Gravitational lensing by black holes in astrophysics and in Interstellar

Interstellar's accretion disc, with and without Doppler shift.

Interstellar‘s accretion disc, with and without Doppler shift. Figure 15 a, c from “Gravitational lensing by spinning black holes in astrophysics, and in the movie Interstellar” Oliver James et al 2015 Class. Quantum Grav. 32 065001

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.

Eugénie von Tunzelmann, Kip Thorne and Oliver James

Left image by Rosie Draper, copyright Double Negative 2014. Reproduced with permission. All rights reserved.
Right image copyright Joe Wehmeyer. Reproduced with permission. All rights reserved.
Oliver James (middle right) is Chief Scientist at Double Negative. Oliver studied Physics at Oxford University, then took a change in direction and became an apprentice in a photographic studio. In 1995 he combined those disciplines by joining the nascent digital effects industry and for the last twenty years has been applying scraps of physics to movies including The Matrix Reloaded, The Matrix Revolutions, Harry Potter, Quantum of Solace and Inception. It’s with this project, however that he’s firmly cemented together these three strands.
Eugénie von Tunzelmann(far left) is a CG Supervisor at Double Negative. Eugénie studied Engineering & Computing Science at Oxford University; her Masters project focused on the use of computer vision methods for image tracking in the visual effects industry. After university she moved into feature film visual effects, working first as an R&D developer before moving over to the artistic side and specialising in volumetric fluid simulation. As well as Interstellar, she has worked on films including Man of Steel, Iron Man 2, Batman Begins, Hellboy II: The Golden Army and four films in the Harry Potter series.
Kip Thorne (middle left) is the Feynman Professor of Theoretical Physics, Emeritus, at the California Institute of Technology (Caltech), and also the Science Advisor and an Executive Producer for the movie Interstellar. He is the author of the books The Science of Interstellar and Black Holes and Time Warps, and co-author of the textbook Gravitation. From 1967 to 2009 he was a professor at Caltech, where he mentored 52 PhD students and carried out research on relativity and astrophysics, with emphasis on relativistic stars, black holes and especially gravitational waves. He was cofounder (with R. Weiss and R.W.P. Drever) of the LIGO gravitational wave project, with which he is still associated.
Paul Franklin (far right) is a visual effects designer working in the film industry. After studying Fine Art at Oxford University he worked in video games and television through the 90s before moving into feature film VFX. In 1998 he helped to co-found Double Negative Visual Effects Ltd which has since grown to be one of the world’s largest and most successful VFX studios. Paul’s credits include Harry Potter and the Order of the Phoenix, Batman Begins and The Dark Knight. Paul’s long-standing working relationship with director Christopher Nolan won him an Oscar® in 2010 for Inception and again in 2014 for Interstellar.

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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