Palm Springs’ Theoretical Physicist on the Science Behind the Movie’s Twists

A spoiler-filled interview with Clifford Johnson, who consulted on (and cameos in) the new movie.

The two look stoned, confused, or both, surrounded by question marks.
Andy Samberg and Cristin Milioti in Palm Springs. Photo illustration by Slate. Photo by Hulu.

This interview contains spoilers for Palm Springs, including the ending.

There are plenty of romantic comedies in which a couple gets a second chance at love—but what about a million chances? In Palm Springs, two wedding guests played by Andy Samberg and Cristin Milioti find themselves stuck in a temporal rut, reliving the same day over and over again, and having company on their endless journey doesn’t always make the going easier. Viewers raised on Groundhog Day will expect that the story is building towards a karmic out, but extricating themselves turns out to involve a dose of hard science, provided courtesy of science advisor Clifford Johnson.

Johnson, a PhD who teaches at the University of Southern California, helped fine-tune the time-travel mechanics of Avengers: Endgame, and he’s also written and drawn a nonfiction graphic novel, The Dialogues, that features “conversations about the nature of the universe.” For this particular conversation, which has been condensed and edited for clarity, he spoke to Slate from his home, near a shelf bulging with hardcover comic books, and answered follow-up questions via email.

Slate: How did you get involved with Palm Springs?

Clifford Johnson: The whole business of science advising and connecting scientists to artists and filmmakers and what have you is completely unformalized and unstructured, in an industry that is extremely tightly structured. That produces all sorts of difficulties, and even the business of starting a conversation can be fraught with difficulty. One thing that has helped a lot in recent times has been a thing called the Science & Entertainment Exchange. There’s a lot of people who think, “I don’t know any scientists and I don’t know how to start,” or “I don’t want to bother the scientists with my trivial thing because they are doing important work in the ivory tower.” Some of us would like that default position to change and to have people instead go, “Hey, there are scientists who would love to help.”

One of the reasons a filmmaker might not reach out for scientific consultation—especially on a movie like Palm Springs, which is closer to romantic comedy than hard sci-fi—is because they fear it might ruin their story. So: Why does scientific accuracy matter?

One of the things I try to avoid when people talk about science advising is words like accuracy, because it makes us sound like we’re the Science Police. That’s what produces increased fear on the part of filmmakers and sometimes on the part of viewers to play with science. My view is that science is a part of our general culture, like music or various other aspects of the arts. It’s part of that fabric that should be in every aspect of our lives, including our entertainment. If you have to have special skills or special knowledge, or have done a four-year degree, or have to have been good at science in high school to engage with science, then we as a society are much poorer for it. How just thinking scientifically as a citizen to make decisions, how those things affect our lives, and how we should all be involved in the conversation that is science—that’s hugely important right now. People can see that, you know, we’re in the middle of a pandemic, right? So if you stick it just in the classroom, as spinach, that’s not so great.

You’ve said, when you advise on a movie like Endgame, what’s important to you is less the accuracy of the science than the way scientific methods are represented.

Exactly. And then crucially, also, who does science? It should be everybody, all kinds of people.

The script of Palm Springs had the scene in there with [Cristin Milioti] talking to an expert in the phenomenon. I know from experience that this usually ends up being a fairly standard “default” scientist, and so I suggested that this would be a great opportunity for them to show that there are more kinds of people doing this kind of science than this sort of movie usually shows. So I said maybe cast the scientist part as a woman, or maybe a person of color. They came back later and said thanks for the note—they were wondering if I’d like to do it! I was hugely surprised, as that was not my intention in making the suggestion, but they convinced me, so I did it. If several million people are going to see that, that can be helpful.

Johnson appears on a MacBook Air via FaceTime, which labels him as “Clifford Johnson”
Johnson’s cameo. Hulu

You’re also not the “default scientist” in the sense that your job isn’t to give a lecture about what’s really going on. She’s already done the research herself, and all you do is confirm that she’s got it right, and tell her, “You don’t really need me.”

I was so delighted to have the opportunity to say that. On that aspect of the script, they were already there. It’s another cliché in these movies, right? At some point, the people in trouble, especially if there are women in this film, they go to some guru, usually male, for help. I love the fact that she just figures it out. She has an infinite amount of time. She can put herself through a full career’s worth of education in any field she chooses to until she’s ready. The other thing that’s really great is that she does an experiment. And that’s something else you don’t see on screen as often. It’s usually just, “Oh, let’s sprinkle some science magic and then it’s all fixed.” But no, you see her trying things out and doing the experiment until she’s sure it has a chance of working. That’s awesome.

You say the script was “already there” on that point. On what points was it not there?

Full credit to them, the key elements were more already where they needed to be. They wanted to understand two things: One, what’s the backstory of this phenomenon. Not necessarily because you’re going to put all of that in the movie, but they wanted to understand it. And then, they wanted to know how much to put into the film in terms of how we solve the problem. Those details were extremely fuzzy, and they didn’t really have a good understanding of what was going on. So I sat down and I wrote some suggestions for the kinds of things the leads might say to each other, about physics, about the bubble of energy, blah, blah, blah.

But also one of the things I suggested was that all they needed to do was have a strong idea of what the phenomenon was, but don’t necessarily be tempted to try to put too much of that in the film. I think one of the reasons the film is so successful in how it gets across its main themes is that you don’t need to worry about so many aspects. You do need a foundation, or at least some sort of rules of engagement for what the phenomenon is, whether it be based on real physics or made-up physics. In this case, of course, it’s made-up physics, because as far as we know, time loops are not possible. It’s still helpful to talk to a scientist to get that, because then it gives you a foundation to do your writing well, to tell you a story.

Often when you have these time loops and stuff, somebody builds a machine that makes the thing. I was pushing the idea that maybe, for whatever reason, this thing was a natural phenomenon. In real physics, there are extraordinary things that mess with the structure of space and time that we do know of that are hugely beyond human scale. They’re either supermassive black holes or things that happened very early in the universe when it was being formed. We’re not going to build some machine that’s going to make that happen. So the earthquake is meant to symbolize that there’s something beyond our human level that is happening.

The other problem with time loops, in theory, is that they’re perfect. Every part of that circle of time that keeps repeating itself depends upon every other part being in place, which means they’ve very hard to enter or leave. And that’s not what you want for this movie. So I was also talking about the idea that maybe you could be creative a little and have a time loop where things repeat, but not quite the same every time. And that’s good because that gives you the idea that things can be adjusted, gives you the idea that perhaps you could escape it at some point. If you do anything to do with time travel into the past, and we don’t know whether that’s possible, it would involve certain kinds of exotic energy localized in a certain way—in the case of black holes, it’s a lot of mass and energy in one place. So what you would do to mess with it is somehow disrupt that energy. So that clearly this is what the lead eventually realizes she has to do.

So in the movie, the C4 explosion is the version of that exotic energy?

No, no, no, no, no. That would be how you disrupt whatever’s keeping that time loop in place. I think the explosion was in place before I came on. They just didn’t know why the explosion was doing anything. So in some ways I’d give them the assurance that it’s a good thing to have, both for the story and the physics. The best science advising relationship should always be that the scientist is there to help the storytellers tell the best story they want to tell.

One question that comes up a lot: What’s up with the dinosaurs?

You know as much, or as little, about that as I do. I think it’s a lovely touch. It didn’t come up in our conversation.

I think the idea might be—and now, I do not know this at all—but once I suggested the idea to them that, for all we know, this loop has been there since the beginning of the universe and people have been finding their way into it. Which means it could involve things that are ancient, including dinosaurs.  A lot more connective tissue is needed there, but it seems to me that that is a nice possibility that maybe they were playing with.

Do you have favorite movies about time travel, whether accurate or extremely not?

I mean, everyone mentions Back to the Future, which for me is utterly ridiculous from the point of view of the science part. They play with a lot of the clichés which I spend a lot of my time trying to undo. Doc Brown is sort of everything we’re trying to avoid in terms of better images of who can be involved in science. But I still love that movie. In my opinion, it is one of the greatest pieces of visual storytelling. It’s just amazing. I think 12 Monkeys is brilliant. And an indie favorite of mine, just because the tone is so perfect, is Shane Carruth’s Primer. It’s utterly confusing, and it doesn’t matter because tonally, it just feels believable.

You’ve seen the diagrams people have done trying to map out all of Primer’s timelines?

Yeah, it’s sort of amazing. Another reason is that at the very beginning where you see the different engineers playing around with the machinery, that had a real feel of authenticity to it. It didn’t feel like the bridge of Star Trek or something like that. There was a very organic feel to it—there’s stuff everywhere. And the way they spoke to each other, that was really well observed. That oftentimes is one of the most important things to get right, just how it really feels to be in a scientific environment, and how they react when they start discovering there’s something not quite right here, that the times don’t match up. And I didn’t mean any disrespect to Star Trek. I love Star Trek.

When you talk to people about this, you know Interstellar is eventually going to come up. And one of my favorite things is the fact that people often point to some of the stuff that’s actually real science and think it was the made-up science. It’s that episode when they go down to the surface of a small planet that’s in the field of a supermassive black hole, and they come back a couple of hours later hours later and 23 years have gone by for their colleague who was waiting for them on the ship. That’s actually a real bit of science. And the point there is that there are extraordinary things that happen with space and time in the in the neighborhood of extreme amounts of mass and energy. We know that happens in our real universe. It obviously isn’t as intense, but you really do need to take into account that time runs a little differently in orbit compared to down here on the surface of the earth, in a measurable way that is important for our everyday lives. You need to take that into account when you’re doing the timing measurements that give you accurate GPS coordinates.

What storytelling can do is give that science more of emotional impact. It’s important in practical terms that we know how time works differently in orbit, but it’s not the same as feeling that you got stuck on a planet too long, and by the time you get back, your little daughter is a middle-aged woman.

There’s a great tradition of storytelling in science fiction, and more broadly speculative fiction, of imagining what would happen to the human condition—which is what our stories are about—if you changed some accepted parameter a little bit. And time travel into the past is just one example. Real science is full of these of things. So that dialogue between scientists and creators of stories should be encouraged, because at the very least, it allows for new ways of telling stories. If you’re a Hollywood executive and you want to be able to tell those old, old stories, but with a new twist: Why not use science?