February 18, 2019

Redefining the Kilogram

“Humanity is on the verge of a weighty achievement. On Friday, representatives of more than 60 nations will convene in Versailles, France, to approve a new definition for the kilogram.

Since the 19th century, scientists have based their definition of the fundamental unit of mass on a physical object — a shining platinum iridium cylinder stored in a locked vault in the bowels of the International Bureau of Weights and Measures (BIPM) in Sevres, France. A kilogram was equal to the heft of this aging hunk of metal, and the cylinder, by definition, weighed exactly a kilogram. If the cylinder changed, even a little bit, then the entire global system of measurement had to change, too.

With Friday’s vote, scientists will redefine the kilogram for the 21st century by tying it to a fundamental feature of the universe — a small, strange figure from quantum physics known as Planck’s constant, which describes the smallest possible unit of energy.

Thanks to Albert Einstein’s revelation that energy and mass are related, determining exactly how much energy is in that unit can let scientists define mass in terms of Planck’s constant — a value that should hold up across space and time — rather than relying on an inconstant metal cylinder. (Mass determines something’s weight, and for most purposes mass and weight are interchangeable.)”

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