Black holes are famously compact objects where the paths of light are drastically bent by the curvature of space-time. While light itself cannot escape the central mass at the event horizon, at further distances light may orbit the black hole. This phenomenon may enable a distant observer to see multiple versions of the same object. While this has been known for years, only now do theoretical physicists have an exact mathematical solution.
From any location outside the event horizon of a black hole there are an infinite number of trajectories for light to an observer; each of these paths differ in the number of orbits revolved around the black hole and in their proximity to the last photon orbit. Image credit: Sci-News.com / Zdeněk Bardon / ESO.
“A distant galaxy shines in all directions — some of its light comes close to the black hole and is lightly deflected; some light comes even closer and circumvolves the hole a single time before escaping down to us, and so on,” said Albert Sneppen, a student at the Cosmic Dawn Center and the Niels Bohr Institute at the University of Copenhagen.
“Looking near the black hole, we see more and more versions of the same galaxy, the closer to the edge of the hole we are looking.”
“How much closer to the black hole do you have to look from one image to see the next image? The result has been known for over four decades, and is some 500 times.”
“Calculating this is so complicated that, until recently, we had not yet developed a mathematical and physical intuition as to why it happens to be this exact factor.”
With a simple numerical and a perturbed analytical solution, Sneppen succeeded in proving why.
“There is something fantastically beautiful in now understanding why the images repeat themselves in such an elegant way,” he said.
“On top of that, it provides new opportunities to test our understanding of gravity and black holes.”
Light from the background galaxy circles a black hole an increasing number of times, the closer it passes the hole, and an observer therefore sees the same galaxy in several directions. Image credit: Peter Laursen.
The new method can also be generalized to spinning black holes.
“It turns out that when the black hole rotates really fast, you no longer have to get closer to it by a factor 500, but significantly less,” Sneppen said.
“In fact, each image is now only 50, or 5, or even down to just two times closer to the edge of the black hole.”
Having to look 500 times closer to the black hole for each new image, means that the images are quickly ‘squeezed’ into one image.
“In practice, the many images will be difficult to observe,” he said.
“But when black holes rotate, there is more room for the extra images, so we can hope to confirm the theory observationally in a not-too-distant future.”
“In this way, we can learn about not just black holes, but also the galaxies behind them.”
“The travel time of the light increases, the more times it has to go around the black hole, so the images become increasingly delayed.”
“If, for example, a star explodes as a supernova in a background galaxy, one would be able to see this explosion again and again.”
The study was published in the journal Scientific Reports.
A. Snepppen. 2021. Divergent reflections around the photon sphere of a black hole. Sci Rep 11, 14247; doi: 10.1038/s41598-021-93595-w