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Natural constants as the main protagonists

The General Conference on Weights and Measures (CGPM) adopts revision of the International System of Units

PTB-News 1.2019
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On the occasion of their 26th General Conference on Weights and Measures (CGPM) in November 2018 in Versailles, the signatory states of the Metre Convention decided to fundamentally reform the International System of Units (SI). This resolution stipulates that, in the future, all SI units will be based on the numerical values laid down for seven selected natural constants. In passing this resolution, the General Conference has followed a recommendation issued by the International Committee on Weights and Measures (Comité international des poids et mesures, CIPM) – the world's supreme expert committee on metrology. The new definitions of the units will come into force on 20 May 2019, World Metrology Day.

In the new International System of Units (SI), seven fundamental constants will be determined as defining reference entities. The seven base units – arranged in the outer circle of the diagram – will lose their prominent role. (Diagram: BIPM)

In principle, the idea of defining units of measurement on the basis of natural constants is not new. What began 50 years ago with the definition of the second by means of atomic clocks, and continued over 30 years ago with the definition of the meter with the aid of the speed of light, will now continue for all of the units in the International System of Units (SI). In this context, four other fundamental constants will be playing the leading roles: Planck’s constant, h, the Avogadro constant, NA, the Boltzmann constant, k, and the charge of the electron, e.

In the major national metrology institutes, elaborate experiments have taken place over the past several years to measure these very constants as accurately as possible. The target measurement uncertainties set for these experiments have been achieved and the requirement that the experiments take place independently of one another has been fulfilled. It was therefore possible to determine the numerical values attributed to the fundamental constants concerned with very high accuracy.

With this fundamental revision, the system of units does away with the old system's deficiencies in terms of definitions. One particularly noticeable deficiency was that the mass of the international prototype of the kilogram and its copies varied by up to half a microgram per year. The new system now has a decisive advantage: natural constants are valid anywhere in the universe and at any time. They lend the new SI a universal character. In contrast to standards such as the international prototype of the kilogram or the triple point of water, the realization of units based on fixed numerical values of natural constants is not restricted to a particular graduation on a unit scale. The SI is thus intrinsically open to technological innovation.


Joachim Ullrich
President of PTB, President of the
Consultative Committee for Units (CCU),
and Vice President of the International
Committee for Weights and Measures
Phone: +49 531 592-1001
Opens window for sending emailjoachim.ullrich(at)ptb.de


Decisions of the 26th meeting of the General Conference of Weights and Measures, Versailles, 13–16 November 2018, published at: Opens external link in new windowhttps://www.bipm.org/ en/worldwide-metrology/cgpm/

The International System of Units, 9th edition, Bureau International des Poids et Mesures (BIPM), published at: Opens external link in new windowhttps:// www.bipm.org/en/publications/si-brochure/