Evolution of the International Metric System of Units SI

Once the New SI is approved by the General Conference on Weights and Measures (CGPM), all base units of the international metric system of units (SI) will be defined in terms of physical constants and atomic properties. In this paper, we consider the rationale and the direction of the possible further evolution of the SI. The idea is to define all base units exclusively in terms of fundamental physical constants, with no reference to specific phenomena, physical theories or properties of material entities (including properties of atoms and elementary particles), so that those definitions would not have to be altered or amended following advancement in our understanding of the structure of matter, emergence of new physical theories or due to the technological progress. New developments in science and technology would then affect only the mise en pratique (realization) of base units, rather than their definitions. Furthermore, we point out the need for including base units for the weak interaction and the strong interaction into the SI and propose a way to do it. The structure of the fundamental-constants-based system of units (the FC SI) is discussed and prerequisites for the implementation of the FC SI are considered.