Legacy · Direct Test
Atomic Clocks
The best clocks are now precise enough to notice that time passes faster at your head than at your feet, just as general relativity says.
Time Is Not Universal
General relativity predicts that a clock deeper in a gravitational field runs slower than one higher up. A clock on a mountain gains on a clock at sea level. A clock on the upper floor of a building gains on one in the basement. The effect is real, permanent, and far too small for any mechanical clock to reveal.
Clocks That Can See It
An atomic clock keeps time by the frequency of light absorbed by atoms, which is the same for every atom of a given kind everywhere. Since 1967 the second itself has been defined this way, as 9,192,631,770 oscillations of a particular transition in cesium.
The newest clocks use strontium atoms held in a lattice of laser light, or single trapped aluminum ions. The best of them would neither gain nor lose a second in the age of the universe. That is precise enough to watch relativity at human scales.
- In 2010, physicists at the US National Institute of Standards and Technology compared two clocks and raised one of them by 33 centimeters. It ran faster, by the predicted amount. They also detected the slowing of time for a clock moving at only a few meters per second.
- In 2022, a team at JILA in Boulder, Colorado, measured the difference in the passage of time between the top and bottom of a single cloud of atoms one millimeter tall.
Relativity as a Tool
The situation has turned around. Because clocks at different heights tick at different rates, comparing two clocks tells you their difference in height, or more precisely in gravitational potential. Researchers are developing this "relativistic geodesy" to measure the shape of the Earth, changes in sea level, and the movement of magma beneath volcanoes.
It also poses a practical problem for timekeeping. The world's reference clocks sit at different altitudes, and each must be corrected for its height before they can be compared.
The Meter
Relativity is in the definition of length as well. Since 1983 the meter has been defined as the distance light travels in a fixed fraction of a second. That definition only makes sense because the speed of light is the same for every observer, the starting assumption of special relativity.
How Direct Is the Link?
Direct. These experiments apply and test Einstein's equations themselves, and the best clocks cannot be used without them.