How can the speed of gravity be measured?
The first accurate measurement ever taken of the speed with which gravity propagates shows that it is equal to the speed of light, agreeing nicely with the General Theory of Relativity. Though fast, light takes time to travel. If the Sun suddenly disappeared, it would take about 8.3 minutes before daylight on Earth would evaporate. With the Sun gone, gravity would cease to keep Earth in a circular orbit, and it would fly away. If gravity works instantly, Earth would fly away the moment the Sun disappeared. If gravity works at light speed, Earth’s course would not change until 8.3 minutes later. Sergei Kopeikin of the University of Missouri, Columbia, and Ed Fomalont of the National Radio Astronomy Observatory devised a clever experiment to test which of the two assumptions is right. On several days last September, they observed a faraway galaxy as the planet Jupiter passed near it in the sky. Jupiter’s gravity would bend the light ever so slightly, they knew. The question was by how mu
