New insights into the effects of black holes from the team responsible for the Oscar®-winning visual effects of Interstellar.
Depicting a super-massive black hole in the movie Interstellar presented a new challenge to our visual effects team at Double Negative. Luckily the Executive Producer was theoretical physicist Kip Thorne who ended up working closely with us to create a new computer code, DNGR: Double Negative Gravitational Renderer. This code traces the path of light past a spinning black hole (Kerr metric) whose immense gravity warps space and time in its vicinity. A hot disk of gas orbiting the hole appears to wrap over the top of the black hole’s shadow and underneath it; distant stars appear to move in complex swirling patterns as the camera orbits the hole; sometimes images of stars get amplified in brightness, split into double images, or a pair of images merge and annihilate in a flash of light. These effects combined to create the iconic images of the black hole seen in the movie.
The code was innovative in computing these visual effects by propagating beams of light to the camera (in contrast to light rays used in all previous visualization codes). Our beams have an elliptical cross-section, initially intersecting a small circle on the camera’s image plane, roughly the diameter of a pixel. As we trace these beams backwards in time to their origin at either a star, or a glowing portion of the accretion disk, the beams’ cross-sections get stretched and squeezed by the warped spacetime. Using light beams and modelling this effect was crucial to creating the smooth flicker-free images needed for the movie.
In creating DNGR, we realised we had a tool that could easily be adapted for scientific research. The critical curves of the spinning black hole, on the camera’s image plane, are the generalisations of the Einstein ring of a non-spinning black hole. These curves correspond to the direction of incoming beams that geometric optics predict collapse to zero cross-section at the light source, so the source lies on a caustic of the camera’s past light cone. By plotting these critical curves and their caustic origins, we were able to gain a deep understanding of the fascinating gravitationally lensed images seen by a camera in orbit around a fast spinning black hole.
We describe the development of DNGR and the results of these investigations in our article.
Read the full article in Classical and Quantum Gravity:
Gravitational lensing by spinning black holes in astrophysics, and in the movie Interstellar
Oliver James, Eugénie von Tunzelmann, Paul Franklin and Kip S Thorne
Oliver James et al 2015 Class. Quantum Grav. 32 065001
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