The second and its definition is fundamental to metrology, the science of measuring things. In fact, the meter, the kilogram, ...

is defined by taking the... transition frequency of the caesium-133 atom, to be 9192631770 when expressed in the unit Hz, which is equal to s⁻¹ . If you're confused, let me elaborate.

at least — is 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of a stationary cesium-133 atom at a temperature of 0K.

Maybe that’s even more than half of the fun. [CuriousMarc] got an HP 5061A Cesium clock, a somewhat famous instrument as the model that attempted to prove the theory of relativity. The reason?