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A revolution in the System of Units, starring the fundamental constants

The International Committee for Weights and Measures (CIPM) has set the course and declared its support for a redefinition of the kilogram, the kelvin, the ampere and the mole


This revolution has been a long time in the making. Behind it are metrologists working in their scientific laboratories, and scientific managers in their decision-making committees. Now the moment is approaching when many of the old quantities (specifically, the definitions behind them) will have to step down to make way for their successors, who are waiting in the wings. The quantities concerned are the kilogram, the mole, the ampere and the kelvin. The foremost expert committee in the world of metrology, the International Committee for Weights and Measures (Comité international des poids et mesures, CIPM), issued its unequivocal recommendation for this development at its recently concluded annual assembly. It is hoped that, at their general conference next year, the international community of signatories to the Metre Convention will heed the experts’ advice and provide the physical units with an especially firm foundation – one that consists of the set values of selected fundamental constants.



Prof. Dr. Joachim Ullrich
President of PTB, President of the Consultative Committee for Units (CCU) and Vice President of the International Committee for Weights and Measures (CIPM)
E-mail: joachim.ullrich@ptb.de, phone: +49 (0)531 592-1001

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Im neuen Internationalen Einheitensystem (SI) werden sieben Naturkonstanten als definierende Bezugsgrößen festgelegt. Die sieben Basiseinheiten – in der Grafik im äußeren Kreis angeordnet – verlieren ihre herausgehobene Stellung. (Grafik: BIPM)

In principle, the idea of defining units of measurement on the basis of fundamental constants is not new. What began over 50 years ago with the definition of the second by means of atomic clocks, and over 30 years ago with the definition of the meter with the aid of the speed of light, is now to continue for all of the units in the International System of Units (SI). Metrologists have taken a shine to four other fundamental constants: Planck’s constant, the Avogadro constant, the Boltzmann constant and the charge of the electron. In metrology laboratories, extensive experiments have taken place over the past several years to measure these very constants as well as possible. The message that these measurements have been successfully completed has been received loud and clear from every domain of metrology.

As a result of these scientific success stories, which have been written at the Physikalisch-Technische Bundesanstalt (PTB) in Germany and at the National Institute of Standards and Technology (NIST) in the United States (among other places), the path to a fundamental revision of the International System of Units is now unobstructed. For this reason, practically no doubt remains that the members of the Metre Convention will adopt this fundamental change to the System of Units when they hold their General Conference in Versailles in November of 2018. All of the SI units will then be derived from a total of seven constants.

The key date on which these new definitions are formally to enter into force is World Metrology Day (20 May), 2019. Nothing about our daily lives is expected to change – the kilogram will be just as heavy on 20 May as it was on 19 May. However, daily life in advanced technological fields will look somewhat different, where the new, highly innovation-friendly definitions are sure to bring noticeable benefits.