A pendulum that swings almost half a quadrillion times per second. This isn't mechanics: it's the pulse of light trapped by ytterbium atoms. Instead of gears, there's an optical lattice — a standing wave that holds atoms at its nodes like peas in a pod. That's how the world's most accurate clocks work: they tick with an error that accumulates to one second only over a period three times the age of the universe.
Two copies of such clocks were placed side by side and made to compare their timekeeping. A laser beam reflected by mirrors in a vacuum chamber creates a perfectly even "comb" of light for the atoms. The slightest frequency shift, and the clock notices. The sensitivity is such that they can detect how the heat from the experimenter's own body presses on the atoms, even through the walls of the vacuum chamber. To eliminate this disturbance, a double thermal shield was built around the atoms — a kind of thermos where the temperature is known to within hundredths of a degree.
The main goal isn't just a record. The clocks are being prepared for an optical "roll call" between Shanghai and Wuhan. A light signal over optical fiber will carry the rhythm by which fundamental constants of nature can be checked, or a drift in the very foundations of physics can be sought. And if a discrepancy shows up in this roll call — then reality itself has thrown us a puzzle.
If such a clock had been started at the moment of the Big Bang, by today it would have lost or gained less than half a second.
In Liu Cixin's novel "The Dark Forest," ultra-precise clocks in orbit help synchronize an attack on distant starships — here, an earthly pair of clocks will link two cities in the hunt for new physics.