Picture the Milky Way not as a silent pinwheel of stars but as something that quietly sings. Scattered through it are millions of pairs of dead stars, mostly white dwarfs, whirling around each other and stirring ripples in spacetime as they go. Individually, these ripples are far too faint to notice . Together, they blur into a constant background hum, and a planned European space mission called LISA is being built to listen for it. ...read more read less