National Geographic reports:
Using two ultraprecise atomic clocks, Chou and colleagues showed that lifting one clock by only about a foot (33 centimeters) above the other creates enough of a gravitational difference that the higher clock ticks slightly faster.
In a second experiment, the team measured the effects of relativity on the time-keeping aluminum atoms inside the clocks.
Atomic clocks work based on the number of vibrations an electrically charged atom experiences as it moves between two energy levels. For the clocks used in the experiments, one second is equal to more than a million billion vibrations.
In one of the clocks, the team nudged the normally stationary aluminum atom so that it gyrated back and forth as it vibrated. As Einstein predicted, the clock with the moving atom ticked at a slightly slower rate than the second clock.
Before anyone rushes to lower elevations, though, the NIST scientists note that these effects are much too small for humans to perceive directly—adding up to approximately 90 billionths of a second over a 79-year lifetime.