Before the spookiness of quantum mechanics, came the queerness of relativity. We learned that, contrary to everyday experience, time and space are dependent on our frame of reference. A consequence is that two clocks positioned at different points in a gravitational field, or moving at different speeds, tick at different frequencies. This time dilation is now discernable between two clocks separated by only one metre in height or moving at a speed of less than 10 ms−1 — if, that is, you have access to world's most accurate time pieces (C.-W. Chou et al. Science 329, 1630–1633; 2010).

Both special and general relativity give rise to a form of time dilation. In special relativity, it is a consequence of the constant relative speed of two observers; each sees time progress more slowly for the other. In general relativity, gravity is the underlying cause. The clock at a higher point in the field runs faster than a lower one.

Credit: ISTOCKPHOTO / SLAVOLJUB PANTELIC

These effects are both tiny. Take the example of the 1971 experiment by Joseph Hafele and Richard Keating. They placed clocks on commercial airliners that were circumnavigating the globe. A clock flying westwards at a few hundred metres per second and at an altitude of approximately 10 km was just 273 ns ahead of a reference clock on the ground after 50 hours of flying time. They calculated that about 65% of this was an effect of gravity and the remainder due to the relative motion.

Atomic clocks are needed to measure such small changes. These keep time using stable atomic transitions. Whereas Hafele and Keating used a microwave-frequency transition in caesium atoms, Chin-Wen Chou and colleagues use state-of-the-art optical-frequency atomic clocks, which keep time at least ten times more accurately.

They compared two aluminium-ion-based clocks connected by an optical fibre. As one was raised by 33 cm with respect to the other, the frequency of the ticks changed by one part in 2.5 × 1016, a change that the optical clocks were just sensitive enough to measure. To measure the effect of motion, rather than move the whole clock, the team applied an electric field, making the aluminium ions vibrate. Again, the apparatus was sensitive enough to measure a time dilation, even when the atoms were moving at just 5 ms−1 — slower than a running athlete.