Hollywood Helped Physicists Cast Light on Spinning Black Holes

The movie Interstellar was the first to use physics to depict what a black hole would look like up close.

Movie-goers probably didn’t expect it to do much more than contribute some stunning visuals. But now, physicist Kip Thorne and his colleagues have published a paper saying that the visual effects generated by the London-based company Double Negative have actually furthered our understanding of the cosmos.

In other words, a tool that was originally designed for cinematic effects became useful for real scientific purposes.

Filmmaker Christopher Nolan and his team wanted Interstellar to include the most accurate and realistic view of a supermassive black hole possible, so he turned to Thorne for advice. After fiddling with code, they realized that existing visual technology to simulate the black hole would create a flickering effect that would look bad in IMAX theaters.

As a solution, they developed a code called Double Negative Gravitational Renderer (DNGR) to solve the equations for how bundles of light (or light beams) propagate through the warped spacetime of a spinning black hole. Most CGI effects calculate individual light rays instead of bundled ones, so this was a novel approach.

black-hole
Visual technology used in Christopher Nolan’s Interstellar helped physicists learn more about the nature of black holes.

Here’s Adam Rogers, writing for Wired in October:

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.

Now, one of those papers is out, published in the journal Classical and Quantum Gravity this morning. The paper explains how DNGR works and illustrates some new astrophysical insights that came as a result.

Here’s Erin Blakemore, writing for Smithsonian.com:

The team soon realized that the images produced using DNGR code could be used for much more than a fictitious interstellar trip. They began to use the code to conduct simulations of how a weird space surface called a “caustic” might affect images of star fields near black holes in a process known as “gravitational lensing.” Their simulations showed that as caustics are dragged around the sky by the spinning force of a black hole, they stretch around the hole again and again, affecting how stars look. This both creates and obliterates images of stars, creating up to 13 images of a star as the caustic flings images out of the black hole.

caustic-texture
A caustic texture is a space surface created by the reflection or refraction of light around massive objects.

In other words, a caustic is a network of light rays reflected or refracted around a surface—in this case, a black hole. An everyday example of a caustic is seen in the projection of light patterns on a surface like water (see above for an example). As light touches the water, space bends the light to produce this effect. The same is true for light near the accretion disk of a black hole. Thorne’s new paper suggests that, based on DNGR evidence, caustic networks affect the appearance of far-off stars and galaxies if you’re looking at them while in the vicinity of a black hole.