Redefinition of Units with Fundamental Constants

The present international system of units (SI) was established in 1960, and the definitions of some units have been changed since then. However, a huge change in the way to define the units will be made in 2018 by the CGPM (Conférence Générale des Poids et Measures). The values of seven defining constants will be fixed with no uncertainty, from which seven SI base units will be derived. Units such as the kilogram, kelvin, ampere, and mole are going to be defined by constants such as the Planck constant, Boltzmann constant, elementary charge, and Avogadro constant, respectively. In this way, the kilogram, which was first defined in 1899, will no longer be associated with an artifact, the International Prototype of Kilogram. The advantage of this change is that we do not need to redefine the units even when new methods or technologies are developed in the future because the definitions and the realizations of the units are separate. Accordingly, many other fundamental constants will have zero uncertainty, and precision metrology is expected to advance with these changes.