Modernizing the SI: implications of recent progress with the fundamental constants

Abstract

Recent proposals to re-define some of the base units of the SI make use of definitions that refer to fixed numerical values of certain constants. We review these proposals in the context of the latest results of the least-squares adjustment of the fundamental constants and against the background of the difficulty experienced with communicating the changes. We show that the benefit of a definition of the kilogram made with respect to the atomic mass constant (mu) may now be significantly stronger than when the choice was first considered 10 years ago.