Feature
Toward the Optical Second
Armed with new kinds of atomic clocks, scientists are planning to overhaul the unit of time.
[Getty Images]
In November 2018, representatives of 54 countries met in Versailles, France, to approve a major revision to the International System of Units (SI). Delegates at the General Conference on Weights and Measures (CGPM) agreed to redefine four of the seven base units—meaning that, for the first time in history, all units refer to constants of nature. Notably, the kilogram had until that point been defined by the mass of a platinum-iridium cylinder held in a basement on the outskirts of Paris.
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