A Black Hole Rings Like a Bell, and a Neural Network Listens express
Strike a bell, and it rings at its own pitch. A black hole, after being disturbed, also rings — except instead of sound, it's the fabric of space itself that shivers. The frequencies of this ring can tell you a lot about the hole: how fast it spins, what its mass is.
Previously, to compute these frequencies, the equation for the trembling space was simplified: it was split into two independent parts, like breaking down a bell's sound into separate notes. For an ordinary rotating black hole, this works. But for more exotic objects (say, a black hole with an electric charge) such a split is impossible.
It's like a tuner who, instead of taking the bell apart, listens to it whole and pinpoints the note. The method isn't as precise as specialized programs, but it's far more flexible: it can be applied to any black hole, even those where old approaches fall short.
The longest-lived note of a black hole's ring is the one that decays slowest. For nearly maximally spinning holes, such notes can sound for a very long time.
In the movie Interstellar, the characters fly near the rotating black hole Gargantua. If it were to ring, the neural network from this article could calculate its note.