We Asked a Theoretical Physicist How Time Travel in the Terminator Movies Works

According to a Hollywood myth, the original Terminator came to James Cameron in a dream. By that I mean, Cameron once said he came down with a fever in the 80s and imagined a killer chrome skeleton-bot. But as for the plot about a time-traveling soldier from the future on a mission to kill a foe in the present? He stole that—yep, stole in the legal sense—from an old episode of The Outer Limits written by sci-fi god-among-men Harlan Ellison. But in the process of creating that amalgam of new and borrowed, he set in motion one of the most elaborate, ostensibly continuous, time-travel plots in the history of the movies.

It's supposed to be so simple: Sarah Connor will give birth to the chosen one who can fight the machines, but the machine-man from the future will stop at nothing to carry out the world's most invasive, pre-pregnancy abortion to prevent that from happening. Sadly, the series can only continue if the premise keeps adding complications.

For instance, the second film added a wrinkle to the closed-loop conceit of the first by informing us that despite the Connor baby successfully being born, spare robot parts left in the present ensured that Judgment Day, the first shot of the robot war, still happened. T2, in turn, went to great lengths to quash Judgment Day's eventuality once-and-for-all. But the thing about "once-and-for-all" is that T3 is a profitable idea, so screenwriters futzed around with time, and Skynet miraculously came online once again.

To understand how all this time-travel jiggery-pokery works, I called Caltech theoretical physicist, author, and science consultant on Terminator: Genisys Sean Carroll. He helped me get the Terminator cinematic universe back into focus, since I tend to lose track of how it all works after the part where lightning strikes Arnold's bare ass.

[Warning: Terminator franchise spoilers abound in this edited interview.]

VICE: As a scientist, how's the science in this movie franchise just generally?
Sean Carroll: There's no question that it's a horrible mess, really, which is very, very common in time travel movies. I think it's very very rare for a time travel movie to work hard to keep everything consistent and sensible. The movie will plod along in whatever way the writers want to get from point A to point B and messing with timelines is definitely a way to do that.

Let's start with the first film: The Terminator. The guy who goes back in time has sex with the mother of the guy who sent him back in time, and turns out to be his father. Does that make sense?
There's nothing illogical about a chain of events that leads to itself. The only illogical thing is when there's a chain of events that prevents the initial event from ever happening? That would be bad.

And does that happen in the series?
It does in subtle ways. It's very clear from the very premise of the first movie that at least Skynet and the resistance in the future think it's possible to change the past. Skynet lives in a world where John Connor exists, and they want to prevent him from existing. So we certainly are operating under the impression that timelines can be changed.

Is that a fatal flaw?
It's not necessarily flawed by itself. A layer that you can try to add to this is, "Can we imagine a set of timelines that are different from each other, and yet the whole shebang is consistent?" So let's say we have a simple world where the first terminator succeeded, and John Connor was killed. Then you can imagine there's a timeline where John Connor was killed, and one where he was not, and they're sort of separate from each other.

So to understand multiple timelines, we have to get into the weeds of quantum mechanics for a second. Can you explain that briefly?
Quantum mechanics is a true theory of the world that helps us explain the subatomic realm. When you have a particle in a certain position, there's no such thing as where the particle is. It's spread out through space until you observe it. And when you observe it, you see it in some location or another, and that's given by a probability that you'll see it in one place versus some other place.

That's the "quantum" part. But where do whole separate timelines come in?
There's a very respectable version of quantum mechanics, which is not yet known to be true, which is called the many-worlds interpretation of quantum mechanics, and it says that when you observe a particle, and you see it in one position, there's another world that comes into existence where it's in some other position, and yet another position, and so forth. So the idea that there are branching timelines, where slightly different things happen is very realistic as far as physics is concerned.

But that doesn't necessarily mean we can time travel, right?
That has nothing to do with time travel. That's just branching because the universe is becoming more and more complicated and varied over time. You could add to that the idea of time travel, where you're going back into the past, but the place you're going back to isn't the place you came from because the world has split into a different timeline.

That sounds like it would give the Terminator films a lot of flexibility. Do they stay consistent enough for the multiple-timelines explanation?
The weirdness in the Terminator films is that it sort of goes beyond that, with people passing back and forth between timelines, not just creating more every time somebody goes back.

Right. Time travel in the Terminator films is all back in time, never forward. Is there a scientific basis for that? Might we be able to go forward someday?
It's easy to go forward in time. I went forward in time yesterday by 24 hours, and here I am. It took me 24 hours to do it, and I'm gonna do it again in 24 hours. We're always going forward in time, and it seems to be a one-way street. The English language isn't really up to the task of talking about this. We say we move "forward in time," but it's not the same kind of movement we talk about when we're talking about moving through space.

So you just stay on course, but faster to go into the future. How would terminators get to the past, though?
If you ask, "If you did go to the past, what would the rules be?" We're making stuff up. We can guess.

Yes, please guess.
The scientific question is, "Can the shape of the universe be distorted so much that you and I can locally, in our neighborhood of space, be moving forward in time just like we always do, but that forward motion in time twists around so much that we end up coming back to a point before we left?" This is what's called a "closed timelike curve" in physics.

Would it look like what we see in the Terminator movies?
If you want to be physically reasonable, one of the very first things you should object to in movies like The Terminator is if you could travel backwards in time, it wouldn't be that you disappear, and then reappear in the past.

Why is that a problem?
Einstein taught us that time and space are both part of one four-dimensional thing called spacetime. So in general relativity, which is Einstein's theory of spacetime, if you want to go backwards in time, you just move through spacetime in a particularly curvy way, so that your path takes you to a point before you left. But there's no part in physics that says I can just disappear and reappear somewhere else. There's nothing even remotely respectable about that.

What should traveling into the past look like using a closed timelike curve?
You'd have to hop in a spaceship and fly around for several years until you finally got back to the year 2011 or whatever. It's a lot less visual, and it gets in the way of the story you're trying to tell. That stuff doesn't bother me. That's just violating the laws of physics. That just goes without saying in movies like this. It's violating the laws of logic that I find to be much more annoying.

Aren't other movies worse about that though?
The classic example is something like Back to the Future, where you see the photograph changing because of something that happened in the past. That doesn't make sense in anyone's version of anything. I mean, what is right now? Why is it changing now? What is this supposed to be? It's something that happened years in the past! It's clearly a mixed-up attempt to change the past and yet only have one timeline. That would not make sense.

Have you figured out why screenwriters keep getting tripped up there?
Storytellers tend to mix up the narrative time with the actual flow of time in the universe as being described in the story. That's what makes it impossible to reconcile this in any scientific way. That only makes sense because there's an omniscient godlike narrator point-of-view who's showing you different things happening. From that narrator's point of view, things are simultaneous, but no physical person in the story's point of view. In some sense, Terminator's not really that bad.

Are there specific problem areas in the Terminator universe, where the timeline is altered?
In the third one, it's very clear that they have affected the timeline. But I think it was still, at least until then, possible that it was just a branching structure, where more and more timelines were being created by all these people being sent into the past.

In the third film they think they've stopped Judgment Day, but the robot tells John, "Judgment Day is inevitable." Does the concept of inevitability have any scientific basis?
You can invent an idea that's not based in physics, or anything we know about the laws of nature, but you could invent an idea [that is] sort of like fixed points in the future. There are choices that we have now. Different things could happen. But no matter what we do now, there's going to be some ultimate outcome. That's not in any way how physics works. You're making up a story. You don't have time-traveling robots either.

What's wrong with the inevitability conceit, science-wise?
If in the Terminator universe there is this rule, how do they know that? Have they been in every timeline? It seems completely impossible to me to verify something like that. Maybe that was someone's point of view, and that point of view turns out to be wrong. The narrator's voice of God can say that, but I don't see how any person living in that universe could know something like that.

There wasn't really any time travel in the fourth movie. So what what was your contribution to this new one?
Alan Taylor and I chatted a couple times. You know this is something where I go into Paramount, and I sit in his office and we chat for an hour and then I leave. And then a year and a half later, the movie appears.

Can you share any of the substance of the conversation?
We talked a little bit about why the Terminators are always naked when they come back.

Oh my God, yeah! Why is that?
Who cares, really? They wanted to show Arnold Schwarzenegger naked.

Fair enough. So as a consultant on these movies, do you have a general mission for filmmakers?
I think that the moviemaking profession, and the moviegoing audience are both becoming a little more sophisticated over time. So these days more than ever, you make a better movie if you try to make it make sense. If you really just slap things together because you thought that every individual scene is adorable, but don't worry about the underlying logical structure, the audience will be bugged by that.

Terminator: Genisys opens in theaters today.

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