A black hole is like a spinning top: the faster it spins, the more noticeable its behavior. It has two boundaries — the outer and inner horizons, the edges beyond which even light cannot return. In the new work, physicists looked inside a rotating charged black hole and multiplied the areas of these two surfaces.
This is not just number crunching. The constancy of the area product, and hence of entropy, a measure of disorder, allows connecting the black hole to a two-dimensional quantum field theory on its horizon. The scientists calculated the central charges of this theory — quantities that govern the symmetries of the system. And here everything reduced to rotation: the charges depend only on the angular momentum. Even for a stationary but charged black hole, they managed to find a nonzero charge using the same approach.
Such universality is rare. For comparison, they took another type of hole, Kerr-Newman, and saw that the difference hides in the "right" sector of the quantum theory.
The product of horizon areas for some black holes does not change under any transformations, like a bank account where the sum on two accounts is always constant.
In the novel "Light of Distant Stars," the heroes calculate a black hole's history from its oscillations — much like in this work.