New definitions for kilogram and mole

Abstract

It doesn’t happen too often, but after a vote that took place Nov. 16 near Paris, science textbooks really will have to be rewritten. At the Congress Chamber in the Palace of Versailles, metrologists from over 50 countries voted to redefine four fundamental units of measure in the International System of Units (SI): the ampere, kelvin, kilogram, and mole. These units will join the meter, candela, and second in being defined not in reference to physical artifacts, but in reference to fundamental physical constants. Scientists say redefining these units to be based on physical constants will make measurements more accurate and stable. The redefined units, which will take effect May 20, 2019, World Metrology Day, are the result of years of work, discussion, and competition to measure the fundamental constants of nature to an incredible degree of certainty. Although most people will not notice the change, the increased precision will