Supermassive black holes with active nuclei and accretion disks sometimes capture a neighboring hole, and the pair emits gravitational waves. These will be heard by the future space-based LISA — a giant interferometer.

Drawing a bow across a string produces sound. Pressing the string changes the pitch, and a mute on the bow adds a new overtone. So it is with black holes: theories beyond Einstein's ideas predict a 'scalar charge'—that very mute. Taking the model of a non-rotating Schwarzschild black hole, scientists performed numerical simulations of thousands of signals, akin to spectroscopy, and found: the charge is noticeable even with inaccurate data. This contradicts Hawking's 'no-hair' theorem — the scalar charge would be an exception.

The discovery accelerates the search for new fields, possibly related to dark energy or axions of dark matter. Similar effects are also sought through gravitational lensing, but it is precisely the music of black holes that gives the purest signal.