A Time Travel Dialogue
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3. Wednesday

The three are seated at a table. The speaker, Dr. Twitchell, has just concluded a talk about the expanding universe. Lunch is being served.

Carlene: It looks like we’ll be gathering a lot more data about our mystery particle in the coming weeks. I might even say I hope it’s time-traveling.

Willie: I’m still thinking about Dr. Twitchell’s talk; it’s always struck me as odd to describe the universe as expanding. What’s it expanding into?

Dr. Rufus and Tad share skeptical glances.

Tad: I don’t know about that, Willie, but something similar has been bothering me about our time-travel hypothesis. I’m wondering where the particle could go.

Carlene: I don’t understand.

Tad: If the psi-lepton reverses its direction in time, traveling from the present to the past, then we should acknowledge that the past exists, but that’s not right. I mean, whatever happened yesterday, or even a moment ago, is done; it happened in another time, and that time no longer exists. This is now; the past—and the future, for that matter—don’t exist. How can the psi-lepton time-travel if there’s nowhere for it to go?

Dr. Rufus looks interested, but Willie is unimpressed.

Willie: That sounds like some arguments against time travel that I’ve heard from proponents of presentism.

Tad: And what’s presentism?

Willie: Well, although it’s defined somewhat differently by different philosophers, presentism is generally taken to hold that only what is present exists.

Tad: Okay, good, that seems pretty obvious to me. But I get the feeling there isn’t general agreement that that’s the way the world works.

Willie: Perceptive as always, Tad. Many philosophers believe in some form of eternalism, which contradicts presentism in holding that some non-present things exist. Eternalists believe that, in addition to what is present, both what is future and what is past exists.

Tad: So, you’re saying that a presentist would claim that Albert Einstein doesn’t exist, which is obvious, but that an eternalist would say he does exist?

Willie: Yeah, but the eternalist would concede that Einstein doesn’t exist now.

Tad: That doesn’t make any sense to me. How can you claim that something exists but doesn’t exist now?

Willie: Well, I’m not claiming that, but the eternalist would just be saying that Einstein exists in the past but not in the present.

The server arrives with three lunches.

Carlene: It actually seems obvious to me that Einstein exists, not now, but in 1905, for instance. We’re talking about him, aren’t we? Willie, I guess you would classify me as an eternalist. But I’m wondering, wouldn’t the presentist admit that as well? Wouldn’t he concede that, though Albert Einstein doesn’t exist now, he does in 1905?

Willie: Well, strictly speaking, no. You have, however, come upon a key difference between presentism and eternalism: the eternalist might say that Einstein exists some years ago, but the presentist would say only that he existed some years ago.

Carlene: That seems to be nothing more than word play.

Willie: In a way, it might be. I’m not entirely sure myself.

Tad: But what about my objection to our particle’s being a time traveler?

Willie: There’s a lot to be said. Let’s make sure we have the argument right. You claim that if there’s only the present, and if time travel requires a destination other than the present to travel to, then there’s nowhere for a time traveler to go—better yet, no when for the time traveler to go.

Tad: Once again, Willie, you know exactly what I said.

Willie: Good. It seems to me that we have a few relevant options: we can give up our time-travel hypothesis while granting that the argument is sound, or we can give up presentism—there’s still eternalism, after all—or we could hang on to both time travel and presentism and then try to uncover some flaw in the argument that doesn’t demand that we reject presentism.

Tad: The argument seems foolproof to me, including the assumption that presentism is true. I lean towards giving up the time-travel hypothesis.

Willie: We know that’s how you see it, Tad, but we might as well give it some more thought.

Carlene: As tempted as I am to outright reject presentism, I suspect there’s something wrong with Tad’s argument. Sorry, Tad. It appears that if Tad’s argument works—if it’s sound, as you philosophers say, Willie—then all forms of time travel must be impossible. I doubt that presentism has such a strong consequence.

Tad: I think it does have such a consequence. Neither the future nor the past exists if only the present does. Any form of time travel requires the existence of a time other than the present, so if only the present exists, then any form of time travel is impossible.

Carlene: Tad, think about the special theory of relativity, though; it tells us that at least a certain kind of forward time travel is possible. Time passes differently for material objects that take different paths through space and time; for example, a twin who makes a roundtrip at nearly the speed of light to a distant location ages much less than his twin who never leaves Earth. We actually know that this kind of time travel takes place; in one experiment, two atomic clocks—one placed on a plane and flown around for a few hours, and the other left on Earth—experience different amounts of time. The ability of atmospheric muons to reach the ground, the phenomenon of Thomas precession, and even the quantum-relativistic effects that give us the glitter of gold are all examples of well-understood physical phenomenon that involve this kind of time dilation. The important point, though, is that it seems that at least one form of time travel is very real.

Willie [looking back at Tad]: A hypothetical case makes the same point. If a woman were cryonically suspended for ten years before being revived, she would experience something like forward time travel. She might be convinced that it’s 2010 when it’s really 2020; she would be able to describe nothing about her trip but would have excellent recall of events immediately prior to suspension. That seems perfectly possible and like a case of time travel, so something must be wrong with your argument.

Tad: Seriously? That’s not time travel!

Willie: Why not?

Tad: I’m not exactly sure. Even the atomic clock traveling at high speeds seems a little too mundane to be time travel, but I’ll grant you that case, Professor. The deep freeze is a different matter.

Carlene: I agree with Tad about the cryonic process. In terms of the physics, there’s nothing interesting going on temporally. The Northern wood frog, a species that regularly freezes solid during the winter, is not a species of time-traveling amphibian; there are intra-cellular and sub-molecular processes taking place even though little or nothing is happening at macroscopic levels.

Willie: Fine, but whether freezing and thawing is a way to time-travel is irrelevant. All I said, by the way, is that it was like a case of time travel. The point I want to make is that what’s important about traveling is not that the destination be there when the traveler starts out, but that it be there when the traveler arrives. It doesn’t matter whether the year 2020 exists when our woman is frozen, so long as it exists when she thaws. Similarly, it doesn’t seem important that the arrival time as experienced by the clock on Earth exists when the plane starts to accelerate, but it’s important that the arrival time exists when the plane returns. Tad’s argument seems to overlook this particularly salient detail.

Tad: I don’t follow.

Willie: Suppose you just graduated from high school. You and some friends have heard of an amazing theme park that’s being built in Zimbabwe, so you decide to go there for an extended graduation trip, figuring that if you start walking when they hand you your diplomas, you’ll get to the park just when it opens.

Tad: Okay, that’s a pretty crazy story, but I’ll play along. I guess my friends and I will need to do some swimming during the journey, too.

Willie: Good point. So, with your first step, and eventually your first backstroke across the Atlantic, you’re traveling to the theme park, right?

Tad: Sure.

Willie: Well, the theme park doesn’t exist yet, so you’re traveling to a place that doesn’t exist. If that’s the case, then one of your premises is false, and your argument is unsound.

Carlene [smiling]: Just a minute, Willie, I think you’re trying to trick Tad. Perhaps the theme park in Zimbabwe doesn’t exist yet, but the space where it will be exists, so Tad and his friends aren’t really traveling to a place that doesn’t exist.

Willie: Okay, okay, I’ll concede that my example has that flaw, but—with Dr. Twitchell’s talk in mind—we might suppose that our universe expands by creating new spatial locations; then you could take a trip to a place that doesn’t exist. Of course, by the time you get there, it’ll exist.

Tad: I still don’t follow. If yesterday doesn’t exist, then I can’t travel there; if tomorrow doesn’t exist, then I can’t travel there. It would be like meeting Godzilla or traveling to the Fountain of Youth; it can’t be done.

Willie: The difference is that the future will exist, and the past did exist. Godzilla and the Fountain of Youth never existed, don’t exist now, and never will exist. You can be traveling to a spatial or temporal destination that doesn’t exist yet; you just can’t arrive until it does. When you’re traveling to some time or place, you’re engaging in traveling behavior, but you don’t need to be simultaneously arriving anywhere.

Carlene: Is that true, Willie? In the expanding-universe example, even if I want to travel to a region of space that doesn’t exist yet, on the way there I would have to travel through—or arrive at—all the intervening space that does exist; the intervening space is what seems to make the trip possible. The analogy doesn’t appear to hold up; if presentism is true, if the present is all there is, then there’s no intervening time through which to travel.

Willie: But in a presentist’s universe all of the intervening times will at some point be present.

Carlene: That’s not really the issue, is it? It seems that in order to travel to some place, we must travel through all the intervening locations; in space, these are readily available, but in a presentist universe there’s only the present. Maybe we could, in manner of speaking, ride the present until a later time exists, but is that time travel? In the same way that the wood frog isn’t time-traveling while it’s frozen, the normal passage of time isn’t time travel.

Willie: I would say that the normal passage of time is a form of time travel—albeit a limiting case of it—but you’re right that we don’t go out of our way to think of it like that. More has to be going on than just riding the present for an interesting case of time travel.

Tad [interrupting]: Professor, Willie admitted you were right!

Carlene: I don’t think Willie was finished, Tad.

Willie: I was going to point again to the twin-paradox case. The traveling twin occupies intervening positions in space and time on his way away from and on his way back to Earth, but what makes it time travel—genuinely interesting time travel—is how so much less time passes for him than for his stay-at-home twin and everyone else on Earth.

Tad quiets down. The server arrives, clearing the table and offering coffee. The offer is eagerly accepted.

Tad [returning to business]: I still think time travel is incompatible with presentism; actual time travel would require arriving at some non-present time, which presentists deny exists.

Willie: Look, how about this. A time traveler enters a time machine now and will arrive in 2020. The presentist should be fine with that.

Tad: There’s still something screwy. This ‘will arrive’ thing bothers me. Until your time traveler arrives in 2020, it isn’t true that she arrives at that destination. So, how can anyone now be time-traveling?

Willie: Sometimes our present-tense statements require for their truth what will or did occur to happen in a certain way. If our waiter is now placing arsenic in your coffee, isn’t he now committing a murder, even though for that to be true it must also be true that, not knowing any better, you will drink the coffee and die? To be time-traveling now, you must be engaging in some sort of traveling behavior that causes that you did arrive or that you will arrive.

The coffee is delivered. Tad scrutinizes the contents of his cup.

Tad: Now it sounds like you’re saying that being a time traveler doesn’t really require that the past or future exist. It seems that all it requires is engaging in a certain behavior like traveling at high speeds or pulling the lever on a time machine. I have to admit that this sounds pretty reasonable. Traveling to ancient Greece implies that ancient Greece exists, but the presentist definitely denies that ancient Greece exists. Traveling at high speeds? Pulling a lever? These things seem perfectly consistent with presentism.

Willie: Then, maybe, we’re in more agreement than I first thought. I was thinking of time-traveling to the past as consistent with presentism because I understood this as requiring only that the past did exist. You say that it also requires that the past exists and so see traveling to the past as inconsistent with presentism. Yet we can agree that I can be time-traveling even if presentism is true just by seeing time-traveling as a matter of engaging in the right kind of behavior. So, we seem to be in agreement on the important point that time-traveling is consistent with presentism.

Tad: So it seems.

Carlene: Setting aside my worry from before, Willie, it might be helpful if you could tell us how some time-travel example fits with presentism.

Willie: Well, if one of you will outline one of your favorite time-travel plots, I’ll show you how the same story can be told in a way that’s consistent with presentism.

Tad: Okay, how about Star Trek IV: The Voyage Home. Kirk and the Enterprise crew travel back to twentieth century Earth, interact with some nobodies like us, find two humpback whales, and then take those whales back to the twenty-third century to stop an alien probe from destroying Earth.

Willie: Sure, I don’t remember any obvious contradictions in that one. And remember, we said that presentists don’t deny past- and future-tensed truths. So, suppose it’s two days after the Enterprise crew picked up the whales sometime in the 1980s. At that time it would be true to say that there was a ship that picked up two whales two days ago, and in 300 some years that ship will appear near Earth.

Carlene [interrupting]: I get the point. All you have to do is express the elements of the story in different tenses to make it fit with presentism. I’m still not convinced this isn’t just a way of playing with the language.

Tad: Hang on, Professor, I just thought of something. Maybe the part of my objection about not having anywhere to go is dead, but I still think there’s disagreement between presentism and the time-travel hypothesis. Willie, what about the causation our particle must be involved in if it’s time-traveling?

Willie: I admit that there must be some strange causation; no matter which view—presentism or eternalism—is correct, backwards causation is troubling. But I don’t think the type of causation we’re worried about here raises any special issues for presentism.

Carlene: Surely, though, if only the present exists, causation must occur only in the present.

Willie: Be careful to remember that causation isn’t an event; it’s a relation between events. Even though we sometimes talk about causation as if it’s the kind of thing that happens in time, it doesn’t actually. More importantly, there are countless apparent examples of causal relationships between present and non-present things; for example, the Big Bang caused the Earth, like every other material thing, to exist. Many of the things we do today will have effects in the future. If presentism is true, then it has to be able to explain how there could be any causal relationships at all, which is something presentism has to do whether or not time travel is possible.

Tad: Okay, so the apparent discrepancy between presentism and causation isn’t limited to issues of time travel, meaning that the issue about causation doesn’t really help us determine whether time travel and presentism are compatible.

The server returns and refills Willie’s empty cup. Willie offers the server a thumbs up.

Carlene: The thing that most worries me about presentism is scientific in nature, but we don’t need to get into that right now.

Willie: No, go ahead, I’m curious.

Carlene: Okay, so what worries me is that presentism seems to presume an absolute frame of reference that distinguishes what’s real from what isn’t; the present seems to consist of all and only those events simultaneous with right now. This is quite a departure from Einstein’s relativity. The hallmark of relativity is that it doesn’t include any notion of absolute simultaneity; simultaneity is a frame-dependent notion. If this is right, then presentism has to be false.

Willie: Yeah, that does seem to be a blow with respect to the truth of presentism, and—as I may have demonstrated earlier—I’m not exactly prepared to defend presentism until its dying day. This issue isn’t really a problem distinctive of presentism, though; lots of our ordinary ways of thinking about time, space, and motion look to be at odds with relativity. Just as these ordinary ways of thinking have to be reconsidered in light of relativity, maybe elements of presentism need to be reconsidered too. Just as I’m not ready to say there’s no such thing as motion and length, I’m not ready to conclude, well, that there’s no time like the present.

The three share a chuckle.

Tad: Good one, Willie.

Willie: Just to be clear, even though I said “no time like the present”, what I mean is that I’m not ready to conclude that times other than the present actually exist.

Tad [interrupting]: Lighten up, Willie. It was amusing; we get it.

Carlene: Once again, this philosophical discussion is well and good, but if it is possible that the particle is time-traveling backwards in time, then it’s up to science to show whether it’s actually doing so. So, let’s table the discussions for a while, until we have some more data to back up our ideas. With any luck—and some computer wizardry from Willie—we’ll make some progress tomorrow.