Astronomers have made significant developments in understanding the reflections of the Universe around black holes. These reflections depend on the observer’s angle and the speed of the black hole’s rotation. Physicist Albert Sneppen from the Niels Bohr Institute in Denmark made this breakthrough in July 2021 (source: Nature).
Sneppen expressed his fascination, stating, “There is something fantastically beautiful in understanding why the images are repeated in such an elegant way. In addition, this provides new opportunities to test our understanding of gravity and black holes.”
Black holes are well-known for their incredibly strong gravity, causing light to be unable to escape their gravitational field. Just outside the black hole’s event horizon, where the gravitational field is immensely powerful and the curvature of space-time is almost circular, photons that enter this space are pulled along this curvature. Consequently, from our perspective, the path of light appears to be curved.
At the innermost edge of this space, right outside the event horizon, there exists a phenomenon known as the photon ring. In this region, photons orbit around the black hole multiple times before either falling into it or escaping into space. As a result, light from distant objects behind a black hole can be amplified, distorted, and reflected multiple times.
Prior to Sneppen’s work, scientists were already aware of this effect, but it was challenging to mathematically describe it. Sneppen reformulated the trajectory of light and quantitatively assessed its linear stability using second-order differential equations. He not only mathematically explained why the images are repeated at distances of E2π, but also demonstrated that this applies to rotating black holes, with the distance between repetitions being dependent on the rotation.
Sneppen stated, “It turns out that when a black hole rotates very quickly, one no longer needs to approach it 500 times, but much less.” Theoretically, there should be an infinite amount of light rings around a black hole. Since we have already obtained an image of the shadow of a supermassive black hole, we can eventually capture even better images, including one of a photon ring.
The possibility of obtaining numerous images near a black hole could serve as a valuable tool for studying not only the physics of a black hole’s space-time, but