The finding is linked to the strange, time-bending effects of Albert Einstein's theories of relativity, which have been shown to affect earthbound distances and time frames.
Specifically, Einstein's special theory of relativity predicts that time does not flow at a steady rate, and it can be affected by acceleration. As a result, a clock speeding away from an observer will appear to tick slower than a stationary clock.
This theory is the basis of a famous thought experiment known as the twin paradox, in which a twin sibling who travels on a fast-moving rocket ship would return home younger than the other twin.
The equations of general relativity also predict that gravity similarly slows down, or dilates, time.
"So if you are experiencing stronger gravitational pull, then your time is going to go slower," said study co-author James Chin-Wen Chou of the National Institute of Standards and Technology (NIST).
Atomic Clocks Show Gravity Slowing Time
The time-slowing effects of acceleration and gravity have been demonstrated in experiments that compare real clocks on Earth's surface with timepieces in high-flying spacecraft and satellites, such as ones used for global positioning systems.
But the study, appearing in the journal Science, shows that these effects are also measurable here on Earth's surface.
The pull of gravity on an object increases closer to the center of mass, so an object on Earth's surface actually experiences a slightly stronger pull than one floating in the atmosphere.
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.
"It's not a road to youth," said Daniel Kleppner, a physicist at the Massachusetts Institute of Technology who was not involved in the study.
Clocks' Precision More Impressive?
In general, the team's findings are not unexpected, because they agree with broadly accepted theories of Einstein's, Kleppner said.
"I think what's impressive here is the incredible precision of the clocks."
With further refinement, ultraprecise clocks such as the ones used in the study could one day allow scientists to measure geographic variations in Earth's gravitational field—a science called geodesy—with unprecedented precision.
Geodesy is important for calculating Earth's mass distribution, which can help determine, for instance, the distribution of water on the planet and how that water moves. (Read about a new fault found in the Adriatic Sea using geodesy.)
"You can envisage having these clocks scattered around the world and comparing time with these precisions," Kleppner said. "That would have a revolutionary impact on geodesy."