…or at least what a kilogram actually weighs.
Doesn’t it weigh a kilogram? Maybe not:
The kilogram is unique in that it is defined by reference to a lump of crude, man-made stuff. Besides aesthetic niggles, that fact leads to an important practical problem. The international prototype kilogram (IPK), the technical name for the cylinder in Sèvres, does not in fact keep a constant mass. Over the years pollutants from the air settle on its surface, causing its mass to rise. Attempts to clean it then cause its mass to fall. As a result, what science understands by a kilogram has varied, but has done so in a way that is, by definition, unmeasurable.
The Economist editors raise a question often debated within the homeland security community:
Science would thus love to be free of this awkward lump of metal, but attempts to define mass objectively—with reference to, say, the mass of a proton—have always foundered on the question: “So how do you measure that?”