The meter is fixed to the speed of light and a second to the radiation of cesium, but the mass of one kilogram is still not defined by a universal constant. Instead, it’s still pegged to an old-fashioned cylinder of platinum iridium alloy kept under lock and key in Sèvres, France.
The method isn’t just old-fashioned, it’s imprecise, which has literal ramifications across the world when the point is to set the kilogram standard. The cylinder is weighed every few decades against official copies that had the same mass when they were all cast in 1899. When they were last weighed in 1988, however, their masses had drifted 70 micrograms apart.
The smallest named unit in the metric system is the yoctogram, equal to 0.000000000000000000000001 grams. (Yes, that’s 24 zeros.) For a scale that can measure differences in mass as small as a yoctogram, which is on the order of the mass of a proton, physicists writing in Nature Nanotechnology turned to the wunderkind of nanotechnology: carbon nanotubes.