In the 1971 Hafele–Keating experiment, elapsed time differed between atomic clocks flown eastward and westward.
Joseph Hafele and Richard Keating placed four cesium atomic clocks on commercial aircraft and flew them around the world, once eastward and once westward. After the flights, the clocks were compared with reference clocks that remained at the U.S. Naval Observatory.
The traveling clocks showed different elapsed times depending on their direction. The results reflected two relativistic effects: motion causes special-relativistic time dilation, while differences in gravitational potential cause general-relativistic time shifts.
The observed differences were tiny but measurable and agreed broadly with relativity’s predictions. The experiment did not show that clocks malfunctioned; it showed that elapsed time depends on motion and gravitational environment.