The Revision of the SI – Towards an International System of Units Based on Defining Constants

Abstract

Following many years of intense investigations, leading to significant progress with the determination of fundamental constants, it is now likely that the General Conference for Weights and Measures (CGPM) will approve revised definitions for the SI base units at its next meeting in November 2018. It is proposed that the definitions for the kilogram, the ampere, the kelvin, and the mole will be based on new physical concepts, whereas the definitions for the second, the meter and the candela will only be reformulated. As a consequence, the units of the revised SI would be derived from a set of seven defining constants. This article describes the principal shortcomings of the present SI, the planned changes and the consequences for the realization and dissemination of units in the different fields of metrology.