Double Negative Builds an Interstellar Camera for Black Holes

Using their code, the Interstellar team says that if a camera were close up to a rapidly spinning black hole, surfaces in space known as caustics would create more than a dozen images of individual stars and of the thin, bright plane of the galaxy in which the black hole exists. They found that the images are concentrated along one edge of the shadow a black hole casts. The shadow is either ring-shaped or an undefined shape, and is as a result of background light that each gravitational body absorbed and prevented from reaching the camera.

Warping Light

The team asserts that multiple images are caused by the black hole dragging space into a whirling motion and stretching the caustics around itself many times. Their work is among the first computations of the effects of caustics on a camera near a black hole and, from there, the resulting images give some idea of what a person might see if they were orbiting around a black hole.

The discoveries were made possible by the team’s computer code, which, as the paper describes, mapped the paths of millions of lights beams and how they were magnified and distorted as they passed through the black hole’s warping effect on space and time. The computer code was used to create images of the wormhole and the story’s black hole Gargantua and its glowing accretion disk with enough smoothness and clarity.

The code also showed portions of the accretion disk swinging up over the top and down under Gargantua’s shadow, and in front of the shadow’s equator, producing the well known image of a split shadow that has come to represent the movie.

This distortion of the accretion disk was caused by gravitational ‘lensing’, in which the black hole bends and distorts light beams from different parts of the disk, or from distant stars, before they arrive at the simulated camera. The theory is that the extremely strong gravitational field literally bends the fabric of spacetime around itself.

Gravitational Renderer

Early in their work on the movie, starting with the black hole encircled within a dense field of distant stars and nebulae instead of an accretion disk, the team found that the standard approach of using just one light ray per pixel in a computer code, in this case a total of 23 million pixels to produce the IMAX picture, resulted in flickering as the stars and nebulae moved across the screen.

Co-author of the study and chief scientist at Double NegativeOliver Jamessaid, “To get rid of the flickering and produce realistically smooth pictures for the movie, we changed our code in a manner that has never been done before. Instead of tracing the paths of individual light rays using Einstein’s equations - one per pixel - we traced the distorted paths and shapes of light beams.” He said that once the code, named DNGR for Double Negative Gravitational Renderer, was reliably creating the images used in the film, the team felt that it could be adapted for scientific research.

In their paper, several research simulations are described that the team used DNGR to create to explore the influence of caustics - creased surfaces in space - on the images of distant star fields as seen by a camera near a fast spinning black hole.

“A light beam emitted from any point on a caustic surface gets focussed by the black hole into a bright cusp of light at a given point,” said Oliver. “All of the caustics, except one, wrap around the sky many times when the camera is close to the black hole. This sky-wrapping is caused by the black hole’s spin, dragging space into a whirling motion around itself like the air in a whirling tornado, and stretching the caustics around the black hole many times.”

Shadow Play

As each caustic passes through a star, it either creates two new images of the star that the camera records, or destroys two old images of the star. As the camera orbits around the black hole, film clips from the DNGR simulations showed that the caustics were constantly creating and destroying a huge number of stellar images.

The team identified as many as 13 simultaneous images of the same star, and as many as 13 images of the thin plane of the galaxy in which the black hole is located.Kip Thornesaid, “This new approach to making images will be of great value to astrophysicists like me. We, too, need smooth images.”

These multiple images were only seen when the black hole was spinning rapidly and only near the side of the black hole where the hole’s whirling space was moving toward the camera, which they decided was because the space whirl was flinging the images outward from the hole’s shadow edge. On the shadow’s opposite side, where space is whirling away from the camera, the team concluded that there were also multiple images of each star, but that the whirl of space compressed them inward, so close to the black hole’s shadow that they could not be seen in the simulations.

A second, related paper called‘Visualizing Interstellar’s Wormhole’is available on botharXivand Double Negative’swebsite, and is to be published in the American Journal of Physics. This second paper is also co-authored by Oliver James, Eugénie von Tunzelmann, Paul Franklin and Kip Thorne. It describes the creation of the ‘Interstellar’ wormhole and suggests various study opportunities the film might be useful for, in elementary courses on general relativity. www.dneg.com