Get Ready For the New, Improved Second

Alanna Mitchell in The New York Times:

Modern civilization, it is said, would be impossible without measurement. And measurement would be pointless if we weren’t all using the same units. So, for nearly 150 years, the world’s metrologists have agreed on strict definitions for units of measurement through the International Bureau of Weights and Measures, known by its French acronym, B.I.P.M., and based outside Paris. Nowadays the bureau regulates the seven base units that govern time, length, mass, electrical current, temperature, the intensity of light and the amount of a substance. Together, these units are the language of science, technology and commerce.

Scientists are constantly refining these standards. In 2018, they approved new definitions for the kilogram (mass), ampere (current), kelvin (temperature) and mole (amount of substance). Now, with the exception of the mole, all of the standards are subservient to one: time.

The meter, for example, is defined as the distance light travels in a vacuum during one-299,792,458th of a second. Likewise, the new definition of the kilogram rests on the second, in a manner too complicated to explain in fewer than several paragraphs. “All the units now are not autonomous units, but they are all depending on the second,” said Noël C. Dimarcq, a physicist and the president of the B.I.P.M.’s consultative committee for time and frequency.

More here.