in order to get from point A to point B in any real-world situation, you have to spend some positive amount of time doing it. That is why we measure speed in units of both distance and time (as in meters/second, or miles/hour)–that is to say, space/time. That’s all speed is: an implicit admission of the “space-time continuum” that Einstein’s theory of special relativity made explicit. Any travel implies a speed at which that travel happened, but speed through time only makes no sense. Though it sounds cute as a tweet or a Facebook post, you do not actually travel through time at the “rate” of one second per second any more than you travel through space at the “rate” of one meter per meter. Even measures of distance in space or time by themselves are completely subjective and tell us nothing about whether we might be able to travel that distance without further context. For instance, is a thousand miles far away? Well, it depends on how far in time you have to travel while crossing those thousand miles. Do you have to be there in the next ten minutes or can you take ten years to make the trip? Can you get there in a rocket or do you have to travel by foot? Similarly for time–is one second “far” away? The answer depends entirely on the distance traveled in space during that time–if you’re just sitting there, one second isn’t very far at all. If you’re traveling near the speed of light, it’s a very long time indeed!
So time isn’t an independent medium we can travel through without also traveling through space. But might time travel still be possible? We can travel through space in any direction we like, after all. Why can’t we travel through time the same way? Why can’t we go to the past or the future the same way we can go up or down? This is the second problem with the false analogy of time travel: although there’s no law of physics that prohibits “turning around” the arrow of time (or even causality!), the concept of “travel” into the past is not well-defined. What most people think of when they think of traveling to the past is hopping into a time machine, “going back” a million years or so, stepping out and looking at some dinosaurs, then hopping back in and traveling “forward” again to the present day so they can tell all the eager paleontologists what color the dinosaurs’ feathers were. So what’s wrong with that? What’s wrong is that in this scenario, everything outside the time machine is traveling backwards in time, but the time machine and its occupant are somehow still traveling forward–otherwise the time machine would be dismantled and its occupant un-born long before any dinosaurs rose from their fossil beds. In principle, we could be traveling back and forth through time constantly–erasing and re-forming memories as we go–but only one direction at once, just as we can go up or down in space, but not both at the same time. The idea of getting into a time machine and stepping out into a world inhabited by dinosaurs is roughly analogous to getting into an elevator, traveling to the 100th floor, then stepping out into the lobby: it’s not that the laws of physics prohibit it so much as our understanding of what it even means to “travel” somewhere makes the scenario nonsensical, like asking someone what 2 plus blue equals.
Although in principle we can “travel” into the past, that act of travel will necessarily erase our memory of the experience, meaning that our subjective perception is of a fixed, unchangeable, and unreachable past. We can’t remember two different versions of history at the same time for the same reason that we can’t be in two different places at the same time. Yet that still doesn’t answer why the past seems so different from the future–if the laws of physics give no special treatment to either direction in time, why does it seem like it only goes one way? Why does the past seem fixed and stable, while the future seems fluid and limitless? This is where entropy enters the picture: time, ultimately, is a measure of change, and change means entropy. Our traditional conceptions of the one-way arrow of time and the fixed and singular nature of the past are reflections of the fact that when things change, they do so by becoming less ordered, simply because there are far more ways in which something can be disordered than it can be ordered. That’s all that entropy is–the idea that it’s much easier to break, say, a piece of glass than it is to fix it, because the glass can be destroyed in an infinite number of ways (scratched, cracked, broken in pieces, ground to powder…), but there is only one way in which it can be perfect. That’s why time seems to flow in one direction only, and why the past seems fixed while the future does not–there’s only a very small number of ways in which something can be ordered, so it seems clear that the past is fixed, but there are infinitely many ways in which it can be disordered, so the future appears malleable.
Okay, so that (hopefully) explains why we experience things happening in a particular order, and why that order seems fixed. But why can we only remember things on one side of the timeline? If there’s no distinction in principle between the past and the future except that the past is more ordered, then why are our memories so preferential? Why don’t we have access to the future in the same way we have access to the past? The surprising answer is that we can remember the future: we call it prediction, and we do it all the time. I can bet you’re rolling your eyes right now–“right, but imagining what might happen in the future isn’t really like remembering what actually happened in the past, is it?”–but in fact, it is! The mechanisms in our brains that predict what might happen in the future are the very same ones that remember what has happened in the past. Memory and prediction, like time and space, are indivisible parts of a single whole–the only difference between the two is that when we remember something we’re predicting what has happened, rather than what will happen. And as our discussion of entropy shows, it’s no surprise that this is easier–there are far more possibilities for the future than there are for the past (remember the glass?), so it’s easier to get predicting the past right. Not that our memories of the past are infallible–far from it. In truth, we don’t really have “access” to the past or the future, only the constantly-changing present. Even supposedly infallible recording devices such as cameras only give us an imperfect and incomplete (i.e., less-ordered) representation of the past. If we could somehow recover that lost order completely–say, through science fiction time travel–we would be able to do all sorts of impossible things like break the speed of light, create infinite amounts of free energy, travel to any location instantaneously, destroy the very idea of causal relationships–basically, we would be able to do literally anything. Depending on your attitude, that probably makes time travel seem either much less or much more appealing, but in either case it should at least make clear that if it isn’t impossible, it will almost certainly never happen–because if it did, even once, it would erase all of history. We would never have existed! Don’t let it get you down, though–the next time you’re watching Dr. Who or Star Trek IV, just think of it as taking place in a world where physics works differently…an alternate timeline, if you will…
See you next week.
I don’t consider myself an expert on time or physics by any means, I just happen to have some very smart friends who have taught me a lot about them, and I wanted to share some of the conclusions I’ve drawn here. If I’ve misrepresented anything or made any factual errors, please let me know in the comments and I will gladly correct or clarify them.
 The only way such same-unit ratios make sense is when comparing relative speed–that is, if I’m traveling at 90% the speed of light relative to some observer, and you’re traveling at half the speed of light in the same direction relative to that same observer, the observer could say we were traveling at roughly the rate of two seconds per second, and about two meters per meter, relative to each other.
 It’s arguably the very definition of intelligence!
 At least, for conscious or so-called “declarative” memories. Things are a bit more complicated for other kinds of learning like muscle memory, habituation, or operant and classical conditioning–but no matter the mechanism, all types of memory and learning ultimately entail prediction as well.
New evidence suggests that time may be an emergent consequence of quantum entanglement, and that an “outside” observer would see our universe as fixed and static–meaning time is even less like travel than we thought!