After botched launch, orbiting atomic clocks confirm Einstein’s theory of relativity

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 part of that warping, time should tick more slowly near a massive body than it does farther away. That bizarre effect was first confirmed to low precision in 1959 in an experiment on Earth and in 1976 by Gravity Probe A, a 2-hour rocket-born experiment that compared the ticking of an atomic clock on the rocket with another on the ground.