Interstellar Science: What Would Really Happen if Your Planet Orbited a Black Hole?

New frontiers in space exploration
Nov. 6 2014 10:27 AM

Interstellar Science

What the movie gets wrong and really wrong about black holes, relativity, plot, and dialogue.

An Interstellar mess.

Photo illustration by Juliana Jiménez Jaramillo. Photo by Melinda Sue Gordon/Warner Bros./Paramount Pictures

I generally enjoy writing movie reviews; they’re a fun way to gather my thoughts about a movie, analyzing its plot, the production, the writing, even the science.

Phil Plait Phil Plait

Phil Plait writes Slate’s Bad Astronomy blog and is an astronomer, public speaker, science evangelizer, and author of Death From the Skies!  

It’s for that very reason I dreaded writing this one. I was really looking forward to seeing Interstellar … but I thought it was awful. A total mess. So if you’re looking for a tl;dr, there it is. I really, really didn’t like it. And I really, really wanted to.

What makes it worse is that the movie could have been truly great. The overall plot isn’t bad (if a rehash of an old science fiction idea), and some of the ideas in it were solid. The special effects were breathtaking. Outstanding. But they can’t carry a movie with leaden dialogue, obvious foreshadowing, ham-fisted philosophy, and a serious but misguided attempt to be deep. And a lot of the critical details in the plot were a mishmash of ideas that made no sense.

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And the science. Oh dear. The science.

From here on out there will be spoilers, so fairly warned be thee, say I.

Plot Boiler

The plot is hard to synopsize, but here’re the bullet points: In some unspecified time in the future, likely more than 50 years hence, the world is in ecological disaster. Crops are failing, food is scarce, billions have died. Matthew McConaughey plays Cooper, an ex-pilot and engineer who is now struggling to grow corn on his farm along with his father-in-law, son, and daughter Murph. His daughter complains of a ghost in her room that’s trying to send her messages. Initially dismissive, Cooper discovers the messages are real, are encoded using gravity somehow, and include coordinates to a location somewhere within driving distance.

Cooper and Murph discover a secret NASA base at those coordinates, and Cooper is told that half a century before, a “gravitational anomaly” was discovered out near Saturn: a wormhole, presumably placed there by aliens, also presumably the same beings who communicated with Murph using gravity. A dozen habitable planets have been detected on the other side, and a dozen humans sent to explore them. One system has three potentially habitable planets, and it’s now up to Cooper to pilot a ship through the wormhole, figure out which planet is best, and save humanity by giving humans a new home.

At this point the movie pretty much falls apart, both scientifically and in its storytelling. For example, NASA, despite being defunded decades before, somehow has the capability of launching dozens of crewed ships that would cost hundreds of billions of dollars each (and does so, inexplicably—get used to that word—from an underground silo that is literally right next to its work offices). It wasn’t clear why the ships had to have a crew as opposed to being robotic, and the idea that only low-bandwidth data could be sent back (thus precluding getting lots of details about the planets) struck me as a brazen and clunky plot device to get Cooper and his crew to go take a look for themselves.

Cooper successfully pilots the ship through the wormhole (which was lovely and quite well-done, even down to the much-used explanation of how wormholes work borrowed from A Wrinkle in Time), and on the other side he and his crew find the three-planet system, which is inexplicably orbiting a black hole. I sighed audibly at this part. Where do the planets get heat and light? You kinda need a star for that. Heat couldn’t be from the black hole itself, because later (inevitably) Cooper has to go inside the black hole, and he doesn’t get fried. So the planets inexplicably are habitable despite no nearby source of warmth.

At this point I could go on and on (and on and on and on and on … ) with the scientific missteps the movie takes from here. Let me just pick one example, since it was crucial to the movie’s plot but shows how much science was tossed out the airlock.

The Planet That Wasn’t There

It turns out that one of the three planets orbits very close to the black hole, so close there will be severe relativistic effects. Relative to a distant observer, time slows down near a black hole (true), so one hour on the planet will equal seven years elapsing back on Earth. Right away, this is a big problem. To get that kind of time dilation (a factor of about 60,000), you need to be just over the surface of the black hole, and I mean just over the surface, practically skimming it. But because of the way black holes twist up space, the minimum stable orbit around a black hole must be at least three times the size of the black hole itself. Clocks would run a bit more slowly at that distance than for someone on Earth, but only by about 20 percent.

In other words, for the planet to have the huge time dilation claimed in the movie, it would have to be too close to the black hole to have a stable orbit. Bloop! It would fall in.

Also, there’s the problem of tides. One side of the planet is much closer to the black hole than the other side. Gravity changes with distance; the farther you are from the source, the weaker the gravity you feel. The change in the force of the black hole’s gravity across a planet’s diameter is very large, creating a tidal force that stretches the planet. That close to a black hole, the tidal force is huge, mind-(and planet-)bendingly huge. So huge, the planet would be torn to shreds, vaporized.