Does a clock go faster if it is lifted?

Placing two clocks at different heights seems a strange way to desynchronize them. However, general relativity predicts that their rhythm differs in the Earth's gravitational field. A clock located lower advances a little slower than the one placed higher.

In 2010, NIST physicists measured this effect with two atomic clocks separated vertically by thirty-three centimeters. In 2022, JILA work pushed the comparison to the scale of a sample of atoms of about a millimeter. It takes extraordinary instruments to make such a small difference perceptible.

Experience does not mean that a stairway can visibly gain years. It reveals that time is not a perfectly uniform background. Even between two almost neighboring heights, nature imposes a nuance. Our ordinary watches ignore it; the most accurate clocks can read it.