Visualising the light-bending dance of binary black holes using NASA's Discover supercomputer
20 Apr 2021
Try to imagine and describe a black hole. A big one! Now try to imagine a second supermassive black hole and bring the two together in a single system. How do you think the gravitational forces of each phenomenon will affect the other? How will their accretion layers luminess? Don't forget to account for relativistic aberration. Mind boggling? Don't feel too bad. It would take your computer ten years to work it all out, so you may be forgiven. Lucky someone else has done it all for us.
Image: In this frame from the new visualisation, a supermassive black hole weighing 200 million solar masses lies in the foreground. Its gravity distorts light from the accretion disk of a smaller companion black hole almost directly behind it, creating this surreal view. Different colours for the accretion disks make it easier to track the contributions of each one. Credit: NASA's Goddard Space Flight Center/Jeremy Schnittman and Brian P. Powell
A pair of orbiting black holes millions of times the Sun's mass perform a hypnotic pas de deux in a new NASA visualisation. The movie traces how the black holes distort and redirect light emanating from the maelstrom of hot gas - called an accretion disk - that surrounds each one.
Viewed from near the orbital plane, each accretion disk takes on a characteristic double-humped look. But as one passes in front of the other, the gravity of the foreground black hole transforms its partner into a rapidly changing sequence of arcs. These distortions play out as light from both disks navigates the tangled fabric of space and time near the black holes.
Video: Explore how the extreme gravity of two orbiting supermassive black holes distorts our view. In this visualization, disks of bright, hot, churning gas encircle both black holes, shown in red and blue to better track the light source. The red disk orbits the larger black hole, which weighs 200 million times the mass of our Sun, while its smaller blue companion weighs half as much. Zooming into each black hole reveals multiple, increasingly warped images of its partner. Watch to learn more. Credit: NASA’s Goddard Space Flight Center/Jeremy Schnittman and Brian P. Powell
The accretion disks have different colours, red and blue, to make it easier to track the light sources, but the choice also reflects reality. Hotter gas gives off light closer to the blue end of the spectrum, and material orbiting smaller black holes experiences stronger gravitational effects that produce higher temperatures. For these masses, both accretion disks would actually emit most of their light in the UV, with the blue disk reaching a slightly higher temperature.
Visualisations like this help scientists picture the fascinating consequences of extreme gravity's funhouse mirror. The new video doubles down on an earlier one Schnittman produced showing a solitary black hole from various angles.
Seen nearly edgewise, the accretion disks look noticeably brighter on one side. Gravitational distortion alters the paths of light coming from different parts of the disks, producing the warped image. The rapid motion of gas near the black hole modifies the disk's luminosity through a phenomenon called Doppler boosting - an effect of Einstein's relativity theory that brightens the side rotating toward the viewer and dims the side spinning away.
The visualisation also shows a more subtle phenomenon called relativistic aberration. The black holes appear smaller as they approach the viewer and larger when moving away.
These effects disappear when viewing the system from above, but new features emerge. Both black holes produce small images of their partners that circle around them each orbit. Looking closer, it's clear that these images are actually edge-on views. To produce them, light from the black holes must be redirected by 90 degrees, which means we're observing the black holes from two different perspectives - face on and edge on - at the same time.
"A striking aspect of this new visualisation is the self-similar nature of the images produced by gravitational lensing," Schnittman explained. "Zooming into each black hole reveals multiple, increasingly distorted images of its partner."
Schnittman created the visualisation by computing the path taken by light rays from the accretion disks as they made their way through the warped space-time around the black holes. On a modern desktop computer, the calculations needed to make the movie frames would have taken about a decade. So Schnittman teamed up with Goddard data scientist Brian P. Powell to use the Discover supercomputer at the NASA Center for Climate Simulation. Using just 2% of Discover's 129,000 processors, these computations took about a day.
Astronomers expect that, in the not-too-distant future, they'll be able to detect gravitational waves - ripples in space-time - produced when two supermassive black holes in a system much like the one Schnittman depicted spiral together and merge.
