In a study published in the journal Scientific Reports, astrophysicists have created a series of equations that can accurately describe the reflections of the universe that appear in light coiled around a black hole.
The convergence of each reflection depends on the observed angle to the black hole and the rotation speed of the black hole, according to a mathematical solution developed by physics student Albert Sneben of the Niels Bohr Institute in Denmark in July 2021.
And that 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 extraordinarily wonderful about understanding now why images repeat themselves in such an elegant way,” Snipin said in a 2021 statement. It also offers 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. In particular, beyond a certain radius, the highest attainable velocity in the universe, that is, the speed of light in vacuum, is insufficient to reach an escape velocity.
And this tipping point 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 almost circular.
And all photons that enter this space must, of course, follow this curvature. This means, in our opinion, that the path of the 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, where photons travel in orbit around the black hole several times before falling to 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 found that the closer you get to the black hole, the more reflections you see from distant objects.
To switch between images, I had to look about 500 times closer to the optical edge of the black hole, or the exponential function of two pi (and 2π), but for this to be the case it was difficult to describe mathematically.
Snipin’s approach consisted of reformulating the path of light and determining its linear stability, using second-order differential equations. He found that his solution of him not only mathematically described why images repeat at e2π distances, but found that it could work with a spinning black hole – and that this repetition distance depends on spin.
“It turns out that when it spins very fast, you no longer have to get close to the black hole 500 times, but a lot less,” Snipin said. “In fact, each image is now only 50, five or even twice as close to the edge of the black hole.”
In practice, it will be hard to notice, at least anytime soon: just look at the amount of intense work that went into the unresolved image of the ring of light around Pōwehi’s supermassive black hole (M87 *).
And in theory, there should be infinite rings of light around the black hole. Since we have only filmed 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 photograph the photon ring.
And someday, near-infinite images of a black hole could be a tool for studying not only the space-time physics of a black hole, but also the objects behind it, repeating themselves in endless reflections in an orbiting eternity.
