For 130 years, a cylinder made of a platinum-iridium alloy and stored in a suburb of Paris called Saint Cloud has been the official definition of a kilogram, the internationally accepted basic unit of mass. But that will change once and for all on May 20, when for the first time all of the basic units of measurement will be officially defined in terms of atomic properties and fundamental physics constants, rather than specific, human-made objects.
The other objects on which physical standards are based, such as the standard meter, were already replaced years ago, but the kilogram — generally known as the kilo for short — turned out to be a harder unit to define in absolute terms. Physicists and engineers have been frustrated, however, by the inevitable imprecision of a unit based on a single physical object.