Franklin knew that his computers would do anything he told them to. That was a problem and a temptation. “It's very easy to fall into the trap of breaking the rules of reality,” says Franklin, a senior supervisor of Academy Award-winning effects house Double Negative. “And those rules are actually quite strict.”
So he asked Thorne to generate equations that would guide their effects software the way physics governs the real world. They started with wormholes. If light around a wormhole wouldn't behave classically—that is, travel in a straight line—what would it do? How could that be described mathematically?
Thorne sent his answers to Franklin in the form of heavily researched memos. Pages long, deeply sourced, and covered in equations, they were more like scientific journal articles than anything else. Franklin's team wrote new rendering software based on these equations and spun up a wormhole. The result was extraordinary. It was like a crystal ball reflecting the universe, a spherical hole in spacetime. “Science fiction always wants to dress things up, like it's never happy with the ordinary universe,” he says. “What we were getting out of the software was compelling straight off.”
McConaughey explores another world in Interstellar (top). Thorne’s diagram of how a black hole distorts light.
Diagrams courtesy of Kip Thorne
Their success with the wormhole emboldened the effects team to try the same approach with the black hole. But black holes, as the name suggests, are murder on light. Filmmakers often use a technique called ray tracing to render light and reflections in images. “But ray-tracing software makes the generally reasonable assumption that light is traveling along straight paths,” says Eugénie von Tunzelmann, a CG supervisor at Double Negative. This was a whole other kind of physics. “We had to write a completely new renderer,” she says.
Some individual frames took up to 100 hours to render, the computation overtaxed by the bendy bits of distortion caused by an Einsteinian effect called gravitational lensing. In the end the movie brushed up against 800 terabytes of data. “I thought we might cross the petabyte threshold on this one,” von Tunzelmann says.
“Chris really wanted us to sell the idea that the black hole is spherical,” Franklin says. “I said, ‘You know, it's going to look like a disk.’ The only thing you can see is the way it warps starlight.” Then Franklin started reading about accretion disks, agglomerations of matter that orbit some black holes. Franklin figured that he could use this ring of orbiting detritus to define the sphere.
Von Tunzelmann tried a tricky demo. She generated a flat, multicolored ring—a stand-in for the accretion disk—and positioned it around their spinning black hole. Something very, very weird happened. “We found that warping space around the black hole also warps the accretion disk,” Franklin says. “So rather than looking like Saturn's rings around a black sphere, the light creates this extraordinary halo.”
That's what led Thorne to his “why, of course” moment when he first saw the final effect. The Double Negative team thought it must be a bug in the renderer. But Thorne realized that they had correctly modeled a phenomenon inherent in the math he'd supplied.
Still, no one knew exactly what a black hole would look like until they actually built one. Light, temporarily trapped around the black hole, produced an unexpectedly complex fingerprint pattern near the black hole's shadow. And the glowing accretion disk appeared above the black hole, below the black hole, and in front of it. “I never expected that,” Thorne says. “Eugénie just did the simulations and said, ‘Hey, this is what I got.’ It was just amazing.”
In the end, Nolan got elegant images that advance the story. Thorne got a movie that teaches a mass audience some real, accurate science. But he also got something he didn't expect: a scientific discovery. “This is our observational data,” he says of the movie's visualizations. “That's the way nature behaves. Period.” Thorne says he can get at least two published articles out of it.
When Thorne discusses the astrophysics that he likes best—colliding black holes, space dragged into motion by a whirling star, time warps—he uses a lot of analogies. He talks about two tornadoes running into each other or rays of light cast about like straw in the wind. But metaphors can be deceptive; they can make people think they understand something when they only understand what it is like. But Thorne's haloed, spinning black hole and galaxy-spanning wormhole are not just metaphors. Most Interstellar viewers will see these images—the wormhole, the black hole, the weird light—and think, “Whoa. That's beautiful.” Thorne looks at them and thinks, “Whoa. That's true.” And from a certain perspective, that's beautiful too.
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