A supermassive black hole

The supermassive black hole and one of its planets in 'Interstellar.' Credit: Paramount Pictures/Warner Bros.

In an earlier blog post about Christopher Nolan’s latest blockbuster movie, Interstellar, I lauded the film for its ambition, its visuals and the strong performances of its cast. However, I also criticized it for its depiction of interstellar travel and a plot filled with details that didn’t seem to make much sense.

Perhaps because I called some of its science “laughably wrong,” my post drew the attention of Kip Thorne, the Caltech physicist who served as science advisor on the film. Thorne sent me a copy of his new book, The Science of Interstellar, and encouraged me to read it and reconsider my criticisms. The book tells the story behind the film’s creation, and provides deep, thorough explanations for many facets of Interstellar that might otherwise seem nonsensical.

Thorne is even-handed in his treatment of the film’s science, admitting where artistic license was substantial and where it was used barely at all. If you enjoyed the film, but found parts of it confusing or puzzling, The Science of Interstellar and the perspectives it provides might be for you.

Thorne and I discussed Interstellar and his book in a telephone interview. A transcript of our conversation, edited for length and clarity, follows.

I want to begin by saying how much I enjoyed your book. It’s given me a deeper appreciation of just how much work has gone in to legitimizing the plot of the film.

Let me just say as we start that I was doing a lot more than simply justifying the science in the film. The story was built from the ground up on the science to a very great extent, through brainstorming sessions I had with the Nolan brothers. There weren’t a great many times where I had to go in and explain things after the fact.

Well, correct me if I’m wrong, but it seems to me that not all of the science is treated equally in the film, with the science behind its visual components being favored. Take Gargantua, the supermassive black hole the astronauts visit in the film. It’s a thing of beauty, not only aesthetically but also quantitatively, because as you’ve shown in the book, it looks like the real thing. That happened through a painstaking back-and-forth process between you and the filmmakers. But in your book you also mention that Christopher Nolan came to you with a “non-negotiable” and rather far-fetched idea for the astronauts to visit a planet orbiting Gargantua where relativistic effects make an hour there equate to seven years back on Earth.

You know, Chris also considered traveling through space faster than the speed of light as “non negotiable” back then, and that’s something that was changed and is not in the final film. He used that phrase in our brainstorms, but in the end after in-depth discussions he came around. We’d always find some way to make things work together, though in this one instance of faster-than-light travel I gave him a series of reasons why we were quite certain the laws of physics prevented it. We went back and forth for several hours on and off over two weeks about it, until he reached the point where he appreciated intuitively that the problems I was pointing out were insurmountable. Then he simply abandoned the idea of faster-than-light travel and moved in another direction.

This business of the enormous time differential between one of the planets orbiting very close to Gargantua and the flow of time back on Earth – the problem seemed to be that no planet could endure the resulting gravitational forces. This was something that even I thought was impossible, intuitively, until I went and slept on it and did a few hours of calculations. I came to the conclusion that in fact it is possible. The black hole needs to be spinning very fast, but is possible for the spin to be fast enough for a planet in the necessarily close, stable, circular orbit to not be ripped apart. I can’t fault anyone for saying, “Hey, that’s not possible,” without having first having the benefit of my book! Unless it’s someone who is very deep into general relativity and who I would’ve expected to go do the calculations!

This highlights, I think, just how much gnarly number crunching you’ve done for this film, whether you’re brainstorming ideas or reverse engineering plot points. Was this a frustrating process for you at all?

The only frustration in the creative process might’ve been the faster-than-light discussion, just in searching for ways that we could speak the same language. My interactions with Chris, and before him with his brother Jonah, the screenwriter, were really quite joyous. It was brainstorming in the best sense of the word, an artist and a scientist coming at a complex issue together, trying to generate ideas in the context of a story that is just coming into being, searching for interesting scientific ideas that would take the story in certain directions. Finding that common ground between two people of very different backgrounds—an artist that has a tremendous intuition into the universe and the laws of nature from self-education, but no real training, and me, a scientist with much more formal training—it was just a heck of a lot of fun.

Other than that, my main frustration has been that we were unable to make my book available earlier, and that so many people in the first week or two were commenting about the science of the film without having the benefit of the book.

That sounds familiar. One of the main criticisms I had of the film when I first saw it was that the accretion disk around Gargantua was energetic enough to provide light and heat for its orbiting planets, but not so hot and bright that it would bathe the astronauts in fatal x-rays and gamma rays. But you’ve explained in your book how this isn’t as implausible as it may seem.

Gargantua’s disk is anemic, meaning it’s not as dangerous as the black-hole accretion disks astronomers can see and study. It has the temperature of the surface of the sun. With our current technology, astronomers can’t really see an accretion disk of that temperature if it’s around a very massive black hole at the center of some galaxy. Instead, the accretion disks astronomers see are much more energetic and emitting lots of x-rays.

I worked out the relativistic theory of thin accretion disks with Igor Novikov back in the early 1970s, so I know this very well. For those energetic disks astronomers see, the accretion is a steady state where the gas is flowing onto the disk and on down to the black hole. You can work out the temperature distribution and where it is emitting hard and soft x-rays. In the film, the disk is orbiting the black hole, not accreting onto it. There is a reason you don’t see any flow of gas onto the black hole in the movie, because if that flow were there it would fry the astronauts. Gargantua’s disk is a remnant of what was in the past an accreting disk. It’s in a quiescent state and cooling down.

This was a crucial detail that actually dovetailed with Chris’s filmmaking point of view. What Chris wanted was something that was visually impressive in optical wavelengths that the astronauts could see. So that’s what he got – something that glows in the optical but isn’t so hot it pours off a lot of dangerous higher energy radiation. Let me say, though, that this particular quiescent and cool disk wouldn’t be in this state for an awfully long time. But, ha, all that the movie needed was a safe, bright environment around the black hole during the crew’s visit, and this disk meets that.

Were there other examples where scientific plausibility dovetailed with what the filmmakers wanted?

There’s one I discuss in the book, where this one planet—Miller’s planet, the same one so close to Gargantua—has these giant ocean waves that threaten the crew. I don’t use this word in the book, but the waves appear to be solitons, solitary waves. They don’t break, and they are probably coming in from a region where the water is somewhat deeper. One possible explanation for them is that they are similar to tidal bores that can run up the long, gentle channels of rivers with the rising of a tide.

As I describe in the book, I imagine this planet is tidally locked, keeping the same face toward the black hole so that tidal forces don’t rip it apart. But it hasn’t been tidally locked for all that long, it was deposited in its orbit relatively recently, so it’s actually wobbling back and forth slightly relative to the tidal-locking position, and as a result huge tides are created in the ocean at the planet’s surface. And these tidal forces are so great that they create the huge waves you see in the film. The fortuitous thing is, in the film’s dialogue we learn these waves come about one hour apart from each other. It just happens to turn out that the planet’s period of oscillation back and forth to make such big waves in the first place also needs to be one hour. I have to confess I didn’t realize this until after the waves’ size and hour-long period was set in stone in the script.

Is there any science-based criticism you’ve encountered or that you have of the film?

I have said this elsewhere, but there was one item in the film that troubled me in terms of the laws of physics, and that was the strength of ice not being sufficient to support the structures seen on one of the visited planets. Most places that have mentioned this, though, tend to leave out the second part of my statement, which is that if the strength of ice is the most egregious error in the film, we’re doing pretty good!

Right. In some ways, it could be seen as a badge of honor of sorts, that some viewers have taken this movie seriously enough to even worry about whether or not parts of its science are correct. But do you think people getting upset about the film’s scientific basis are missing the point?

I think there are a variety of points of view you can take on a film like this, and I think it’s perfectly valid if someone wants to go in and search for scientific flaws.

What I maintain, and what I believe Chris would maintain as well, is that to a very great extent real science can give rise to wonderful ideas for a film that can in most cases be better than what was created from whole cloth out of the brain of a screenwriter. You know, this is the first time Chris has ever aspired to make a film that has real scientific accuracy. His films have always had their own internal logic. He always lays down a cohesive rule set for each one about what can and cannot happen, and this is part of his pact between himself and the viewer. But this is the first film where that rule set closely corresponds to the known laws of nature, and some truly wonderful things came out of this. Real science can be an absolutely fabulous foundation for great filmmaking.

I should add that there are some portions of the film for which the science is beyond the frontiers of our present knowledge. The issue of time travel is one. There’s been a lot of research that’s been done on whether the laws of physics permit travel back in time or not, and we’ve got interesting results but no firm answers. In that area Chris made his own rule set, which we discussed at length when he described it to me early last year. And it’s a rule set for which I then could find a scientific rationale, but it was a rule set that was much less constrained by the laws of physics because we don’t understand the laws of physics in that domain yet!

You know, the point of my earlier piece on Interstellar wasn’t to attack the science at all, so much as it was to say that wormholes and warp drives and other exotic ideas might not be our best path to the stars. And, in fact, some very august scientists seem to think interstellar travel is so difficult as to be effectively impossible. What do you think? Do you think our descendants or we will ever leave the solar system, and how would we do it, if so?

I’m quite sure we will, if we live that long as a species. I have this chapter in the book on interstellar travel and give far-out examples of how one could undertake interstellar voyages. Nuclear-pulse propulsion, laser-powered light sails, binary black hole slingshots, things like that. And as crude and far-out as they may be, they convince me that the time will come when humans travel between the stars. But it’s a long time off, a very long time – centuries away, at least.

So, for these kinds of far-out proposals, is it fair to say they’re more plausible or even easier in some ways than the notion of wormholes or warp drives?

I think the laws of physics very probably forbid warp drives and traversable wormholes. The research that has gone on over the past 25 years trying to determine whether its possible all point in negative directions, but it’s not a firmly closed door. So there are two issues here. One is that the laws of physics probably forbid it, but, gee, if they don’t, it would be great to have! The other is that the technology required to make a warp drive or a traversable wormhole is so far, far, far beyond the technology needed for a laser sail or a nuclear-pulse rocket that I would not be in favor of putting any significant resources into trying to develop it.

Now, you may have small amounts of money—tens or hundreds of thousands of dollars—spent on this, but nothing is wrong with that. Peer-review, at least in the United States and in Europe, is too strong for there to be any danger of millions or billions of dollars being spent on these things. The technology required for wormholes is so far removed from our current and plausible near-future capabilities that to throw lots of money at it would almost certainly be a total boondoggle.

So why focus on wormholes at all if they’re so out of reach? What should someone do if they’re really inspired by the film and want to personally contribute to bringing something as wild as interstellar travel closer to reality?

To a great extent, my motivation here was to try to use the movie as a lure to get people who might otherwise not have much interest in science curious about it, by exposing them to strange, exotic phenomena like wormholes. The film is the bait, and the book is the hook I want to use to draw them in even further, to get them to dig in and learn something new. If they are young, maybe they will consider careers in science rather than in finance or law. If they are older, I still think it’s tremendously important that larger fractions of our citizenry possess enough understanding of science to appreciate its powers and its limitations.

What this is really about is inspiring others to learn enough about the laws of nature and about engineering and technology so that they might take advantage of those laws in order to make some real contribution to our society, perhaps ultimately toward interstellar and, for the moment, interplanetary travel. The central thing is to get people excited by this so that they focus on real science and technology and on making a big difference in our world with those tools.