Physicists know that if you and I are sitting in a room together, you exert a gravitational force on me. It’s almost nothing—I can’t feel it—but it’s the same force that binds our planet to our star. That’s the very simple, very elegant purpose of the rotor: to hold a pair of tiny stars. Each time one of them swings toward the laser, spacetime bends just slightly; the laser must travel a slightly longer path to reach the mirror; a line spikes on one of LIGO’s screens. Even that silent signal could be enough to let us reimagine gravity, and the way it moves us on the human scale.
