Black holes are compact objects around which light bends sharply due to the curvature of spacetime. And although the light itself cannot leave the central mass beyond the event horizon, at great distances it can orbit a black hole. This phenomenon can allow a distant observer to see multiple versions of the same object. Despite the fact that this fact has been known for many years, theoretical physicists only now have an exact mathematical solution for this phenomenon. Study published in Scientific Reports.
Here’s what says Albert Sneppen, student at the Cosmic Dawn Center and the Niels Bohr Institute at the University of Copenhagen:
A distant galaxy shines in all directions – some of its light passes close to the black hole and is slightly deflected; some of the light goes even closer and goes around the hole once before running in our direction, and so on. Observing the vicinity of a black hole, the closer to the horizon we look, the more of the same galaxy we see.
The more light from the background galaxy revolves around the black hole, the closer it passes next to it, thus the observer sees the same galaxy in several directions. © Peter Laursen
Each next image is 500 times closer to it than the previous one – this has been known for over 40 years. However, until recently, the calculations were so complex that scientists had neither mathematical nor physical intuition regarding this coefficient.
Sneppen was able to prove why this coefficient is exactly the same with a simple numerical and perturbative analytical solution. The same method can be applied to rotating black holes.
It turns out that with a very fast rotation of a black hole, you no longer need to approach it 500 times, but much less.
Moreover, each next image in this case is now only 50, 5, or even only 2 times closer to the edge of the black hole.
In the case of static black holes, when each image is 500 times closer to the event horizon than the previous one, they all very soon “collapse” into one image. In other words, multiple images are very difficult to observe in reality. However, when the black hole rotates, there is more room in its vicinity for additional images. Sneppen hopes to confirm his theory through observations in the near future.
Thus, we can study not only black holes, but also the galaxies located behind them. The more light has to fly around the black hole, the longer the travel time increases, so that the images become more and more delayed. If, for example, a star explodes in a supernova in a background galaxy, the observer will be able to see this explosion over and over again.