The ticking of atomic clocks is like the beat of a metronome, but instead of a pendulum, it's the oscillation of electrons in an atom. Two such clocks — one using an ytterbium ion, the other using strontium atoms — were made to tick side by side, and their rhythms were compared.
The measurements went on for nearly two years. The portable clock was taken out of the lab several times, but every time it returned, it showed the same rhythm, with an error of only 4.3 × 10⁻¹⁸. If such a clock had been running since the beginning of the universe, it would have gained or lost less than a second.
This stability isn't just a record. It opens the door to a new definition of the second, based not on a single type of atom but on comparing several. Moreover, clocks sensitive to gravity can measure altitude: time runs slightly slower in a basement than in an attic. Portable clocks become a standard you can drive to another institute, even where you can't stretch fiber optics. So the rhythm of the atomic metronome connects fundamental physics to everyday space.
Over nearly two years of measurements, the portable clock traveled outside the lab several times, yet each time its ticking matched to the 18th decimal place.