He reworked the equations of general relativity to express the gravitational field of a moving body in terms of its mass, velocity and the speed of gravity. If you could measure the gravitational field of Jupiter, while knowing its mass and velocity, you could work out the speed of gravity. Such an opportunity arose last September when Jupiter passed in front of a quasar that emits bright radio waves. He and Ed Fomalont, of the National Radio Astronomy Observatory in Charlottesville, Virginia, combined observations from a series of radio telescopes across Earth to measure the apparent change in the quasar’s position as the gravitational field of Jupiter bent the passing radio waves. From that they calculated that gravity does indeed move at the same speed as light. This result should help narrow down the possible number of extra dimensions and their sizes.