by Tim Sommers
Stuck inside? Unable to travel? Have you considered traveling through time instead of space? Time travel is impossible, you say? Wrong. We are, each and every one of us, time travelers, traveling forward second by second, hour by hour, day by day into the future.
Why not consider traveling in the other direction? If you are afraid of traveling to the past because you fear paradoxes – killing your own grandfather, stepping on a butterfly while fleeing from a dinosaur, etc. – I have great news for you. My new theory of time travel will allow you to travel into the past without creating any paradoxes – guaranteed! (Time machine not included.)
Let’s not go straight to the big payoff. Let’s work our way up to it. Hannibal Buress has pointed out that when people go through something rough in life, they sometimes say, “I’m taking it one day at a time.” Hannibal says, “Yeah. So is everybody. That’s not a philosophy. That’s how time works. If there’s a way to go faster, let me know?”
I am here to help. You can move through time more quickly than your friends and family (that is, travel to the future faster than they do). Here’s how. Either go very fast or stand close to something really massive.
You might say, Tim, I know that it follows from Einstein’s general theory of relativity that there is no universal “now”, that simultaneity is an illusion, and that we all travel through time at different rates depending on our speed and proximity to extremely massive objects, but do you have any empirical evidence that this actually, really happens in the real world?
I’m glad you asked.
There are at least two kinds of evidence for this. If you take a relatively short-lived particle whose half-life is predictable and you accelerate it to a significant fraction of the speed of light in a particle accelerator it will persist for an unexpectedly long time. (Well, not really unexpected. It’s typically expected by the kind of people who would accelerate it like that, but not necessarily by you, which is what I meant.) (https://www.scientificamerican.com/article/einsteins-time-dilation-prediction-verified/)
Too far from home? Get this. NASA put one twin brother into orbit – at 28K/h for 520 days on the International Space Station – and kept the other on Earth as a control. Guess what? The brother on Earth, who used to be six minutes older, is now six minutes and five milliseconds older. Crazy, right? (https://www.space.com/33411-astronaut-scott-kelly-relativity-twin-brother-ages.html)
That’s all well and good, Tim, you might say, but the fact that we can vary the rate at which we travel into the future doesn’t show we can travel into the past! Why think that? I was going to say, well, the time-reversibility of all the laws of physics would seem to indicate that travel in both directions is at least theoretically possible. But why get into all that? Haven’t scientists actually succeeded in sending particles back in time? (https://www.nytimes.com/2019/05/08/science/quantum-physics-time.html) No. Not really. (https://www.technologyreview.com/2019/03/14/103311/no-ibm-didnt-just-reverse-time-with-a-quantum-computer/) But, okay, I should have just said, trust the physics. Traveling back in time is, at least, theoretically, physically possible.
Perhaps, you’re thinking, Tim, if one day people were able to travel back in time, certainly they would do so, and if they would, then they have, and if they have, then where the heck are they? In the whole history of the world, we have not one confirmed instance of a time traveler. (https://www.snopes.com/fact-check/time-traveling-hipster/) This is the Fermi Paradox for time travel. Well, I can explain it. Bear with me.
So, what about those other paradoxes? If you could travel to the past, you would be able to change things in the past which would cause changes in the very future that you yourself come from. That can’t be right. Forget killing your grandfather, you could be your own father and mother (https://gist.github.com/defunkt/759182/ad44c6135d168ae54503a281bb7e1a24c6c2ea0c). If you, to use another example, go back in time and destroy all life on Earth, then where did you come from? It just doesn’t seem to make sense that you could causally affect the past. So, logically, you can’t.
But what does that mean? If you can travel in time, how is it that you can’t change the past?
Obviously, I don’t know. But I have a theory. You might say, Tim, would you please just get to that theory. But, no. I want to discuss alternatives first.
So, maybe, you can go to the past and do what you want and whatever that is must turn out to be what you have already done even though you didn’t necessarily know that before you traveled back. But suppose that’s true. Suppose, for example, I travel back in time and save General Washington from dying of pneumonia at Valley Forge causing it to be the case that the North American Commonwealth successfully rebelled against British rule in the 18th Century and is now the United Territories of America or something. Well, then, I can go back again and assassinate him and make everything again as it actually is in our world. I’ll bet you a pound on that. So, saying you can, but then already have, doesn’t save us from paradox.
What if the Everettian or Many-Worlds Interpretation of quantum mechanics is correct and the universe is constantly splitting into many separate, distinct worlds? Some have thought that it would avoid paradoxes, if you are only able to travel to the past of other worlds or (as is sometimes said) travel to different, alternative time-lines. The problem with that is that, of course, it is not time travel. It’s just being able to travel to parallel worlds. By definition, this leaves me unable to travel into my, or our, actual past.
So, what about this? What if there is some physical constraint that demands that for every second into the past you travel, you also must move, or are moved, one light second away from where you started? For every year you travel into the past, you also move one light year away from where you started?
If you are unable to move into the past without simultaneously moving away from the point you start from, then paradoxes become impossible. If you want to go one day back in time to win the lottery yesterday with today’s numbers, for example, you will end up, spatially, one light-day away from any gas station that sells lottery tickets, out past the edge of the solar system, further away than Voyager 1 (https://www.dailysabah.com/science/2018/12/10/nasas-voyager-2-reaches-interstellar-space-18-billion-kilometers-from-earth), the human object farthest away from Earth ever. Even if you have a rocket-propelled time machine, the universal speed limit, the speed of light, 299,792,458 m/s, will ensure that you cannot get back to Earth fast enough to play those sweet, sweet numbers.
So, there it is. My new theory of time travel. The spatial-displacement, causal paradox avoiding, theory of temporal displacement or the theory of spatial/temporal displacement or, as I prefer to call it, STD.
Some have already said to me, Tim, that’s not a theory. To these I have said, What do you mean? They have said that there’s no reason to think that time travel simultaneously creates spatial displacement in the way I say it does. You have no underlying physical mechanism, they say, nor any mathematical model, they say. It’s not a theory, according to them.
STD does make testable, empirical predictions. It’s falsifiable. It’s a theory, dammit.
Or, anyway, part of a theory. When Einstein was developing general relativity, at some point, he said, “What if we assume that the speed of light is somehow constant from any and every frame of reference?” He, like me, had no real evidence for his assumption. Not yet. It was just a theory. So, as you can see, in this analogy, I’m Einstein, which makes it seem more likely that I am right. And STD is maybe not a complete theory. But it could be a piece of a complete theory. It’s a postulate that solves one problem for any future theory of time travel: how to avoid paradoxes.
That’s no way to build a theory! you might say. Well, tell that to David Lewis. In his hugely influential book, “On the Plurality of Worlds”, he argues that all possible worlds exist concretely. His evidence? It makes modal logic easier to just assume that they do. Or, rather, if possible worlds are real things that that are simply isolated from each other, this gives us a way of understanding counter-factuals that helps our understanding of, for example, causation by giving us possible worlds to compare ours with. We want to know the closest possible world to ours, for example, which is also different from ours, in the way we want to understand. (Interestingly, Lewis argues that in explaining singular events, worlds with different laws of nature are farther away from us than worlds with the same laws of nature but in which miracles sometimes occur. (To me, the miracle is that he doesn’t stop to wonder how he knows for sure that that world is, in fact, not this (that is, our) world. But that’s a topic for another day.))
So. Hey. You know what? You don’t like David Lewis? You don’t like Einstein? You say it’s not a theory? Give me this much. It’s good advice. It explains how to avoid paradoxes and, as a bonus, it explains why we haven’t met any time travelers. (Just to be clear: If there were time travelers from around here, they’d be very far away from us now, and if they were from very far away, they would just be part of our now now).
If you are planning to travel back in time, to avoid paradoxes, please make sure that you always move a distance, spatially equivalent to your temporal distance, away from your starting point. STD may, or may not, be borne out by future physical and cosmological models. In the meantime, though, like I said, it’s damned good advice.