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 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.

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