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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


On the occasion of their 26th General Conference on Weights and Measures (Conférence Générale des Poids et Mesures, CGPM) on 16 November 2018 in Versailles, the signatory states of the Metre Convention resolved to fundamentally reform the International System of Units (SI). This resolution stipulates that, in the future, all SI units will be based on the 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)

Using nearly perfect monocrystalline silicon spheres, the Avogadro experiment allowed not one, but two constants to be determined: the Avogadro constant and Planck’s constant. After laying down the constants, such silicon spheres represent one possibility of realizing the kilogram. (Photo: PTB)

A few minutes before the new SI was accepted by all 60 delegates from all over the world: Nobel prize winner Bill Philips giving a lecture on the long way to fundamental constants as the new basis for the units. (credit: A. Nicolaus/PTB)

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 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 metrology laboratories, extensive experiments have taken place over the past several years to measure these very constants as well as possible. These measurements, which were mainly carried out at the leading national metrology institutes such as PTB (Germany), NIST (USA), NMIJ (Japan) and NRC (Canada), have been successful: the target measurement uncertainties have been achieved and the requirement that the experiments take place independently of one another has been fulfilled. Based on these measurements, it was therefore possible to determine the numerical values attributed to the fundamental constants concerned very accurately.

The new SI will not be subject to variations caused by the definitions, since the fundamental constants have been attributed binding fixed values. This means that the new kilogram defined in this way will remain stable “for all times”. The international prototype of the kilogram, whose mass changes, will then become a thing of the past, once and for all. All electrical units (including the ampere) will be included in the system via quantum realizations (via the Josephson and the quantum Hall effects or “simply” by counting electrons per units of time). Last but not least, the mole will now also be defined via a fixed number of particles (the Avogadro constant) of a specified substance.

Thus, the following applies to the new SI: if measurements can be more precise, then the units can be realized with greater accuracy as well without changing the definition they are based on. In a high-tech world where length subdivision does not end with nanometers and nor time subdivision with femtoseconds, this technical openness of the new SI is a huge bonus for all future progress in terms of accuracy. The revision of the system of units thus provides better preconditions for innovation wherever very high accuracy is required, for example, when developing quantum technologies, enhancing diagnostic capabilities in the medical sector, or improving the efficiency of energy harvesting or the analytical methods used in climate research. And this openness applies to the whole scale of the unit concerned, since the fundamental constants do not emphasize any particular section of a scale. This differs somewhat from the present situation, where the kilogram merely fixes one precise point on the mass scale, namely the 1 kg graduation mark, or where the triple point of water merely fixes a single value on the temperature scale, namely the 0.01 °C graduation mark.

The definitions in the system of units have been entirely revised in order to remedy the previous system’s deficiencies. However, the changes will go unnoticed in everyday life. In contrast, progress for the scientific community will take effect as soon as the new definitions have been adopted. In technological fields, progress will become evident in the long run. Another advantage speaks for itself: natural constants are universally valid. The new SI has thus established a universal language that the international community has now agreed to use.


Background information
In the middle of the 19th century, international trade with technical goods, which had been triggered by the industrial revolution, started thriving. At the time, the industrialized nations quickly realized that a uniform, international system of measurement would be crucial to promoting international trade. In 1875, 17 leading industrialized nations (with Germany among them) decided to introduce an international system of measurement based on the meter (m), the kilogram (kg) and the second (s). Their decision was sealed in the form of a diplomatic treaty, the Metre Convention. In this treaty, it was agreed that a permanent scientific institution, the International Bureau of Weights and Measures (BIPM) was to be set up to realize the international weights and measures in the form of material measures, disseminate them to the member states and conduct research & development in order to improve these measurement standards – an activity this institution is still pursuing today. The International Committee for Weights and Measures (CIPM) was established as a supervisory entity; it is composed of 18 international experts. The International Conference on Weights and Measures (CGPM) was created to represent the member states. A supreme committee, the CGPM, elects the members of the CIPM, decides upon the working program and budget of the BIPM and makes the fundamental decisions regarding international metrology.

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: Opens window for sending emailjoachim.ullrich@ptb.de, phone: +49 531 592-1001


Further information is available from

Decisions of the 26th meeting of the General Conference of Weights and Measures, Versailles, 13-16 November 2018, available from: Opens external link in new windowhttps://www.bipm.org/en/CGPM

The International System of Units, 9th edition, Bureau international des poids et mesures (BIPM), available from: Opens external link in new windowhttps://www.bipm.org/en/publications/si-brochure/

Opens external link in new window“The new International System of Units (SI)”, PTB Info Sheet, November 2017

Opens internal link in current window“Research on the new SI”, an entire chapter of PTB’s website dedicated to the new SI, with publications such as the PTB-Mitteilungen, facts and figures and much more information about the new SI