The science of 'Interstellar': A primer on black holes, wormholes, and more

Interstellar is sci-fi that goes heavy on the “fiction,” but still has a healthy dose of science—in part because Kip Thorne, a renowned astrophysicist, signed on as the film’s scientific advisor. Although this doesn’t mean Interstellar is 100 percent accurate, it does mean that it tries to be accurate when it can—and that some parts of the film run the risk of flying right over viewers’ heads.

Because we’re not all experts on wormholes, EW turned to some people who are to help explain what went on in the Interstellar universe: Mark Jackson, founder of Fiat Physica, and Fiona Harrison, a physics professor at California Institute of Technology (and one of the lead investigators on NASA’s NuSTAR black hole mission). The two talked to EW about black holes, wormholes, and why—spoiler alert—Cooper’s daughter is older than him by movie’s end.

This post discusses specific plot details of Interstellar.

The astronaut crew goes into hibernation for the years-long trip to Saturn. But with no one to command the ship, how does it reach its destination?

If you throw something in space, it will continue on in a straight line until or unless something—space debris, an asteroid, etc.—pushes it off that course. So even though McConaughey, Hathaway, and co. are fast asleep, their ship—which has rocketed off in the right direction—doesn’t need any steering help. But there’s still the risk of that “something,” the space junk or asteroids or whatever else might be hanging out in space, pushing the ship off-course. Thanks to telescopes and other technology, though, it’s possible to detect those kinds of obstacles pre-flight.

“I assume that whoever programmed the spaceship would be smart enough to figure out the correct path so it would go in a straight line unless something really unexpected happened,” Jackson says. His assumption is right: Getting pushed off-course is about the onlyproblem the Interstellar crew doesn’t deal with.

The astronauts slip into those funky chambers that will put them to sleep for the duration of the trip to Saturn. How do they work?

It’s called suspended animation, and it’s not entirely possible yet—at least not for lengthy periods. It’s basically hibernation for humans: Theoretically, inducing hypothermia in a human can pause that human’s life while still allowing them to be resurrected at a later time. This is useful not only for humans who have to travel great distances through space, but also for those with serious injuries here on Earth.

Usually when someone has a life-threatening wound or injury, the increasing loss of blood and oxygen that results takes time off the clock—but the colder the body is, the less oxygen that body needs. So a patient who is put in suspended animation won’t suffer the consequences of a loss of oxygen, and surgeons can operate while the patient is in this limbo state.

So far, suspended animation trials have involved temporarily replacing animals’ blood with a cold saline solution, and only for very, very short periods. In Interstellar, there’s no blood-swapping evident: The astronauts just lie down in a chamber full of water, zip themselves up in a plastic bag, and then wait for their alarm to go off, whether it’s two or 10 years from their entrance. That water is most likely freezing cold, so it should have the same effect that replacing blood with cold saltwater would have—but without the pesky needles.

Part of their plan involves traveling through a wormhole to another galaxy. What?

This is exactly what wormholes are for: They’re shortcuts to regions of space that would otherwise take years and years and years to get to. In the movie, Romilly demonstrates this by drawing two dots on either side of a piece of paper and then folding that piece of paper in half, therefore decreasing the distance between those two dots.

There was all that talk about how “they” made the wormhole. What did that mean?

Wormholes are unnatural phenomenons, meaning someone has to make them for them to exist. So when Interstellar‘s NASA identified the wormhole near Saturn, they assumed someone had to put it there for them. Although this part of the film is correct, it’s not possible for us real humans to make wormholes (yet) because it requires a material we don’t know how to construct called exotic matter. “We mathematically can describe them, but we wouldn’t know how to actually build one because we don’t know how to build this exotic material that would keep it stable,” Jackson says.

Cooper goes into a black hole, and in that hole, he basically time travels. Is that possible?

In short, probably not. At least Harrison isn’t buying it. “I think this is a construct of the movie,” she says, “that it’s a self-consistent set of rules. That to communicate backwards in time, that can only be done through gravity or what we call ripples in space-time. I think there’s no real basis for it. But there’s no physical law that rules it out.”

Then he ends up outside of the black hole. Aren’t black holes supposed to keep you inside forever?

Yes. Black holes, are, as Jackson calls them, “a one-way ride.” “If you’re inside a black hole, that’s actually very bad news, because you’re never, ever coming out,” Jackson says.

That’s because once you enter a black hole, the roles of space and time are reversed. “Just like you can’t go back in time right now—you can move back and forth in space but you can’t move back and forth in time—that’s why you can’t leave a black hole,” Jackson says. “It would be like trying to go back in time, and we can’t do that for whatever reason.”

That Cooper makes it into the black hole is a feat in itself. Its incredibly strong gravitational field should have actually stretched him apart through a process called (no joke) spaghettification. “Your feet are getting pulled toward the black hole much more than your head is, and so you get stretched out,” Jackson says. “So it’s actually kind of a gruesome thing that would happen to you as you went near a black hole.”

While Cooper is galavanting around space and not aging, his daughter, Murph, is aging. How is that?

Time can be experienced in different ways; it all depends on the strength of gravity because, as Harrison says, “gravitational fields change the rate at which clocks run.” Here on Earth, someone who is on Mount Everest’s peak will age faster (however slightly) than someone down at sea-level because the person on Mount Everest is closer to the gravitational field of Earth. But Cooper is hanging out in a galaxy full of planets orbiting around a black hole, which, in theory, has a much stronger gravitational field than Earth. So compared to her dad’s position, Murph is near a relatively weak gravitational field. Therefore she experiences time faster—and, as a result, ages faster—than her father.

Still confused (or just want more answers)? Thorne wrote an entire book on the science of Interstellar, appropriately titled The Science of Interstellar, out now.