If a pair of black holes ever hit Earth, you would literally hear them coming: not by sound, which cannot cross the vacuum of space, but by waves of gravitational force, which, as the holes approached, would knead the bones of your inner ear by a perceptible amount, producing a whine like a camera flash charging up. Under normal circumstances, such waves are utterly inaudible, even though astronomers think they reverberate through our bodies all the time. Weakened by vast cosmic distances, the waves that reach us change the length of bones and other objects by less than the width of a proton.
To hear these waves, you need a supersensitive microphone such as the Laser Interferometer Gravitational-Wave Observatory (LIGO), whose twin facilities in Washington State and Louisiana are basically fancy laser range finders that look for wave-induced oscillations in the length of the instrument. But there are other ways to detect such oscillations, and a new study argues that one of those techniques might even test whether Einstein’s general theory of relativity is the full story when it comes to gravitation.