Proposed redefinition of SI base units

A committee of the International Committee for Weights and Measures (CIPM) has proposed revised formal definitions of the SI base units, which are being examined by the CIPM and which are expected to be adopted at the 26th General Conference on Weights and Measures (CGPM) on 16 November 2018. The metric system was originally conceived as a system of measurement that was derivable from nature. When the metric system was first introduced in France in 1799 technical limitations necessitated the use of artifacts such as the prototype metre and prototype kilogram. In 1960 the metre was redefined in terms of the wavelength of light from a specified source, making it derivable from nature, leaving the kilogram as the only unit still defined by an artifact. If the proposed redefinition is accepted, the metric system (SI) will, for the first time, be wholly derivable from nature.

The proposal can be summarised as follows:

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.

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