Erik Benkler

In the basements of four European institutes stood seven devices: each can be compared to a tuning fork of unimaginable precision. But instead of sound, they emit 'ticks' made of light: oscillations of a laser locked to a specific frequency by atoms of strontium, ytterbium, or mercury.

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.