<em>Interstellar</em> Science: 'That's Relativity'

Have you seen Christopher Nolan's Interstellar yet? It's generating a lot of discussion both about the story and the science, and as one of the characters explains in the movie ("that's relativity"), the science focuses around Einstein's theory of relativity. So as the author of a new book about that theory -- What is Relativity? (Columbia University Press, 2014) -- I thought I should weigh in on the movie.

First and foremost: I really liked it. The story isn't perfect; for example, while I loved the father-daughter themes, I felt kind of bad for the son, who wasn't always portrayed in a very flattering way. And I'm not quite sure why Matt Damon's character was in the film -- for me, that sequence distracted from the main story line. That said, the movie made me think -- and not many movies do that these days, especially ones that concern topics that I've been studying for many years. So go see it...

Second, as a parent, I'm always looking for guidance on what movies are appropriate for children, so here's my advice: I'd take kids from about fifth grade on up. There's nothing particularly wrong with taking younger kids. Indeed, this movie is remarkable for Hollywood in that it contains no sex, no real violence beyond a fist fight and a couple explosive accidents, and I can't even recall a bad word appearing in the dialogue. But the story line is pretty sophisticated, so kids younger than about fifth grade will have trouble following it, which may leave them bored in a movie that runs nearly three hours. The movie also begins with a fairly dark theme -- it takes place in the relatively near future and shows an Earth that has been environmentally ravaged -- and this may be troubling to younger children.

But my main goal in this post is to talk a little about the science, so the rest of this is really aimed at those of you who have already seen the movie (though I'll still try to avoid spoilers). Here is my brief list of what's real, what's speculative, and what seems fairly unreal. I make no claims for this list to be comprehensive, but I believe it covers the main points.

What's Real

From a science standpoint, the good news is that there's a lot in Interstellar that is real, solid science. This probably stems largely from the fact that Caltech professor Kip Thorne -- who has long been one of my physics heroes -- served as an executive producer on the film.

So in no particular order, here are some things done in the movie that are really true:

"That's Relativity": It really is! For background: Einstein published his theory of relativity in two parts. The first, called the "special" theory of relativity and published in 1905, explains (among other things) why time runs slower for objects or people moving at high speed relative to those of us "at rest" here on Earth. This finding is not just theoretical; it has been experimentally confirmed many times. The second part, called the "general" theory of relativity and published in 1915, is really a theory about gravity and it includes the fact that time slows down in strong gravitational fields, like those near the black hole called "Gargantuan" in the movie. Worth noting: Next year (2015) will be the 100th anniversary of Einstein's publication of general relativity, so the release of a movie in which relativity is a central theme seems particularly timely.

Black holes don't suck. Despite common sentiment to the contrary, black holes don't suck. That is, if you are at some reasonable distance from a black hole, you will orbit it just like you would any other object of the same mass. The movie does this very well, as you'll notice that both the planets and the spaceship have no difficulty remaining in orbit around the black hole. The only subtlety occurs when they decide to enter the black hole, and one of the characters actually uses the words "sucked in" to describe what will happen. But it's not really "sucking"; rather, as the visuals show well, when you get close enough to the black hole the distortions of space and time become so great that no stable orbits are possible, and you will inevitably fall in. For those who are interested in understanding how this is different from "sucking," you'll find the explanation on p. 137-138 of my book (What is Relativity? ).

A supermassive black hole in another galaxy. I don't recall the mass of the black hole ("Gargantuan") being given explicitly in the movie, but backup for the film states that Thorne based his calculations on a rotating black hole with a mass of about 100 million times that of the Sun. Black holes like this really do exist, as supermassive black holes are found in the center of almost every galaxy. Even our own Milky Way has one, but ours (often called "Sagittarius A-Star") has a mass only of about 4 million times that of the Sun -- which explains why the movie had to place this one in another galaxy. Why did they use such a supermassive black hole rather than a more modest one? Because of tidal forces. For a more "ordinary" black hole (seems a bit of an oxymoron, but oh well), tidal forces would kill anyone trying to enter it and destroy any planets orbiting close enough for significant effects on time. But the tidal forces are weaker for supermassive black holes, and at 100 million solar masses, it would indeed be possible to pass safely across the event horizon. As to rotation, we expect most or all black holes to be rotating, probably quite fast. The calculations for rotating black holes become very complex, but I trust that Dr. Thorne's involvement means they were done correctly. Click here for a nice infographic showing some of the black hole features.

Black hole and worm hole visualizations: As has been much advertised, the visualizations shown in the movie are based on actual calculations by Dr. Thorne and his team, and therefore show our best scientific guesses at what it would really look like if we took a trip like that shown in the movie.

Artificial gravity through rotation. The movie shows both a spaceship and a space station that rotate to create artificial gravity, and correctly shows what it looks like inside. As an aside, note that this is one of many things in the movie that are reminiscent of the movie 2001: A Space Odyssey, which apparently was a major influence on director Nolan.

The trip to Saturn. The movie spaceship needs two years to get to Saturn, which is within the realm of plausibility with current technology. The depiction of Saturn is also realistic, presumably based on images from the Cassini mission.

The unknowns of physics around "singularity." The movie states correctly that the center or "singularity" of a black hole represents one of the great unknowns of physics. The movie also correctly gives the reason: Two of the most successful theories in physics -- general relativity (which explains gravity and the large-scale universe) and quantum mechanics (which explains atoms and the small-scale universe) -- give different predictions for what should happen at singularity. We therefore presume that there must be some grander theory of physics that we have yet to discover and that would explain singularity. The fact that there is this "known unknown" territory in physics gives the movie license to move into the realm of speculation, which brings us to the next part of this list.

What's Speculative

The movie also includes some true "science fiction" ideas; namely, ideas rooted in real science, but which go beyond what current science can say with great confidence. I counted three major areas in which the film dove into this type of speculation.

Wormholes that connect distant parts of the universe. The idea for wormholes comes from the equations of Einstein's general theory of relativity. However, under current physics, there is no known way to make a stable wormhole that would allow objects or people to travel through it. In essence, the movie speculates that future discoveries in physics will mean that this type of stable wormhole travel is possible, but for now there is no way to know whether that will really prove to be the case. On a related note, whatever might make a wormhole stable is presumably the same thing that the movie is referring to when it talks about anomalies in gravity, and hence explains why the word "gravity" is used so many times.

Inside the black hole. The event horizon of a black hole is the point of no return, from which no material or light can ever emerge back into the surrounding universe. (Be sure to notice that the event horizon, which is like the "surface" of the black hole, is different from the singularity, which is in the very center of the black hole.) Therefore, we do not know what happens inside a black hole, so everything in the movie that happens within the event horizon is clearly speculative. The movie also suggests that some unstated quantum phenomena might allow communication across the event horizon, which is another speculative idea that, for now, we can neither prove correct nor wrong.

A "dying" Earth. In some ways, the most interesting speculation in the movie is about what may happen to Earth in coming decades, since the movie is clearly set in a future not more than a few decades away. The movie does not tell us exactly what happened, but it clearly involves some sort of environmental devastation, and much of what it shows is consistent with some of the worst-case scenarios around global warming. This alone should get everyone thinking: While worst-case scenarios are by definition not the likeliest case, we should always keep them in mind when deciding how to deal with a problem such as global warming. For more on distinguishing truth and fiction when it comes to debates about global warming and how we should deal with it, I encourage you to read and view the video at my previous post The Truth About Global Warming.

What's Probably Not Real

The movie is of course a work of fiction, so itself contains plenty of things that are not real, or that serve as plot devices that may not make sense if you analyze them too deeply. But that's OK; it's a part of story telling. In terms of scientific ideas that seem unrooted from real science, I noticed only two things, both concerning the planets that they visit:

Habitable planets around a black hole. The movie has planets with varying degrees of habitability orbiting the black hole. Perhaps the filmmakers have some justification for this that I'm unaware of, but it seems to defy scientific plausibility. In order for planets to have conditions like those shown in the movie, they would need a source of heat and light; indeed, they need plenty of visible light, since they could see on the surfaces, and must not have a lot of higher-energy light (ultraviolet and X-rays) that might prevent a planet from having a stable atmosphere. But the only heat and light around a black hole will be coming from its accretion disk (material falling into the black hole from surrounding space). We've observed plenty of radiation from black hole accretion disks, including from ones around supermassive black holes like "Gargantuan," and their output is primarily in high-energy light, which means they could not provide the type of heat and light shown on the planets in the film. So here, I think, they just made it up for their story line. There are several other specifics that seem unrealistic, such as the huge waves in the very shallow ocean, but those concern me less, since the mere existence of the ocean doesn't make sense on these worlds.

Oxygen without life. For one planet in particular, the movie suggests that it has an atmosphere with enough oxygen for us to breathe, but they at least suggest that it may have no life on it, only lots of hydrocarbons and other things that would help us out. But oxygen is a highly reactive gas, and as far as we know, there is no way for a planet to have this level of oxygen content unless the oxygen is being actively produced by life through photosynthesis (or some similar process).

To learn more:

--If you want to understand relativity at a very basic level, suitable for anyone in high school or older, I humbly suggest that you try my book What is Relativity? It's fairly short at under 200 pages, and I worked hard to make it understandable even if you have very little background in physics.

--To learn more about worm holes and other more exotic ideas that come from relativity, one of my favorite books is Kip Thorne's Black Holes and Time Warps: Einstein's Outrageous Legacy.

-- Dr. Thorne also has a new book, The Science of Interstellar. I have not yet seen it, but I would presume that it will be very good.

-- And, of course, you can find much more on the web. Try a simple search on the "science of Interstellar" and you'll get may articles, and more are likely to be coming soon.