Thought Process The Science of Interstellar OBERLIN ASSISTANT PROFESSOR OF PHYSICS ROB OWEN WAS PART OF THE RESEARCH GROUP LED BY PHYSICIST KIP THORNE, THE LEAD SCIENCE CONSULTANT FOR THE FILM INTERSTELLAR, WHEN THE FILM WAS IN ITS FIRST STAGES OF DEVELOPMENT SEVEN YEARS AGO. INTERVIEW BY BEN JONES ’96 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 under- stood at this point. 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. When the lead character 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? 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 stretch- ing phenomenon that relativists call “spaghettifica- tion.” 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, 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—involv- ing existence in a five-dimen- sional “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 math- ematically 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 from 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. Turning our attention to the plot of Interstellar, 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 rela- tivity 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 1 0