A figure skater glides across the ice, gradually approaching the center of the rink. A slight twist of the torso, and the smooth spiral breaks, the trajectory becomes unpredictable. Something similar happens when a small black hole with its own rotation (spin) falls toward a giant black hole. For a long time, it was thought that such chaos required unrealistically fast spin. New research shows that chaos robustly arises even at the most ordinary, astrophysically plausible values.
The key result of the work is a method that allows us to hear this chaos in gravitational waves. When the orbit remains regular, the waves it produces resemble a clean musical chord: a few bright, clearly separated frequency peaks. Chaotic motion, on the other hand, creates a noisy, dense spectrum where many additional, weaker peaks appear between the main ones. The authors introduced a measure of "local spectral flatness"—something like a check of how evenly energy is distributed among neighboring frequencies. For chaotic signals, this flatness turned out to be hundreds of times higher than for regular ones.
The discovery is important for future space detectors like LISA, which will catch gravitational waves from black hole mergers in the centers of galaxies. The ability to distinguish chaos from order directly from the signal shape will give astronomers a new tool for measuring the spins of these invisible objects and testing how gravity works in strong fields. So the skater's dance, which began with a slight twist, gains a voice, and that voice can be decoded.
The chaos effect in such systems is so subtle that two nearly identical black holes, differing in rotation speed by just a hundredth of a percent, can end up at completely different points in their orbits after thousands of revolutions.