Fundamental constants will soon go from being things that are measured to things that are defined, and then used as reference for measurements
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,Measuring anything means comparing it with an agreed standard.
But all that is now to change. From Monday onwards, several other fundamental constants will go, like the speed of light, from being things that are measured to things that are defined, and are then used as references for measurement. A kilogram, for instance, will be derived from Planck’s constant, which relates the energy carried by a photon to its frequency. An ampere will depend on the charge on an electron, a kelvin on Boltzmann’s constant and the mole on Avogadro’s number—6.0221409x10, originally measured as the number of atoms in a kilogram of a particular isotope of carbon. Only the metre, the second and the candela remain unchanged.
With luck, this will be the last change ever needed to the system. By definition, the fundamental constants of the universe do not alter with time or place. Neither, even in America and Britain, need thesection of the print edition under the headline
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