I used to have a tee shirt, that had a picture of a garden mole and exterminator. The caption was “Mole problems? Call Avogadro 6.022 x 10^23.”
It’s a chemistry joke.
OK, it’s geeky. Avogadro’s number is the number of atoms in a mole, which allows us to convert the unit of mass to number of atoms, and vice versa, by converting the atomic number to grams. Carbon 12 (the most common carbon isotope) is the reference–a mole of carbon 12 atoms will, in theory, mass exactly twelve grams. Similarly, a mole of hydrogen atoms will mass one gram.
Obviously, for this to work, we have to know pretty accurately just how many atoms there are in a mole. In fact, if we knew it accurately, and precisely enough, we could use it as an atomic basis for mass (just as the meter was defined in terms of wavelengths of a specific chemical laser, and more recently as the distance light goes in a certain time interval measured by a cesium clock). The current (crude) standard for mass is a lump of metal, a kilogram by definition, kept in a bell jar in Paris.
Recent research indicates that the traditional number, first identified by Avogadro, may be a little off. If they can refine the number sufficiently, it can be established as the basis for mass, and we can free ourselves of one of the few areas in which we’re dependent on the duplicitous French…