When two black holes merge, the resulting black hole 'rings' for a while, emitting gravitational waves — like a dying bell. This ringing is captured by giant detectors created by Rainer Weiss and Kip Thorne. Within the sound, you can distinguish the fundamental tone and overtones, which reveal the mass and spin of the black hole. But the quiet overtones usually get drowned out by the strong main signal.
Physicists found a way to isolate these faint notes, using spectroscopy — a method akin to fine-tuning an equalizer: it boosts the desired frequencies and suppresses noise. For the loudest recorded event, GW250114, they confirmed the first overtone with a confidence of 99.9% (previously 82.5%). No deviations from relativity — the black hole rang flawlessly.
Surprisingly, this quiet overtone told us as much about the black hole as the loud fundamental tone — we just hadn't heard it before. The cosmic eavesdropping session continues.
Every black hole rings with a unique set of frequencies, like a fingerprint, from which you can precisely determine its mass and spin.