Since 1889, Le Grand K, a sleek cylinder of platinum-iridium metal, has ruled from its underground vault in Paris. An absolute monarch, it was the very definition of 1 kilogram of mass. Scientists ...
The kilogram isn't a thing anymore. Instead, it's an abstract idea about light and energy. As of today (May 20), physicists have replaced the old kilogram — a 130-year-old, platinum-iridium cylinder ...
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