The Science Behind the Movie “Interstellar”

If you've seen the film Interstellar, you may now find yourself with many new questions about the laws of the universe. Kip Thorne served as executive producer and lead science consultant for the film; Oberlin Assistant Professor of Physics Rob Owen was a student in Dr. Thorne's research group when the movie project was in the first stages of development seven years ago, and the science in the film aligns closely with his current area of research.

Professor Owen answered several of our questions about the accuracy of science in the film and entertained some of the more mind-bending physics questions we had after seeing the movie.

I understand you were a student in Kip Thorne's research group when the project was first developing. What was that like?

It was very exciting, but not all that far from the ordinary. Kip’s notoriety with the general public has grown substantially with this movie, but he's long been a celebrity in theoretical physics circles. On the hallway near Kip’s office, at least when I was at Caltech, were a series of framed documents representing famous wagers that Kip and others in his group have made about difficult problems in physics. The most famous of these are wagers with Stephen Hawking about the existence of black holes, about the “cosmic censorship conjecture,” and about the “black hole information loss” problem. Recent students in Kip's group (myself included) would have grown up reading about these famous wagers. And there they are, just sitting on the wall. It imbues all of our research with a feeling that, yes, we're dealing with very deep questions, questions that have attracted the attention of very smart people for many, many years.

When the movie development began, the level of excitement got even higher. When Kip would mention that he had to run to a meeting with Stephen, suddenly it wasn't clear whether he was talking about Stephen Hawking or Steven Spielberg (Spielberg was planning at the time to direct Interstellar, before contractual conflicts got in the way). More important than the name-dropping was the fact that our field of research was about to be represented, faithfully and centrally, in something designed for broad public consumption. Physicists of the future, I hope, are currently being inspired by this film in the same way that I was inspired by A Brief History of Time, or in the same way that a large number of astronomers and physicists have been inspired by Carl Sagan’s books and the movie Contact.

This film aligns closely with your area of research and you mentioned that you've been waiting eagerly for seven years to see it. Did it meet your expectations?

It absolutely did. The central vision, as Kip described it from the very beginning, was to produce a movie where issues in general relativity (the science of spacetime, black holes, and wormholes) were central to a compelling story, and were communicated accurately. I think they met that goal spectacularly well. Somebody actually sat down and calculated how massive the black hole would need to be, and how rapidly it would have to spin, for the time dilation effects in the movie to come out right. The special effects team actually rewrote the software underlying the computer graphics to trace the paths of light rays in curved spacetime, to give accurate visual images of what the black hole and the wormhole would actually look like to the eye.

Would you describe the science in the film as fairly accurate, overall?

Overall, I would say it’s quite accurate, at least with regard to the issues that I have the expertise to judge. It should be noted that the story moves in on realms of theoretical physics where our current knowledge is incomplete. The physics of wormholes is at this point incomplete—most physicists would agree that they likely don’t exist as natural phenomena, but the question of whether they could be “built” by a civilization with extremely advanced technology, remains open. Similarly, the events at the end of the movie—involving existence in a five-dimensional “bulk” universe, and the idea that what looks like time in four dimensions might be traversable like space in five dimensions—these ideas are quite speculative, but they are rooted in serious attempts at understanding quantum gravity.

How did you feel about the portrayals of things that in real life are purely theoretical at this juncture?

It’s a dangerous business, but I think there’s a lot of value in it. For all we know, someone might come along in just a few days and prove mathematically that the basic laws of physics completely forbid the creation of a wormhole, for example.

But it’s important in science to remember that there are always phenomena that lie outside the realm of our current understanding. Some phenomena, such as wormholes and extra dimensions, are not fully understood yet but our incomplete knowledge is enough for us to carry out a kind of informed speculation about what kinds of effects are possible. At some level, that is the very definition of science: we build a hypothesis about new phenomena by extrapolating our knowledge of past phenomena. We can’t say whether such a hypothesis is true until we’re able to test it, but working through the implications of our hypotheses can provide us with a means of establishing those tests. As one of many apocryphal Einstein quotes reminds us: imagination is an important element of good science.

Were there any parts that you found to be simply ridiculous or absurd?

There isn’t much that comes to mind. There was a shot in the movie where the ship is descending on a planet and it bounces off of a solid, frozen cloud. Kip Thorne has admitted in interviews that he's not convinced that there are any real materials that could freeze like that.

Also, as someone who lectures to introductory physics students about rocketry, escape velocity, and related phenomena, I get a little queasy seeing a little star-trek-style shuttlecraft simply taking off from a planet like an airplane and reaching into the interplanetary space above. With current technology, it would take a very substantial rocket to blast off of a planet like the ones in the movie. But I must admit, I haven't thought it all through deeply enough to be sure whether the depiction in the movie is completely impossible.

I’ve been struggling with a logic question: how could the wormhole near Saturn have been placed there by future humans when they would have needed it in order to survive long enough to put it there in the first place? Does the addition of the fifth dimension mean that time is no longer linear?

The short answer is “yes.” The longer answer: This is actually a really interesting question, and I think it gets at some very deep issues in relativity theory. In our ordinary experience, we think that all events have some set of events that precede them (“causes”), and some other set of events that follow them (“effects”). In our ordinary experience, the causes are completely distinct from the effects, the past is completely distinct from the future.

In general relativity, causality is not so simple. A wormhole, for example, is a phenomenon where space bends back on itself. But Einstein’s relativity theories tell us that time and space are not distinct entities; that our three dimensions of space and one dimension of time are just particular cuts through a four-dimensional continuum that we call “spacetime.”

The idea of space looping around on itself hopefully isn't too surprising if we think about it a bit. For example, if I walked due East, and never stopped (and helpful people provided me with boats whenever I needed them), then I would eventually come back to where I started, moving in from the west. I would have simply walked around the Earth (not an easy task, by any means, but certainly consistent with the laws of physics).

What Einstein’s general relativity shows us is that it’s possible for spacetime to fold up on itself in such a way that one can march toward the future, but then eventually end up somewhere in what we would have originally called “the past.”

This possibility leads to a slew of time-travel paradoxes, and to interesting questions about how the fundamental laws of physics might intervene to enforce a self-consistent history of events. How that might happen is not fully understood at this point.

But the important thing is that in general relativity “causes” don't necessarily have to precede “effects” in the sense of our ordinary experience. All that is required, as far as we know, is for history to be self-consistent. According to the story in Interstellar, humans survived long enough to develop the technology to build the wormhole because the wormhole was there to save them, and the wormhole was there to save them because they survived long enough to build it. Either event can be considered the “cause” of the other, and there isn’t a contradiction.

The idea seems purposely designed to throw our usual sense of causality off kilter. And when paradoxes like this one appear in other time-travel stories the reason is usually either lazy writing or some kind of empty mysticism. In this case, however, I would bet that the apparent paradox was quite intentional, because one of the physicists who has been preeminent in the exploration of these issues of causality is Kip Thorne.

Can you tell us a bit more about the fifth dimension, and how humans might make the leap from existing beyond four dimensions as they do in the film?

There is a long history (going back at least to the 1920s) of trying to explain deep questions in physics by positing that the universe might "fundamentally" have more (or perhaps fewer!) dimensions than we see in everyday experience. Our ordinary experience might make the world appear four-dimensional because the extra dimensions are curled up so tightly that we can't see them. For example if you see a piece of string from across the room it might look like a one-dimensional object (length but no width or depth). Getting up closer, though, you can see an intricate three-dimensional structure. This is the way the extra dimensions were presumed to work, for example, in the early days of string theory.

Another idea for why the universe might appear four dimensional is because most of the matter in the universe (electrons, quarks, photons, ...), might be “stuck” to a four-dimensional subspace in, say, a five-dimensional universe. The most famous models along these lines are known as “Randall-Sundrum models,” named after their discoverers Lisa Randall and Raman Sundrum. These models seem to have inspired the speculations about future physics in Interstellar, because in these models only gravitational fields are able to propagate off of the four-dimensional subspace (our universe) and into the five-dimensional “bulk” universe. This behavior of gravitational fields is mentioned at a few points in the film, and it explains why communication with the past was only possible using simple codes delivered through gravitational effects.

When McConaughey falls into the black hole, we enter a scene that is obviously pure fantasy. Any thoughts on what one might actually experience in that situation? (That is, assuming one is not immediately compressed into a singularity.)

The standard answer from general relativity is that not much changes until you start to approach the singularity. Relativity theorists have shown that no experiment can be designed that would establish whether the experimenter is inside or outside of an event horizon. We could be inside a black hole right now and not even know it!

One thing we do know about black holes is that (like all localized sources of gravitational fields), they produce a phenomenon called “tidal stretching.” This is the same phenomenon by which the moon raises tides on the Earth’s oceans. If you were to fall into a black hole feet-first, your feet would be closer to the black hole than your head, and would therefore experience stronger gravity. This effect—your feet constantly being tugged more strongly than your head—inevitably leads to a stretching phenomenon that relativists call “spaghettification.” For extremely massive black holes, like the one in the movie, this effect doesn’t become strong until one is well inside the event horizon, but assuming our current non-quantum picture of spacetime can be trusted, this stretching would become infinitely strong as one approaches the singularity. There are also effects where the direction of the stretching might jump around chaotically, causing a kind of violent mishmash as the singularity is approached. One of my former students, Ben Lemberger ’14, now a graduate student at the University of Wisconsin, researched this phenomenon in an honors project at Oberlin last year.

Is there any scientific basis to McConaughey's ability to manipulate past events from within the black hole?

The idea they put forward in the movie is that information can be carried in the gravitational field (through an effect called gravitational radiation that both I and my Oberlin colleague Dan Stinebring spend a great deal of time studying), along paths that exit the four-dimensional universe, then traverse the five-dimensional bulk universe (where our “time” dimension acts as a space dimension, and is hence traversable in all directions), and then land back in the four-dimensional universe at an earlier time. This is an extremely speculative idea, but it is rooted at least qualitatively in the Randall/Sundrum models where gravity is the only field that is allowed to propagate freely into the five-dimensional bulk universe.

The film seems to care deeply about accurately portraying the relationship between gravity and time. In one section of the movie, the astronauts are on a planet impacted so significantly by gravitational force that each hour to them is the equivalent of seven years on Earth. As such, wouldn't McConaughey’s few minutes inside the black hole have translated to thousands of years on Earth? And would it really be possible for McConaughey’s character to escape the black hole at all?

In order to escape a black hole, one would need to travel faster than the speed of light. One way to do that is to find a shortcut that would allow you to cover distance faster than light even though your instantaneous speed is always slower than the speed of light. A wormhole would provide just such a shortcut. My (entirely unofficial) interpretation for the escape from the tesseract is that the very same wormhole that led to the black hole in the first place might have acted as an escape route. If the wormhole throat near the black hole eventually falls into the black hole itself, even long after McConaughey’s character fell in, then it could reach him before he’s destroyed by the singularity, and he could ride it back out into the outside world. The film even shows that his travel back to Saturn went through this very same wormhole.

Now, as for the time dilation effects: wormholes aren't just shortcuts through space. They're shortcuts through spacetime. If the two ends of a wormhole move relative to one another, they get out of sync, causing even more exotic time dilation effects. Kip Thorne pioneered the study of these effects with a series of papers in the late eighties and early nineties.

My family attended your talk a few weeks ago and mentioned that you have quite a talent for programming simulations. Do you have one or two you'd be willing to share with our readers?

In my talk, I showed a movie that I produced of what it would look like to travel through a wormhole. The video was just a simple proof of concept, meant to demonstrate some of the techniques that would have had to be used by the special effects team. It certainly won't win me any Oscars, but you're welcome to show it. (To give a sense of scale, I superimposed a couple of images on a starfield, one an image of Saturn taken by the Cassini spacecraft, the other a composite galaxy image. The travel through the wormhole looks quite a bit different in my little video than in Interstellar. My visualization used a very simple kind of wormhole, one with a very short “throat.” The wormhole shown in Interstellar has a much longer throat, so the travel through the wormhole takes more time and more interesting optical effects are visible. When I get a bit more free time (meaning, when the semester comes to a close) I hope to extend the computer code I used for my visualization to use the same kind of wormhole that was in Interstellar.

Some more impressive videos, some a bit technical, some a bit less so, can be found on a youtube channel created by my research collaboration. If anyone would like more information on the research that I and my collaborators do, we have a public website.

As a theoretical physicist, what is your take on the film's question of love as a force? Does love have a role to play in the laws of the universe, as the film implies? Or is that just Hollywood being Hollywood?

Well, it's important when communicating these subtle and sometimes arcane concepts that they be embedded in a human story. That was part of the original vision for this movie, going all the way back to 2006. A compelling human story is important for the drama, obviously, but it's also important for fully communicating the science. So often when we learn and lecture about these deep physics concepts, we treat it as a story of "person A" interacting with “person B,” as if we're just working through a series of mathematical theorems and formulas. If we give a little more context than just the one-letter signifiers, we can emphasize that this isn’t just about mathematical formulas, and it isn’t just about a set of phenomena that one hears about in a physics class and then promptly forgets. It's about our world, my home and yours. It's about how that world works, and about what we’re experiencing when we experience it. Physics is about the structure of our experiences, and when we clothe our discussion in human terms it helps us to more fully come to terms with it.

As for love being one of the fundamental forces in the universe, well yes, sometimes Hollywood just can't help itself.