Interstellar’s Scientific Fact-Checker

A talk with physicist Kip Thorne.

Physicist Kip Thorne on the set of Interstellar. (Paramount/Warner Bros./Legendary Pictures)
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In the new science-fiction film Interstellar, just released last week, Earth is dying. NASA has detected a wormhole leading to a galaxy with three planets potentially suitable for human habitation, and launches the spacecraft Endurance to investigate these planets in the hope of transplanting humankind to at least one of them.

The movie comes from an A-list collection of principals: Christopher Nolan, who made Memento, Inception, and The Dark Knight trilogy, directed the film and co-wrote its screenplay with his brother Jonathan (or “Jonah”) Nolan. Matthew McConaughey, Ann Hathaway, David Gyasi, and Wes Bentley star as the crew of the Endurance, with Michael Caine and Jessica Chastain playing NASA scientists seeking a solution to the crisis back on Earth.

The elaborate equations seen on blackboards in the movie behind Caine and Chastain were all written by Kip Thorne, a CalTech theoretical physicist who conceived of Interstellar almost a decade ago with his friend Lynda Obst, the producer of a varied roster of hits including The Fisher King, Sleepless in Seattle, The Siege, and How to Lose a Guy in Ten Days. Obst and Thorne had been set up on a blind date in 1980 by their mutual friend Carl Sagan. They didn’t work out as a couple, but they remained friends and occasional creative partners.

Thorne is credited as an executive producer on Interstellar, but his more specific role was as the film’s science adviser, one who worked closely with Christopher and Jonah Nolan throughout the writing, production, and editing of the film to ensure that its fantastic story remained rooted in actual science. His book The Science of “Interstellar,” a 324-page volume explaining his scientific rationalization for every aspect of the film’s story—each of its sections clearly labeled as “Truth,” “Educated Guess,” or “Speculation” —has just been published. This interview, like that book, discusses aspects of Interstellar’s story in detail, so if you’re wary of spoilers, we’d advise waiting until after you’ve seen the movie to read it.

Air & Space: A day before Interstellar’s wide release, Phil Plait wrote a Slate post attacking the film’s scientific foundation. He published a follow-up three days later recanting his critique of the science—though he still didn’t like the movie—and admitting he assumed you were brought in late in the development process. Were you expecting to get this kind of reaction when you got involved with a science fiction movie?

Thorne: I expected that people, including my colleagues, would not understand what the science was underlying the film without reading my book. There are not enough hints in the film for a physicist to really know what’s happening in terms of underlying science. Phil wasn’t aware of my deep involvement from the outset. As he says in his pullback, he had not thought of the possibility of spinning black holes as being central to this. It’s natural that they will make public statements and get it wrong compared to how I would interpret things, because they don’t have the benefit of my very long involvement in the film. I think it’ll all shake out after people read my book.

You’ve been working on this movie, off and on, since 2006. Did you always intend to publish a companion volume explaining its scientific basis?

It was always my intention either to do a book or a Blu-ray about the science. I made the decision last January. By that time I was knowlegable enough about how things would go with a Blu-ray. Although I might reach a broader audience, I wouldn’t be able to go into things in the depth that I wanted. So I made the decision to do a book instead of a Blu-ray around New Year. I started writing January 10th and I had my first good draft to the publisher about March 20th.  But then polishing it off and getting this enormous number of figures done, including a large number that I did by hand myself, took an enormous amount of time. It ate me up this year, into September.

I was wondering why the book wasn’t available before Interstellar’s release. Usually ancillary volumes to a movie come out first. Now it makes sense: You talk in detail about a lot of elements that are intended as surprises for viewers seeing the film for the first time.

Yes, there were long, arduous negotiations with Warner Brothers, which was in the driver’s seat on the production and has the foreign rights. Fortunately, I have a very good entertainment attorney in Hollywood. But I’m very pleased with the cooperation I’ve gotten from Warner Brothers and Paramount in this phase of the publicity connected to the book and the movie.

The book deals with complicated concepts in physics, but it’s intended to be entry level. Was it difficult for you to gauge how simple to make it?

Yes. I have some experience with this, but I couldn’t get much feedback because of the amount of secrecy surrounding the film. I had to rely on a very small number of people to read through it and give me feedback. I wasn’t really able to run it past any of my physicist colleagues. On the other hand, I knew the subject inside out. I knew what I wanted to convey. I figured I could probably pull it off.

I don’t know whether I’ve succeeded, but I would really like this book to be understandable, with some effort, to high school science geeks and to the intelligent adult population who don’t have any training in physics.

The movie is extremely fast-paced, especially considering the amount of complex information it’s conveying. I’ve seen it twice, and I certainly caught some scientific explanations and story details that had elided me the first time on my second viewing.

In many cases, the only hint of the physics you get is in a few words of dialogue. Romilly [an astronaut played by David Gyasi] refers to “a gentle singularity” inside a black hole. Which is one key to what goes on when Cooper [the astronaut played by Matthew McConaughey] falls into the black hole. Recent insight says there are actually three singularities inside a black hole, not just one. The more recently discovered ones are much more gentle than the one we’ve known about a lot longer.

This one word, “gentle,” in the movie—the foundation for that was a long discussion between me and Christopher Nolan, in which we discussed which singularity of the three Cooper should fall into, and what the consequences of that would be for his ability to communicate back in time. There was a whole range of discussions between me and Christopher Nolan that underlie all this, and all you hear in the film is this phrase: a gentle singularity. Period.

You also point out in your book that there’s a moment when Dr. Brand, the astronaut played by Anne Hathaway, argues for using the ship’s limited fuel to investigate Edmunds’ planet rather than the closer Mann’s planet by saying, “When you’re orbiting a black hole, not enough can happen; it sucks in asteroids and comets, other events that would otherwise reach you. We need to go farther afield.”

You say her scientific justification is wrong there, and that Christopher Nolan knew that but declined to change it. Anyway, her reasons for wanting to go to Edmunds’ planet become a thematic element of the story. Is that typical of how you and Nolan negotiated the demands of the story with the demands of scientific accuracy?

Yes. I had similar discussions with both Chris and Jonah. Another example is the accretion disk around Gargantua. If you do it fully right, including the Doppler shifts, due to the motion of the accretion disk towards you on one side and away from you on the other, the side where the gas is moving towards you gets shifted to the blue in color and becomes far more intense in brightness. The side going away from you gets shifted to the red in color and becomes very dim. If you look at the images that are produced by that, you say, “How is a general audience going to make any sense out of this without some explanation?”

So the bottom line was, leave out the Doppler shift so this accretion disk can be understandable and still intriguing and scientifically accurate, aside from leaving out the Doppler shift. So there were compromises made, but they were conscious compromises made for good, cinematic reasons.

The visual effects company Double Negative used Thorne's equations to create images of Gargantua, <em>Interstellar</em>'s black hole. (Paramount/Warner Bros./Legendary Pictures)
The visual effects company Double Negative used Thorne's equations to create images of Gargantua, Interstellar's black hole. (Paramount/Warner Bros./Legendary Pictures)

A WIRED article about how Paul Franklin and the effects team at Double Negative used your equations to generate the wormhole and the black hole we see in the movie made it sound like both were purely scientific. But in the book you recall getting an urgent phone call from Nolan saying that the Endurance’s trip through the wormhole looked too familiar compared to other movies, and that they would need to take some artistic license to make it more visually compelling.

Everything else about the wormhole comes from ray-tracing in the spacetime metric of a wormhole. The one place where the simulations based on general relativity equations were abandoned was the trip through the wormhole. I wrote down a spacetime metric. This is the thing that describes the warping of space and time around the wormhole and inside it. I wrote down one that had three “handles” that could be changed: The diameter of the wormhole, the length of a wormhole going from near Saturn to near Gargantua, and how flared-out the opening of the wormhole was. Does it bend sharply to join on to the external universe, or does it bend gently?

The Double Negative team made images of what it would look like to the camera orbiting the wormhole with various values for these three parameters. And Chris Nolan and Paul Franklin made a decision as to what values to use for the parameters of this wormhole in order to get something Chris really liked for a mass audience. That’s what they used. There was no compromising. There was nothing except ray-tracing with light-funnel propagation through the wormhole, with Chris making the choice of these three parameters.

You suggested to your friend Carl Sagan that his novel Contact use a wormhole instead of a black hole as an element of the story. I can’t remember if we see a wormhole in the movie made from that novel in 1997—which also starred Matthew McConaughey, coincidentally.

No, we don’t. With Contact, George Miller [the director of the Mad Max apocalyptic sci-fi films, plus Babe and its sequel] was the original director. He was working hard on the screenplay to get the wormhole sequence into a form he and Carl Sagan were happy with. He got canned by the studio because he was taking too long to perfect the screenplay. They brought in Robert Zemeckis [the director of the Back to the Future trilogy, Cast Away, and Flight, among many others], and Zemeckis just cut that out, basically. You see Jodie Foster traveling through the wormhole but you don’t see the wormhole itself in any way, shape, or form.

The wormhole as the Endurance approaches it is one of the signature visuals of Interstellar. Just putting something into the vocabulary of images that the public can associate with this concept of a wormhole feels like an achievement.

It’s been surprising to me that there have not been widespread images of what a wormhole would really look like, as seen from the outside. This is what I knew it would look like if you had a short wormhole, which is what Chris chose, and if you had a rather sharp bend at the opening. You haven’t seen images like this before, and I find it a bit bizarre that you haven’t.

Why hasn’t anyone attempted this kind of visualization before? Is it a matter of money—do you need the resources of a movie studio behind you to do this?

Physicists have not done it because we’re skeptical that wormholes can really exist in the real universe. The focus of physics research has been trying to pin down whether they’re allowed by the laws of physics. What kind of material would you require to hold one open so it doesn’t pinch off? Is it possible to create one if you don’t already have one? These are all deep issues in fundamental physics that don’t involve visualization at all.

I suppose visualization isn’t of much value as a research tool.

Sometimes it is, as part of coming to grips with something. In this case, it hasn’t been. It wasn’t necessary as a foundation for trying to sort out these very deep issues. Filmmakers just never had a physicist involved in the film from the outset like this before.

Are you aware of the visualization of Sagittarius A* that’s just been conducted at the University of Arizona?

I’m just a little bit aware of their computer modeling. Sagittarius A* is of enormous interest now in terms of imaging what the black hole there would look like, and what the accretion disk around the black hole would look like, because astronomers are close to having the technology to be able to image it from Earth using millimeter and sub-millimeter interferometry. This is called the Event Horizon Telescope. So the folks at the University of Arizona and other research groups around the world have been computing the visual appearance of what they expect to see from Sagittarius A and its accretion disk. It’s a very important area of research, and in some sense it’s complimentary to what’s happening in Interstellar.

In Interstellar, we were focused on what the black hole would look like seen from up close in a spacecraft. We were dealing with a very anemic accretion disk. If it were a normal accretion disk, it would be so hot that the X-rays would fry Cooper the moment he emerged from the wormhole. So this is an unusual accretion disk: It has the temperature of the Sun, say.  It’s emitting light but not much X-rays. It has no jet. The jet would also be the source of huge amounts of X-rays that would fry the crew.  And as I’ve said, for cinematic reasons we left out the Doppler shift.

Also for cinematic reasons, the Double Negative folks put in lens flares. Parts of a movie like this are filmed with IMAX cameras and parts are done with computer graphics. In order for it to have continuity from one to the other, they generally impose on the computer graphics images the effects you would get from the light going through the IMAX cameras, scattering and refracting. The astronomers doing the modeling for the Event Horizon Telescope are not going to put that glare in. They want to see what it would look like without light being scattered through an IMAX lens. Nevertheless, their images look somewhat similar to ours, and that’s gratifying.

The <em>Endurance,</em> the spacecraft that ferries the astronauts in <em>Interstellar</em> to another galaxy, with one of its "Ranger" planetary landing ships docked. Its modular design was loosely modeled on the International Space Station. (Paramount)
The Endurance, the spacecraft that ferries the astronauts in Interstellar to another galaxy, with one of its "Ranger" planetary landing ships docked. Its modular design was loosely modeled on the International Space Station. (Paramount)

If Interstellar were to awaken a massive public yen for physics, space exploration, or one of the other areas of science that figure into its story, what sort of research would you hope to see funded?

I would not advocate for anything in particular at this moment, off the top of my head. I think the key thing is to have a process for making wise decisions. There are many areas where we really need funding to do research that will in some cases have a big impact on human welfare, and in other cases will have a big impact on our understanding of the universe, though the impact on human welfare may be a century or two away.

Your book also talks about how a global crop failure like the one posited in Interstellar is unlikely, but none of the critiques of the movie that I’ve seen have pounced on that point at all. Do you think we’re just more willing to accept a pessimistic projection than we are to accept something crazily optimistic, like a traversable wormhole built for us by benevolent, advanced beings?

The issue of climate change is a huge issue. A lot of people focus on that as being tied to what’s in the film. The film is deliberately opaque on what has happened, aside from the issue of blight.

I am surprised that people haven’t focused in on that more, aside from the ones saying, “Hey, you’ve got to pay attention to climate change and here’s an example.” I hope that my chapter on the blight will open people’s minds about this. I have four brilliant biologists discussing what kinds of things could go wrong with our food supply. I would hope that there would be some discussion of that as a result of this film, or of the film plus my book!

Are you working on anything else that you’d like to tell us about?

My big push in the next half-year will be getting out a textbook on modern, classical physics. I want to broaden physics education in the United States through a very different kind of textbook from what’s been seen before. I will be spending at least four months checking every equation in the book in page proof before it comes out next spring. So I am about to go into hiding and do this fascinating job. That’s “fascinating” in quotation marks.

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