In a study published in the journal Scientific Reports, astrophysicists created a set of equations that can accurately describe the reflections of the universe that appear in the light twisted around a black hole.
The convergence of each reflection depends on the observed angle relative to the black hole, and the black hole’s rotation rate, according to a mathematical solution developed by physics student Albert Sneben of the Niels Bohr Institute in Denmark in July 2021.
And this was really cool, sure, and it’s also likely to give us a new tool for examining the gravitational environment around these extreme objects.
“There is something fantastically wonderful about now understanding why images repeat themselves in such an elegant way,” Snipin said in a 2021 statement. Moreover, it provides new opportunities to test our understanding of gravity and black holes.
And if there’s one thing black holes are known for, it’s their intense gravity. Specifically, beyond a certain radius, the fastest achievable speed in the universe, that is, the speed of light in a vacuum, is insufficient to achieve an escape velocity.
And this point of no return is the event horizon – defined by the so-called Schwarzschild radius – which is why we say that not even light can escape the gravitational pull of a black hole.
Just outside the event horizon of a black hole, the environment is also dangerously noisy. The gravitational field is so strong that the curvature of spacetime is nearly circular.
And any photons entering this space must, of course, follow this curvature. This means, in our view, that the path of light appears distorted and curved.
And at the inner edge of this space, outside the event horizon, we can see the so-called photon ring, in which photons travel in orbit around the black hole several times before falling towards the black hole or escaping into space.
This means that light from distant objects behind the black hole can be magnified, distorted and “reflected” many times. We refer to this as a gravitational lens. The effect can also be seen in other contexts and is a useful tool for studying the universe.
We have known the effect for some time, and scientists have discovered that the closer you are to the black hole, the more reflections you see of distant objects.
To go from one image to the next, I needed to look about 500 times closer to the optical edge of the black hole, or the exponential function of two pi (e 2π), but why this was the case was difficult to describe mathematically.
Snipin’s approach was to reformulate the path of light and determine its linear stability, using second-order differential equations. He found that his solution not only described mathematically why images repeat at e2π distances, but he found that it could work with a rotating black hole – and that this repetition distance depends on the spin.
“It turns out that when it spins very fast, you no longer have to get close to the black hole 500 times, but much less,” Snipin said. “In fact, each image is now only 50, five, or even two times closer to the edge of the black hole.”
Practically speaking, it’s going to be hard to notice, at least anytime soon – just look at the amount of intense work that went into the unresolved imaging of the ring of light around the Pōwehi supermassive black hole (M87*).
And in theory, there should be infinite rings of light around the black hole. Since we’ve only imaged the shadow of a supermassive black hole once, we hope it’s only a matter of time before we can get better images, and there are already plans to image the Photon ring.
And one day, near-infinite images of a black hole could be a tool for studying not just the space-time physics of a black hole, but the objects behind it—repeating in infinite reflections in an orbiting eternity.