In recent years, the LIGO, Virgo, and KAGRA gravitational-wave detectors have "heard" hundreds of black hole mergers. Each such cataclysm is not just a burst of spacetime ripples. After the collision, the newborn black hole oscillates, like a bell, emitting a characteristic "ring" — a sequence of quasinormal modes that gradually fades. And all these signals, down to the smallest detail, correspond to the predictions of the general theory of relativity.
Imagine two massive objects, each tens of solar masses, that in the final stage of their spiral approach each other at speeds close to the speed of light. At the moment of merger, they release energy equivalent to several solar masses in the form of gravitational waves. The remaining black hole does not calm down immediately: it "rings," emitting waves whose frequencies and damping are strictly determined only by its mass and angular momentum. This is the famous "no-hair theorem" — black holes are surprisingly simple.
Astronomers have now accumulated hundreds of such events. Each new "ring" is tested for compliance with the theory. And so far, there have been no discrepancies. Even the most powerful mergers, where the energies are colossal, fit within the framework of Einstein's predictions with high accuracy. This is one of the most stringent tests of the general theory of relativity under extreme conditions of a strong gravitational field.
But the true future of gravitational astronomy lies with the next generation of detectors. Current instruments primarily capture the dominant mode. Future ground-based giants like Cosmic Explorer and Einstein Telescope, as well as the space antenna LISA, will be able to resolve several oscillation modes from the same black hole. This will allow for much more precise tests: measuring not only the fundamental frequency but also the overtones, and even the nonlinear interactions between modes.
Such multi-mode observations will open up the possibility to test the "no-hair theorem" even more rigorously and search for possible deviations from the general theory of relativity — for example, traces of new physics or quantum effects at the event horizon. Today, we are moving from simply detecting mergers to using black holes as precision laboratories for fundamental physics. Spacetime itself is telling us about its laws through these fading "rings.".
Each new detection adds confidence: a theory created more than a hundred years ago on paper works brilliantly in the most violent corners of the universe. And yet, it leaves room for questions. What if, with even greater sensitivity, we finally notice a barely perceptible crack? Or, conversely, we become convinced that black holes are exactly as Einstein described them — perfectly simple and mysterious objects.
Gravitational waves continue to sound, and we are learning to listen to them carefully.
