Making lemonade from lemons, two teams of physicists have used data from misguided satellites to put Albert Einstein’s theory of gravity, the general theory of relativity, to an unexpected test. The opportunistic experiment confirms to unprecedented precision a key prediction of the theory—that time ticks slower near a massive body like Earth than it does farther away. As Einstein explained, gravity arises because massive bodies warp space-time. Free-falling objects follow the straightest possible paths in that curved space-time, which to us appear as the parabolic arc of a thrown ball or the circular or elliptical orbit of a satellite. As...