Higher-dimensional black holes hide an exact symmetry in their ringing, and string-inspired gravity breaks it

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…