Summary

Strike a bell, and it rings with a pitch and a fading that tell you about the bell: its size, its shape, the metal it is made of. Black holes ring too. When two merge, the newborn black hole shivers and sheds gravitational waves in a brief, dying chord, and since 2015, gravitational-wave detectors have been listening. The notes of that chord, which physicists call quasinormal modes, depend only on the black hole's mass and spin and on the law of gravity itself.

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