The tick-tock of a super-accurate atomic clock has been sent over a longer distance than ever before. This paves the way for a global network of synchronised clocks for testing fundamental physics.Atomic clocks use precisely tuned electromagnetic waves to track the vibrations of atoms. Satellite transmissions can synchronise older atomic clocks, which track atoms that vibrate at microwave frequencies, but are not precise enough for optical clocks that use faster, optical frequencies.
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