Imagine two precise clocks in the same room. Raise one by a single step. According to general relativity, the higher clock should tick slightly faster. The change is too small to notice on an ordinary clock.

In 2010, NIST researchers raised one of two optical clocks by thirty-three centimeters. That is about a foot. They compared the ticks and measured the expected difference. The experiment used aluminum ions to keep time.

In 2022, JILA researchers went smaller. They compared the ticking across different heights inside one cloud of strontium atoms. The separation was about a millimeter. A tiny height difference can leave a measurable difference in time.