Classical and Quantum Gravity, 2015. Reproduced by permission of IOP Publishing

Even black holes wear makeup in Hollywood. Last year’s hit film Interstellar used real scientific equations to depict what happens when a team of space farers venture near a supermassive black hole. Now, a joint paper published in the journal Classical and Quantum Gravity from the movie’s visual effects team and scientific consultant reveal that the real black hole (see above) was deemed too confusing for audiences, and some of the science had to be toned down.

Interstellar’s premise was first conceived by physicist Kip Thorne of the California Institute of Technology, who wanted to make a realistic movie about black holes. He got together with director and co-writer Christopher Nolan, and also with London-based visual effects studio Double Negative to create the movie’s black hole, Gargantua.

“I’d ask him a question and maybe a week later, sometimes a month, I’d get a beautifully presented paper that he’d laid out with references going into the history of the problems I’d been asking about,” says Oliver James, chief scientist of Double Negative.


“We base it in science, but we always give control so that artists can change it”

It’s not the first time physicists have used Albert Einstein’s equations of general relativity to produce images and movies of a black hole’s space-warping properties. But these were much lower resolution and less detailed than a Hollywood production, so the team had to make a few changes. To avoid flickering discontinuities, rather than tracing the paths of individual light rays to generate an image, they used bundles of rays, which serve to smooth out the resulting movie. “That involved quite a lot of research to calculate what would happen,” says James.

Black and blue

The most striking element of Interstellar’s Gargantua is its accretion disc, the glowing ring of matter that encircles it. The team started out using a simple, rainbow-coloured flat disc to figure out how it would be warped by the black hole, then exchanged it for a more wispy disc with realistic colours.

The result looked good, but the central black hole seemed to be squashed up against one side. That’s because the movie’s time dilation effects meant the black hole had to spin very fast, causing it to drag the light to one side. Nolan didn’t like this asymmetry and thought moviegoers wouldn’t understand why, so the team slowed it down, says James.

Classical and Quantum Gravity, 2015. Reproduced by permission of IOP Publishing

Gargantua’s disc in the movie is also redder and brighter than it would be in real life (see above). As the team worked on the movie, they added levels of scientific detail. They found that the black hole’s rotation turned the glowing red matter a cool blue, thanks to the Doppler effect shortening the wavelength of the light it gave off. It also made one side of the disc much darker, to the point of almost being invisible. Again, Nolan vetoed these details.

“We base it in science, but we always give control so that artists can change it,” says James. “The first images we gave him didn’t have the Doppler shift, and I think he fell in love with them.”

Far from realism

“When I saw the movie, I immediately saw that the black hole did not look as it should for a near maximally spinning black hole,” says Andrew Hamilton of the University of Colorado in Boulder. Now that he has read the paper, he’s glad to see they slowed it down for a reason. “I had not realised just how careful the Interstellar team had been with their renderings.”

Alain Riazuelo of the Paris Institute of Astrophysics says he appreciates the team’s efforts, but a pure science project would have done things differently, because astronomers want to create models of what their telescopes might see from afar. “From an astrophysics perspective you would want to simulate different configurations of matter around the black hole, then try to predict what your observations would give you,” he says – the team just choose a disc they thought looked nice.

Riazuelo met with Thorne a few years ago and gave him some early visualisations, so was slightly disappointed when the film wasn’t totally realistic. “I understood after a few minutes why they had done this, but I would have preferred they stick a little more close to realism,” he says, though it could have been much worse. “You should keep in mind there was nothing that obliged Christopher Nolan to try to stick to realistic science.”

The techniques developed for Interstellar could have unexpected benefits beyond black holes. James says he’s been emailed by researchers on a NASA project planning to study spinning neutron stars who say the team’s equations could help them interpret real astronomical data. “Initially when the film came out everyone was really excited that real science was being used to make films,” says James. “As film makers we’re now really excited that our science might get used in NASA projects to do things we’ve never thought of.”

Journal reference: Classical and Quantum Gravity

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