A massive top spins so fast that space itself twists into a spiral around it. This is a black hole — only instead of wood and metal, it's made of pure gravity. Add an unusual electromagnetic field that slightly changes the rules of the game, and you get a 'deformed' black hole: its shape and behavior differ a bit from the familiar ones described half a century ago.
A group of physicists calculated how such a hole would look through a telescope and how it would 'sound' after collisions. They took the deformation parameter g and found that if it exceeds about 1.26 times the hole's mass, the shadow on the sky, that famous ring, would become noticeably larger than what is observed for the black hole at the center of the Milky Way. It was photographed with record precision in 2022. Interestingly, this constraint does not depend on spin — only on mass.
Next, the authors turned to the 'ringdown'—the quasi-normal modes (the frequencies at which the hole vibrates after a disturbance, like a bell after being struck). In the high-frequency regime, they derived simple formulas for the shifts of these frequencies. It turned out that here too, the allowed g is about the same. This is no coincidence but a cross-check of two completely different methods.
The most surprising result concerns energy. A spinning hole can release part of its rotational energy through magnetic fields — a process called ergosphere reconnection. The authors calculated how much energy can be extracted from a deformed hole and compared it with an ordinary Kerr hole. If the difference is too large, the reality of such an object is unlikely. This gave an even tighter limit on g, and one that depends on spin: the faster the hole spins, the stricter the constraint.
In the end, two independent approaches, shadow and vibrations, give a consistent upper bound on deformation, while the energy method tightens the allowed region even further. Even simple, single-layer models can provide useful constraints that are testable with today's data and future horizon observations. So the top-like hole, slightly deviating from the ideal, reveals its secrets through shadow, sound, and rotational energy.
A black hole's shadow is actually larger than the horizon itself — by about 2.6 times. That's because light bends around the hole, and we see photons that narrowly escaped falling in.
In the movie 'Interstellar,' the supermassive black hole Gargantua is depicted with real physics of rotation in mind, and its shadow looks exactly as calculations predict.