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 the deflection of light can be significant. So significant that you need a powerful supercomputer to calculate how light will behave.
Recently, NASA Goddard Media Studios released several videos that show us what a black hole binary system might look like under gravitational lenses. The simulation follows the light paths from the accumulation discs of two black holes that orbit close. One with a mass of 200 million Sun, the other with half the mass. The simulation was run on the Discover supercomputer at NASA’s Center for Climate Simulation and lasted about a day.
This new simulation takes into account some of the more subtle effects. For example, next to a rotating black hole, the light coming from the side that rotates toward us will appear brighter, while the light from the side that rotates away from us would appear dimmer. This effect is known as Doppler boost. Another strange effect is known as relativistic aberration, where black holes appear smaller as they move toward the viewer and larger as they move away.
Perhaps the biggest computational challenge is that you can’t just do a simple first-order lens simulation. When two black holes are visually close to each other, the light in black hole A can be distorted by black hole B to the point where it is twisted back into black hole A. It can then be re-lensed before it has a chance. Light paths can sometimes be so distorted that it is difficult to determine from which storage disk the light comes. To make this effect easier to see, the view uses a bright red color for the larger black hole’s accumulation disc and a bright blue color for the smaller black hole’s disc. In the video and pictures, you can see the reflections of one black hole accumulation disk in the other. The proximity of the black holes also distorts the visual shape of the accumulation discs, making them look more oval than they actually are.
Even if this is not a simulation of a real black hole system, it tells us a lot about how binary black holes can appear. This is especially important because we discover several binary black holes after their gravitational waves. Although black holes themselves do not emit light as they join, their accumulation 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 the actual fusions of black holes.
Reference: AGA N. AGA. “NASA’s visualization probes the double distorted world of binary black holes.” https://svs.gsfc.nasa.gov/13831. Published on April 15, 2021.