Brian Resnick / May 20, 2019 / VOX
For more than a century, the kilogram had a very simple definition: It was the mass of a hunk of platinum-iridium alloy that’s been housed at the International Bureau of Weights and Measures in Sèvres, France since 1889.
And for those thinking the kilogram doesn’t matter in the US, which uses imperial units like pounds, feet, and gallons, our measurements are derived from (metric) units. Officially, in the US, 1 pound is defined as 0.45359237 kilograms.
The problem is that Big K is a man-made object, and therefore, it is imperfect. If Big K changes, everything else has to adjust. And this has happened. Big K is not constant. It has lost around 50 micrograms (about the mass of an eyelash) since it was created. But, frustratingly, when Big K loses mass, it’s still exactly one kilogram, per the old definition.
That’s no good. So, what’s better?
The new definition anchors the value of the kilogram to a constant in nature, which can never, ever change
Starting Monday, the kilogram will be defined by the Planck constant.
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