Cinema Peer Review: Astrophysicist Katie Mack Reviews ‘Interstellar’

I watched Interstellar and came out deeply disappointed. Since then, I’ve read many other reviews, of both the science and the filmmaking, and I keep thinking maybe I’m being too hard on the film. Yeah, some of the science was bad. A lot of movies have bad science, and this one also had a lot of really good science. And it’s just a movie! So maybe I should give it a pass. Maybe I should say, “It did a really great job explaining relativity!” (which it did) and leave it at that.

But I can’t. For one thing, this film was hyped like crazy for its supposedly accurate science. It was compared with 2001, which really did have great science (excluding the stuff about weirdly supernatural alien powers because you really do have to give that a pass if you want to enjoy science fiction at all). And it was promoted to the physics community as if it would be our one chance to watch a film and be blown away by its fidelity to physical law. Even worse than the scientific failings, though, is that it misrepresented how science is done. And when I see that, I start to get a lot less charitable. In any case, I now have an excuse to tell you some true and totally mind-bending things about relativity and black holes, and who could resist that?

Scientific Accuracy is Relative

All you need to know about the non-scientific aspects of Interstellar’s plot, for the purpose of this review, is that Matthew McConaughey plays a pilot-turned-farmer named Cooper, and he’s recruited to go into space and save the world by (among others) a physicist named Professor Brand (Michael Caine). And somewhere out there he encounters a planet orbiting a really massive black hole.

Even for a science fiction film, Interstellar contains a lot of science. It starts with ecology, agriculture, and atmospheric science. It touches on robotics and aeronautics. There’s orbital mechanics and planetary science and, of course, relativity. I’m not going to be able to talk about all of that, so I’ll stick with a couple examples dealing with the science I know: physics.

The scientific topic that looms largest is general relativity, or GR. There are sort of two kinds of relativity. Specialrelativity was Einstein’s first big breakthrough in this area, and it all hinges on the idea that light can only travel at a constant speed (through a vacuum anyway), and that speed is the ultimate speed limit in the Universe. Starting from there you can (as Einstein did) derive that the passage of time is relative, and depends on how you’re moving. If you’re moving at a speed that’s close to the speed of light, your watch will tick much more slowly than the watch of someone who has waited at home. Special relativity was the first theory of space-time—the essential melding of space and time, and the idea that time is its own dimension, though one we can only travel through forward.

General relativity is an extension of special relativity that includes gravity, and how it affects space-time. In GR, you can think of massive objects as bending the space-time in their vicinity. The more massive an object is, the more space-time curves around it, which is why planets orbit stars: they’re trying to go in a straight line, but they’re curved around by the distorted shape of the space they’re travelling through. GR also tells us that the way time passes is affected by space-time curvature. The closer you are to a massive object, the slower your time passes. This is called time dilation, and it’s a major plot point in Interstellar. It really is true that if you’re close to a black hole, your time will slow down, and the closer you are, the more extreme the dilation is. Theoretically, you could get close enough to a black hole for only hours to pass for you while decades pass on Earth.

The Spaghetti Factor

But there’s a really important thing about gravity that Interstellar leaves out which doesn’t depend on GR at all–it comes from long-standing ideas in classical physics, and as far as I can tell,* complicated GR effects don’t fix it.

Let’s say you’re falling towards a black hole, feet first. The black hole has tremendous gravity, because it is both massive and compact—the defining feature of black holes in general. When you’re falling toward the black hole, your feet are just a little bit closer to the hole than your head is, and that means they feel a stronger gravitational pull, and it stretches you out. This is true on Earth as well, but since Earth isn’t all that massive, and you’re not very close to the center of mass, it has a negligible effect. For a black hole, this head-to-toe gravitational discrepancy, called a tidal force, is not negligible anymore. Tidal forces are responsible for an effect known in physics as “spaghettification”—the technical term for objects (or astronauts) being stretched into thin spaghetti-like strands.

Spaghettification should appear, but doesn’t, in two contexts in the film, and both times it should completely destroy the plot device in question. The first time the concept is glossed over is to allow a planet to orbit very close to a black hole. Let’s just ignore for the moment the fact that anything that close to a black hole would be violently assaulted by high-energy radiation emitted by things swirling into the black hole in the form of an accretion disk. (And we know this black hole has an accretion disk, because we see it, in one of the most stunning visualizations in the film.) Supposing somehow the planet survives the x-ray blast from the disk, it would be ripped apart by just being in the hole’s vicinity. The point of disintegration is called the Roche Limit, and it’s why you can have large moons orbiting planets if they keep their distance, but only thin rings of tiny rock and ice clumps really close in. Do a bit of light number crunching and you can work out that the “safe” distance for the black-hole-orbiting planet would be at many times farther away than it appears in the film, and at that distance, the time dilation effect would be fairly small.**

The second time tidal forces should come up is when someone goes into a black hole, beyond the event horizon, and is not even slightly spaghettified. There is nothing that could save a person or a spacecraft from tidal forces, and by the time you go past the event horizon, you’re not only lost forever, you’re also doomed to be stretched and crushed to oblivion. So you really shouldn’t try it.

Did Physicists Go Extinct?

I could go on about other scientific inaccuracies, but I won’t – the spaghettification issue is the most egregious thing, and a lot of the others (like the weird orbital mechanics and various things to do with planets visited) have been covered in detail elsewhere. The thing that ultimately killed my suspension of disbelief the most was the movie’s depiction of how science is done.

One of the key plot points has to do with Professor Brand’s attempt to solve “the gravity equation.” Brand claims that if only he can solve gravity, he can create a new kind of spacecraft and literally save the world. At some point he says that if he has a few years, he’ll sort it out. When I heard that, I had to stop myself from blurting out, “What, by yourself?!” In science, nobody does that. Physicists don’t work like that. We talk to each other. We read the literature. We compare notes and work in teams and divide up problems amongst ourselves. We don’t sit in a room with a blackboard and maybe one assistant and just think it out. The character who drags Cooper kicking and screaming into the space program to leave Earth and maybe never come back apparently can’t even manage to find a single PhD student to talk through his equations? I know this is the apocalyptic future, but even so, physicists can’t be totally extinct. I think the trope of the “lone genius scientist” is actually kind of damaging for the way science is understood by the public, so seeing this kind of depiction is a sore point for me.

Anne Hathaway also plays a scientist in the film, but by the time I got to the point in the film when she starts hypothesizing that love is a dimension transcending space and time I had already given up on seeing an accurate portrayal of how scientists talk and think. Remember that this is a film that expects us to believe that Cooper—an engineer who certainly does not come off as un-curious—would have agreed to fly through a wormhole without asking even the most basic questions about relativity and spacetime before setting off. It means we get a nice explanation of wormholes with a folded piece of paper, but you’d think they could have done that back on the ground.

Fade To Black (Hole)

I don’t know if I would recommend Interstellar. Some of the visuals are gorgeous, of course, and it legitimately rivals 2001 and Gravity in that regard. The spacecraft is depicted in a reasonably realistic way, as are a lot of little details about space travel. The spectacular visualizations of the black hole and wormhole are apparently based on such accurate physics that they are being written up for a scientific paper. I don’t think the scientific inaccuracies alone would be enough to make me not recommend the film, if I liked it for its plot, dialog, pacing, etc. Unfortunately I felt it failed on those criteria as well, which was a real disappointment for a film I wanted to love.

It’s a visually impressive film with some very good explanations of general relativity. If you know nothing about the science of time and gravity, you probably will learn something, which is a statement I wouldn’t have expected to be able to make about any film. But you’ll learn nothing about how science is done, and if you do know a bit about space travel and planetary science you’ll probably end up more frustrated than enlightened. So, sure, go ahead and watch it, on the biggest screen you can find, but be prepared for some cringing. And keep in mind that it’s almost three hours long. The spaceship isn’t called “Endurance” for nothing.

Rating: 2 out of 5 supermassive black holes

* It’s been claimed that some kind of complicated GR issue could possibly resolve this, but (a) I don’t see how, and (b) I have yet to be shown that GR corrections, should they exist, could change the answer by the necessary order(s) of magnitude. Since the planet is probably within the ergosphere, the shearing of spacetime probably makes the disruption happen even faster.

** In his first (now corrected) review, Phil Plait said you wouldn’t be able to get close enough to a black hole for the time dilation the film requires without being too close for a stable orbit. This is true for a non-rotating (Schwarzschild) black hole, but a rotating (Kerr) black hole can actually have stable orbits right on the edge of the event horizon. If we assume the black hole in the film is rotating very rapidly, the time dilation might be plausible, but the immunity to tidal forces is not.