It's not news that it happens - but it's rather amazing to be able to measure it. The National Institute of Standards and Technology has been able to demonstrate that time passes faster just a foot above the ground.
Einsteinian theory suggests that time moves faster at higher elevations because of the tiny decrease in gravitational force. It's a tiny effect - 90 billionths of a second over a 79-year lifetime - so it's probably not worth junking your anti-wrinkle cream and just moving downstairs instead.
NIST scientists used two of the world's best experimental atomic clocks for the experiment. Each is based on the 'ticking' of a single aluminum ion as it vibrates between two energy levels over a million billion times per second.
The aluminum clocks — also known as quantum logic clocks, because they borrow logical decision-making techniques from experimental quantum computing — work by shining laser light on the ions at optical frequencies.
They can detect the tiny relativity-based effects because of their extreme precision and high 'Q factor' — a measure of how reliably the ion absorbs and retains optical energy in changing from one energy level to another.
"We have observed the highest Q factor in atomic physics," says NIST postdoctoral researcher James Chin-Wen Chou. "You can think about it as how long a tuning fork would vibrate before it loses the energy stored in the resonating structure. We have the ion oscillating in sync with the laser frequency for about 400 thousand billion cycles."
The team also tested another scenario predicted by Einstein's theories of relativity.
Under the so-called 'twin paradox', a twin who travels on a fast-moving rocket ship would return home younger than his sibling. The crucial factor is the acceleration of the travelling twin.
NIST altered the physical motion of the ion in one clock so that it gyrated back and forth at speeds of several meters per second - making it equivalent to the traveling twin in the thought experiment. That clock ticked at a slightly slower rate than the other, as predicted by relativity.
The team says such experiments could eventually be useful in geodesy - the science of measuring the Earth and its gravitational field - with applications in geophysics and hydrology, and possibly in space-based tests of fundamental physics theories.