One of the strangest predictions of general relativity is that gravity can divert the path of light. The effect was first observed by Arthur Eddington in 1919. Although the bending effect of the sun is small, slight deviation at a black hole can be very important. So important that you need a powerful supercomputer to calculate how the light will act.
Recently, NASA Goddard Media Studios released some videos that show us what a binary black hole system might look like under gravity lens. The simulation follows the light paths coming from the growth disks of two black holes orbiting close together. One with a mass of 200 million Sun, the other with half the mass. The simulation was performed on the Discover supercomputer at the NASA Center for Climate Simulation and it took about a day to complete.
This new simulation takes into account some subtle effects. At a rotating black hole, the light that turns from our side will look brighter, while the light from the side that turns from us will appear dimmer. This effect is known as Doppler boosting. Another strange effect is known as relativistic deviation, where black holes appear smaller as they move toward the viewer, and larger as they move away.
Perhaps the biggest computer challenge is that you can not just do a simple first-order simulation of the lens. If two black holes are visually close to each other, the light from the black hole A can be twisted through black hole B to the point that it is turned back to the black hole A. It can then become lens again before it gets the chance way. Light trails can sometimes be twisted so that it is difficult to determine from which drive disk the light is coming from. To make the effect easier to see, the visualization uses a bright red color for the larger black hole’s growth disk and a bright blue color for the smaller black hole. In the video and images you can see reflection of one black hole growth disk in the other. The proximity of the black holes also distorts the visual shape of the growth discs, making them appear more oval than they actually are.
Although this is not a simulation of an actual black hole system, it does tell us a lot about how binary black holes can occur. This is especially important because we are discovering more binary black holes through their gravitational waves. Although black holes themselves do not emit light as they merge, their growth discs 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 mergers in black holes.
Reference: GMS N. GMS. “NASA visualization sins the twisted world of binary black holes.” https://svs.gsfc.nasa.gov/13831. Published April 15, 2021.