Ordinarily, two spiraling black holes pump out gravitational waves concentrated at a single frequency: double the rate at which they orbit each other. That doubling arises because of the matched masses of the black holes. Every half orbit they return to a position that’s effectively identical to their original one. But if the black holes have distinctly different masses, then general relativity predicts that they should also generate weaker waves at higher frequencies, or overtones.
The next-strongest note sung by the pair should vibrate at three times the orbital frequency, or one and half times the main gravitational-wave frequency. If the main frequency were a C on a piano, the overtone would be the next higher G—a perfect fifth, and the interval of the first two notes in the melody of Elvis Presley’s hit “I Can’t Help Falling in Love with You.” That is what the LIGO and Virgo researchers detected, says Maximiliano Isi, a physicist and LIGO member at the Massachusetts Institute of Technology, who also spoke at the meeting. The overtone rang roughly as loudly as predicted by general relativity, Isi says. "Einstein prevails again."