In October 2011, at its 24th meeting, the General Conference on Weights and Measures (CGPM) adopted Resolution 1 “On the possible future revision of the International System of Units, the SI”. This marked a historic step in efforts to bring the SI in line with contemporary science and technology. Although Resolution 1 comes short of declaring a date for adoption of the “New SI”, it gives a clear indication of how the New SI will be defined and structured. In particular, there will still be the same seven base units (second, metre, kilogram, ampere, kelvin, mole, and candela). Of these, the kilogram, ampere, kelvin and mole will be redefined by choosing exact numerical values for the Planck constant, the elementary electric charge, the Boltzmann constant, and the Avogadro constant, respectively. The second, metre and candela are already defined by physical constants and it is only necessary to edit their present definitions. The new definitions will improve the SI without changing the size of any units, thus ensuring continuity with present measurements.

Defining the SI base units by specifying exact numerical values for the associated physical constants, a formulation known as “explicit-constant definitions”, has many advantages. Nevertheless, such succinct and abstract definitions must be supplemented by documentation on how to realize each definition in practice at the highest metrological level—the so-called mise-en-pratique documents. Note that a realization does not define a unit but, crucially, it explains how traceability to the definition can be achieved. Each mise en pratique can be revised as technology advances without affecting the definition of the unit in question.

How well the Planck and Boltzmann constants can be measured in the present SI will determine the lowest uncertainties with which the kilogram and kelvin can be realized when the New SI takes effect (hence the session on Boltzmann constant determinations at ITS9). The achievement of sufficiently low uncertainties will ultimately determine when the New SI will be adopted and its impact on various metrological communities. Somewhat different considerations apply to the redefinitions of the ampere and mole. In addition to documentation for specialists on realizations of base units in the New SI, it is also desirable that the new definitions and their realizations be understood, at least in a general way, by the scientifically literate public.[1]