The kilogram may need to go on a diet. The international standard, a cylinder-shaped hunk of metal that defines the fundamental unit of mass, has gained tens of micrograms in weight from surface contamination, according to a new study.
As a result, each country that has one of these standard masses has a slightly different definition of the kilogram, which could throw off science experiments that require very precise weight measurements or international trade in highly restricted items that are restricted by weight, such as radioactive materials.
But ozone and ultraviolet light could be used to clean the kilograms without damaging them, the research suggests.
You don’t really need an actual “thing” to define the weight, just a description of the thing in terms that are truly constant. Length is now (as compared to ages ago when the standard was some dead king’s shoe) described in distance between waves of light of a certain frequency, which in theory are constant across space and time. You could similarly describe weight by describing an object of certain dimensions (measured in wavelength derived units) molded in a certain immutable element—1 cubic centimeter of gold perhaps.
A physics prof would rap your knuckles for saying “weight” when you meant to say “mass.” But other than that, there’s surely a lot to be said for defining basic units in terms of things that have invariable properties.
And BTW, the USA is officially “soft metric,” that is, our traditional units (pounds, pints, feet) are defined (by NIST) in terms of SI units.