The world is about to get a new way to measure itself

ON MAY 20THSISI the world gets a new kilogram. It also gets a new ampere, kelvin and mole. And, more important, it gets a new way of defining all these units—which lie, along with the metre and the second, at the heart of the Système International d’Unités () that human beings use to measure things. Even the pounds, miles, gallons and so on, clung on to by a few benighted Anglophones, are, , defined in terms of the .Measuring anything means comparing it with an agreed standard. Until now, for instance, the standard kilogram (see picture) has been the mass of a lump of metal sitting, nestled under a series of bell jars, in a vault in a suburb of Paris. However, the best sort of standard by which to define a unit is a constant of nature, such as the speed of light in a vacuum. And the metre is indeed so defined—or, rather, the speed of light is defined as 299,792,458 metres per second, and the second itself is defined as the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom at absolute zero. The calculation is therefore a simple one.