You’ve probably heard that time is relative. It sounds like a banal cliche, akin to “time flies when you’re having fun.”

But it’s no mere cliche: Time is relative. Though it’s not noticeable in daily life, time passes slightly more slowly when you’re moving vs. when you’re standing still. It also moves slightly more slowly when you’re at sea level than when you’re on top of a mountain.

We know this thanks to a remarkable timekeeping technology invented at the National Institute of Standards and Technology (NIST): the atomic clock. During the past half-century, scientists have designed a series of increasingly ambitious experiments to show that atomic clocks tick faster when moving and slower in stronger gravity. Now, a generation of clocks more precise than any that have come before is allowing physicists to push such measurements to new extremes.

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