Technical Note 1: A Cesium Atomic Clock Was Used To Measure the Lengths of
Versions of 'M M'
A 'cesium atomic clock' is a device that uses as a reference the exact
frequency of the microwave spectral line emitted by atoms of the metallic
element cesium, in particular its isotope of atomic weight 133 (Cs-133). The
integral of frequency is time, so this frequency, 9,192,631,770 hertz (Hz =
cycles/second), provides the fundamental unit of time, i.e., 1 second, which
may thus be measured by cesium clocks.