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.