New Kilogram Standard

“Officially, in the US, 1 pound is defined as 0.45359237 kilograms. A foot is defined as 0.3048 meters…

[As of today, the kilogram is being redefined in terms of Planck’s constant, which we only recently measured accurately enough to derive kilograms from it to an acceptable level of accuracy:]

The kilogram, symbol kg, is the SI unit of mass. It is defined by taking the fixed numerical value of the Planck constant h to be 6.626 070 15 × 10-34 when expressed in the unit J s, which is equal to kg m2 s -1 , where the meter and the second are defined in terms of c and ∆νCs.

[And this has happened before with the meter:] By 1983, physicists had gotten really good at measuring the speed of light. So they used it to fix the length of the meter forever, to make it permanent. Here’s how: They redefined the meter to be equal to the distance light travels in a vacuum in 1/299,792,458 of a second. Essentially, the definition of the meter is now baked into the definition of the speed of light…”

—Brian Resnick, “The world just redefined the kilogram.” Vox. November 16, 2018.

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