[math-fun] Hollywood renders GR visible

FYI -- A Hollywood movie that really, really tries hard to get a correct visualization of general relativity... http://www.wired.com/2014/10/astrophysics-interstellar-black-hole/ Wrinkles in Spacetime: The Warped Astrophysics of "Interstellar" by Adam Rogers Kip Thorne looks into the black hole he helped create and thinks, “Why, of course. That's what it would do.” This particular black hole is a simulation of unprecedented accuracy. It appears to spin at nearly the speed of light, dragging bits of the universe along with it. (That's gravity for you; relativity is superweird.) In theory it was once a star, but instead of fading or exploding, it collapsed like a failed soufflé into a tiny point of inescapable singularity. A glowing ring orbiting the spheroidal maelstrom seems to curve over the top and below the bottom simultaneously. All this is only natural, because weird things happen near black holes. For example, their gravity is so strong that they bend the fabric of the universe. Einstein explained this: The more massive something is, the more gravity it produces. Objects like stars and black holes do this so powerfully that they actually bend light and pull space and time with it. And it gets weirder: If you were closer to a black hole than I was, our perceptions of space and time would diverge. Relatively speaking, time would seem to be going faster for me. What does Thorne see in there? He's an astrophysicist; his math guided the creation of this mesmerizing visual effect, the most accurate simulation ever of what a black hole would look like. It's the product of a year of work by 30 people and thousands of computers. And alongside a small galaxy of Hollywood stars—-Matthew McConaughey, Anne Hathaway, Jessica Chastain, Bill Irwin, Casey Affleck, John Lithgow—-the simulation plays a central role in Interstellar, the prestige space travel epic directed by Christopher Nolan opening November 7. Thorne sees truth. Nolan, the consummate image maker, sees beauty. Black holes, even fictional ones, can warp perception. Thorne Isn't your average astrophysicist. Sure, he's a famous theorist, but even before his retirement from Caltech in 2009 he was deeply interested in explaining the heady ideas of relativity to the general public. Just before his retirement, Thorne and film producer Lynda Obst, whom he'd known since Carl Sagan set them up on a blind date three decades earlier, were playing around with an idea for a movie that would involve the mysterious properties of black holes and wormholes. Before long, Steven Spielberg signed on to direct; screenwriter Jonathan “Jonah” Nolan wrote a script. Eventually Spielberg dropped out; Jonathan's brother Chris—-known for directing mind-bendy movies like Memento and Inception (plus Batman) dropped in. And while Chris Nolan was rewriting his brother's script, he wanted to get a handle on the science at the heart of his story. So he started meeting with Thorne. Over the course of a couple months in early 2013, Thorne and Nolan delved into what the physicist calls “the warped side of the universe”—-curved spacetime, holes in the fabric of reality, how gravity bends light. “The story is now essentially all Chris and Jonah's,” Thorne says. “But the spirit of it, the goal of having a movie in which science is embedded in the fabric from the beginning—-and it's great science—-that was preserved.” The story the filmmakers came up with is set in a dystopian near future when crops have failed and humanity is on the verge of extinction. A former astronaut (McConaughey) gets recruited for one last flight, a desperate attempt to reach other star systems where humans can once again thrive. And therein lies a problem. See, other stars are really far away. Reaching even the nearest ones would take decades at speeds we humans have no idea how to attain. Back in 1983, when Sagan needed a plausible solution to this problem for the story that would become the movie Contact, Thorne suggested the wormhole, a hypothetical tear in the universe connecting two distant points via dimensions beyond the four we experience as space and time. A wormhole was a natural choice for Interstellar too. As Thorne talked about the movie with Nolan, their discussions about the physical properties of wormholes led to an inevitable question for a filmmaker: How do you actually show one onscreen? That's not the only headache inducing bit of physics that the film's special effects team had to grapple with. Nolan's story relied on time dilation: time passing at different rates for different characters. To make this scientifically plausible, Thorne told him, he'd need a massive black hole—-in the movie it's called Gargantua—-spinning at nearly the speed of light. As a filmmaker, Nolan had no idea how to make something like that look realistic. But he had an idea how to make it happen. “Chris called me and said he wanted to send a guy over to my house to talk to me about the visual effects,” Thorne says. “I said, ‘Sure, send him over.’” It wasn't long before Paul Franklin showed up on Thorne's doorstep. 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.” 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.