Time is intangible but stubbornly persistent. We can’t grab a moment in time, nor can we point to it once it’s gone. In 2014, I spoke to science writer James Gleick about the nature of time travel, as part of the ongoing Scientific Controversies series. Gleick, whose bestselling books include Chaos (1987) and The Information (2011), was working on a history of time travel—a concept that’s a staple of science fiction but also a vital lens, as our discussion revealed, through which to examine physicists' understanding of reality. Our conversation was first published in Intercourse 3.

I thought we should talk about time because I know you’re thinking about it a lot.

The more I immerse myself in the history of time-travel culture since H. G. Wells, the more I think the clear-headed people—the people who never for a second believe in time travel—are the science fiction writers. More and more, as the century goes on, the physicists—the smartest physicists in the world—are willing to entertain the idea. I don’t know where you stand, Janna, but we’ll find out.

Well, I’m agnostic on certain things.

Okay. So I want to have the conversation where I am the man on the street, and you are the modern physicist who understands, as you texted me moments ago, that time is an illusion.

I would say that the problems posed by things like determinism and the arrow of time are questions that are totally crucial to how we’ll eventually understand the next step in physics. It might be as big a revolution as when Einstein first looked at Newton’s idea of permanent time existing out there as a stage, and space existing out there as a stage. Einstein came along and did a subtle analysis of this, and overturned everything.

Nobody, including Einstein, predicted that from relativity comes the Big Bang and black holes and an expanding universe. He didn’t foresee the revolution in some sense. This could be that kind of a juncture. Maybe not; perhaps it’s just a tiny philosophical crisis. But it could be huge and when we pick it apart, it’ll be the same kind of thing. It’ll be like, “Boom. Now we understand quantum gravity. We understand emergence and all this stuff.”

David Albert, Janna Levin, and James Gleick at the 2014 event.

Do you mean this question about time? Or do you mean the foundational question?

I mean the questions about time. So let’s lay out what the questions about time are. One theory is that time is like a hill that exists, and that we are like little marbles rolling down the hill. We only experience falling down a hill. We never go back up the hill, and our path of least resistance down the hill is determined. Our experience is that we move through time, we never go back, and we never recover it—it’s asymmetric. But, from a bird’s eye view, the whole hill exists. And I would say that’s an Einsteinian way of thinking—that space and time exist even if they’re relative, and that we move through them. The past is there even if I can’t access it.

To clarify, and correct me if I’m wrong, it’s not that space and time exist, it’s that space-time exists.

That’s right.

It's a combination of the two that you can’t tease apart.

That’s right.

You have to put it all together into a four-dimensional block.

That’s right. So there’s a space-time, and that’s one way of looking at time. The contrary way of looking at it is to say, “Nonsense. There is no hill. It’s a carpet rolling up behind you. You walk and it’s gone. It’s rolling up as you’re moving forward in time, and beyond the instant, time doesn’t exist.” And that’s very interesting. I can’t quite land personally on either of those two conversations. I want to hear more about what people say who think about it all the time. Maybe there is somebody who’s going to sway me. But so far, neither perspective defeats the other. Maybe it’s just two different ways of looking at the same thing. Maybe it’s unresolvable.

But I was going to add that the arrow of time says, “Why can I only roll down a hill? Why can’t I turn around and go back up if time is there? If I can turn left and right, why can’t I go forward and backward?” And that’s a separate issue, in some sense, although they’re folded together.

The next question is whether the future is determined, or whether, at this moment, there are things that could happen that are not forced to happen from the set of initial conditions.

I feel that the future is open. That’s not a very scientific thing to say. It’s a caveat to everything I’m saying. I am well aware that it was intuitively obvious for most of human history that up and down were absolute directions. It is built into us. I mean with my eyes closed, if I’m waking up from sleep, I can point up, and I can point down, which is more than I can do for east and west.

But physics and astronomy have told us that that’s an illusion—those directions are arbitrary and earthbound and have to do with our sensation of gravitational fields. So I should be open, one would think, to the notion that my feelings about past and future are the same kind of illusion. But I’m not.

Except I can take you some place where up and down are reversed. Why can’t I take you some place where past and future are reversed?

We’re not there yet.

Well, even that’s interesting. Why is it so resistant? If it’s possible, why is it so hard? That seems to say time is special in some way. Like you said, I have knowledge of the past, but for some reason don’t have knowledge of the future.

Right, that was my caveat: I think the future is open. The laws of physics are not deterministic. If we’ve learned anything from quantum mechanics, we have learned that the future is open, because quantum mechanics is entirely in terms of probabilities.

But some things are fully deterministic.

What’s fully deterministic?

Well, the wave function—which determines the probabilities—is completely determined, in the same way that the marble rolling down the hill is determined.

But the wave function isn’t real. The wave function is an abstract concept.

Who’s to say it isn’t real? Before quantum mechanics, you would say, “Here is a marble. It’s rolling down a hill. Here is its position. And here is how fast it’s going.” And those are real things, and those are completely determined. Meaning, once I tell you how I kicked the marble, it’s just an inevitable fact how it will roll down the hill. So there we have two things, determinism and the reality, in some sense, of the marble.

Then quantum mechanics comes along and says, “Well the marble is not real. What’s real is the probability of the marble to be someplace.” The marble might be a little wiggly in the left or the right or it might be moving at different speeds. Those things aren’t real. They don’t exist. What exists is the probability of finding the marble at some location and some speed. And that probability is mathematically the wave function squared.

This wave function, which sets the probability, was discovered in the history of quantum mechanics as the thing that is deterministic. You put it into an equation. You grind the crank, you get one determined result.

Okay. Let me challenge that. Why shouldn’t I say, “The marble is real. The marble was always real, and it’s still real. The moon is real." The wave function is a better description of what the marble is going to do. It is a probabilistic description. It’s a better, more useful, more efficient description than the Newtonians had. It works better. But it’s an abstraction. The marble is real. The wave function is an idea.

Probably neither of us wants to get into the philosophical nomenclature of Real. I think we exploded right into the most difficult, abstract questions, and I want to take a step back to one of the more manageable aspects of this, which is time travel.

When you say you do not believe in time travel, what do you think about the fact that people have found possibilities in a hypothetical space-time for time travel? It’s not the universe that we live in, but a universe that is consistent with the laws of physics as we currently understand them, in which you could make a loop in time. Do you think that those will somehow prove to be impossible to construct? That no such universe could be made?

Of course. [laughter]

Why?

Well, I want to turn this back on you, Janna, and make the following argument: I believe that it’s the scientists who have been influenced by the sci-fi writers, because scientists’ arguments and their imaginations have been freed, in a good way, to explore possibilities that the imaginations of people in the nineteenth century could not extend to these imaginary universes where there was closed time like curves.

The reason we know that that’s impossible in our universe is, first of all, because of the notorious paradoxes: If you could go back in time, you could kill your grandfather. But you’re doing that before your father has conceived you. And so, where are you now in this picture?

Okay. Here is a way in which it might not be impossible to construct without getting into the paradox. I spoke to David Albert about this recently, and he gave me a very nice version. The dumb version is: you would not have been free to make the choice to kill your grandfather if you were on a closed path that went back in time creating a paradox. Our assumption that our free will transcends the laws of physics is silly. If you were to go back in time, you would be a person who doesn’t kill his grandfather, because you’re a person who was born from your grandfather.

So the smarter version of this is, let’s say you go back with a gun. And you shoot your grandfather in the head. It turns out that he survives this but, because of the bullet in his head, he makes some bad decisions in his life, and you’re a little wonky. As a result, you’re the kind of person to go back in time and shoot your grandfather in the head. So it gets very subtle.

If we trace those consistent possibilities literally as trajectories in this infinite space-time picture, the one in which you actually kill him dead is just not possible. You can’t reach it, any more than I can stretch my arms to the ceiling or will myself to speed faster than light.

That’s magic. What kind of adjustments do you have to make to the laws of physics to rule out the particular exercise of will that involves pulling that trigger on that particular day? It’s so ad hoc.

No, but who says I had that free will in the first place?

You’re saying we grant that the paradoxes can’t happen. We don’t know exactly why they can’t happen. Maybe they can’t happen because free will was an illusion all along…

I’m saying that’s an alternative to saying that the ability to go into the past is impossible. The alternative is to say, “No, the trajectory that takes you into the past is one that’s inconsistent with you exercising a choice, inconsistent with that path.”

But all of this assumes a kind of deterministic universe. And I still think that your people have made it clear that we’re not in that universe.

Let me try it this way. A classic physicist’s response to the weather would be to say, “The universe was created a certain way and in some sense, the laws of physics are like a computer code that crunches the numbers that specify the conditions of the birth of the universe. And it inevitably crunches those numbers. And so whatever happens on that day—whether or not the weatherman can predict it—was going to happen as soon as the universe was created.”

Then everything that happens is determined, whether it be the wave function or the actual positions or locations is kind of irrelevant at that level. It’s just the idea that things are launched. And I think you’re recoiling against that argument.

I’m saying that was naïve. And didn’t we learn that? Didn’t not just Heisenberg, but also Gödel, tell us that it's naïve?

Gödel showed that there are some numbers so complex, that in some sense, the computer code that crunches them is every bit as complex as the number itself. And, in that sense, the next digit is like a toss of a coin. If the code I write is every bit as complex as the number, I have no predictive power. And I think to some extent, yes, there’s something there. I don’t quite have a handle on what’s there, but things like uncomputability are intriguing. I don’t know where they leave us, because I can still imagine the code crunching them.

That’s the code in that imaginary computer. And the world of mathematics that Gödel was living in—that’s an abstract world. We’re living in a real world. The abstract world is something we create to help us make predictions about the real one. And, as long as we always remember that it’s not perfect, we’re fine. But, as soon as we fall into the trap of thinking that because this abstract world is determined, the wave function is perfectly determined; therefore, the marble must be determined—that’s our mistake.

I have a crazier way of restating something like this, which I’m kind of excited about. If we think about the initial conditions of the entire universe there’s no way for them to be specified because they don’t exist at some numerical level. And so, the code that crunches them starts to eat into the randomness of the universe's details. I don’t know what to say about that, but I tried once to do a formal project where there was a universe that creates its own initial conditions. And I got nowhere. So I don’t know what to tell you.

That’s exactly what I’m getting at. Look. All of determinism boils down to this famous statement of Laplace. And he starts by saying, the past and the future, they are the same. It’s an illusion to think that there is a difference between them, because if you note the initial conditions, the position of every particle, the laws of physics, all of the motion, then, a supreme intelligence, great enough to comprehend all that, would know—he used the word formula—would know the formula. And he may or may not have been thinking about God.

In modern interpretations everybody thinks you just need a computer that is powerful enough to do the computation, but if you just do that math, simple math, you discover that a computer powerful enough to do even an infinitesimal part of what you need to do is bigger than the entire universe.

Well, wait. The computer doesn’t have to be bigger if all it’s doing is crunching the data and throwing it away, unless we’re in the area where the code is as complex as the number. If I can crunch the numbers and throw them away, your laptop computer can be the one that’s crunching the universe.

Given infinite time. But we don’t have infinite time. Physicists are very well aware that there are all kinds of things in nature that don’t make sense without an arrow of time. The ink in a swimming pool doesn’t separate itself, the egg doesn’t unscramble, you know, all the things with the film running backwards. And physicists don’t understand (at least not very well) that all of those things have something in common. They have to do with complexity, with masses of particles, not individual particles. They have to do with probabilities and group behavior, with all the things you would say are emergent. I understand that they are emergent, but then, some physicists seem to say, “Well, the stuff that’s genuine, the stuff that’s real, are the fundamental laws, the microscopic laws. And there is no time in those things.” And so, we are left with a mystery of where the arrow of time comes from. I want to say, “Wait a second. You know that there is an arrow of time, and you know where it comes from—it comes from stuff like the second law of thermodynamics.”

But some people say, “If the microscopic laws don’t distinguish between past and future, but the second law of thermodynamics emerges from complex systems and does, then the arrow of time has an explanation only for complex systems.”

The universe for some reason began in an ordered state. All we have to do is answer that simple question of why the universe began that way, and it might not be hard. Then, the universe evolves to a state of greater disorder, which sets the arrow of time. When we watch a movie and things get disordered, that’s our familiar experience of the forward arrow of time. Glasses smash into disordered shards. Disordered pieces don’t spontaneously reassemble into glasses. So most people who invoke disorder as an explanation of the arrow of time consider it a cosmological process, rooted in the fact that the universe was created in a pretty ordered state.

Why do we have to care about how the universe was created? Why can’t we just talk about—

Because if it was in a state of total disorder, there would be nowhere to budge, and we’d have no experience of the passage of time. If we were in a state of maximal disorder, according to the second law, nothing much happens.

Oh, so you’re saying, “Where did the order come from in the first place?”

That’s the only question we have to answer. And that might not be that hard. Because, after all, chickens give birth to eggs, not scrambled eggs. So they might be able to imagine, for some reason, why universes are born in an ordered state.

It feels to me as though, when physicists talk about the possibility that the universe is deterministic, and that, in reality we are living in a block universe where the past and future are essentially the same, it’s just that our puny consciousnesses are not able to see it. When physicists talk that way, they are temporarily forgetting about all of this stuff. They’re forgetting about the second law of thermodynamics. They’re forgetting everything they know about the behavior of complex systems, because they know very well that as soon as you start to talk about complex systems, and ensembles of things, time does have a direction.

Well I would say not all physicists subscribe to the view that you just described. There is that sort of space-time that works in a certain domain. But when pressed, I think a lot of physicists would sort of say, “Damn, there’s something really complex there. We don’t really understand it. What’s going on with this?” I think physicists understand this works in this domain, this works in that domain. We don’t really get what’s going on in the process. When we’re at the frontier, we don’t know which path is right. And even the wrong idea contributes to the success, ultimately, by pushing the right idea forward. So I love that we’re in a place that we don’t understand. ♦