“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 … The problem is that Big K is a manmade 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.” Vox: The new kilogram debuts today. It’s a massive achievement.