One of the strangest predictions of general relativity is that gravity can deflect the path of light. The effect was first observed by Arthur Eddington in 1919. While the bending effect of the Sun is small, near a black hole light deflection can be significant. So significant that you need a powerful supercomputer to calculate how light will behave.
Recently the NASA Goddard Media Studios released a few videos showing us how a binary black hole system might look under gravitational lensing. The simulation traces the paths of light coming from the accretion disks of two close-orbiting black holes. One with a mass of 200 million Sun, the other with half that mass. The simulation was run on the Discover supercomputer at the NASA Center for Climate Simulation and took about a day to complete.1
This new simulation takes into account some of the more subtle effects. For example, near a rotating black hole, light coming from the side rotating toward us will appear brighter, while light from the side rotating away from us would appear dimmer. This effect is known as Doppler boosting. Another strange effect is known as relativistic aberration, where black holes appear smaller when moving toward the viewer, and larger when moving away.
Perhaps the biggest computational challenge is that you can’t just do a simple first-order simulation of the lensing. When two black holes are visually close to each other, light from black hole A can be distorted by black hole B to the point that it is twisted back to black hole A. It can then be lensed again before it has a chance to head our way. Light paths can be so distorted at times that it is difficult to determine which accretion disk the light came from. To make this effect easier to see, the visualization uses a bright red color for the larger black hole’s accretion disk and a bright blue color for that of the smaller black hole. In the video and images, you can see reflections of one black hole accretion disk in that of the other. The proximity of the black holes also distorts the visual shape of the accretion disks, making them appear more oval than they actually are.
Even though this isn’t a simulation of an actual black hole system, it tells us a great deal about how binary black holes can appear. This is particularly important as we discover more binary black holes by their gravitational waves. Although black holes themselves don’t emit light as they merge, their accretion disks do. As we better understand how this light is distorted by gravity, we can better combine optical and gravitational data to give us a detailed understanding of real black hole mergers.