Toward the SI System Based on Fundamental Constants: Weighing the Electron

A modified International System of Units (SI) based on simply specifying exact numeric values of seven physical constants is described. This "set of constants" approach fixes the scale for all measurements, and the result is that both base units and derived units have equal footing. The seven quantities are the Cs transition frequency, the speed of light, the spectral luminous efficacy, the electron charge, the Avogadro constant, the Planck constant, and the Boltzmann constant. The first three quantities ensure that the second, the meter, and the candela are, in practice, the same as in the present SI. However, this approach requires that the definition of the ampere, mole, kilogram, and kelvin be changed to provide consistency with these constants. A major challenge in ensuring acceptable continuity is in advancing the measure of the kilogram and kelvin in terms of fundamental constants of nature