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.
Courtesy Paramount