Universal Units Reflect Their Earthly Origins

Universal Units Reflect Their Earthly Origins

On November 16, 2018, in Versailles, the General Conference on Weights and Measures officially endorsed the new basic units of the International System of Units (SI; Système Internatiopnal d’Unités). Michel Van Camp, Head of the Operational Directorate “Seismology and Gravimetry” of the Royal Observatory of Belgium, and his colleagues at the Federal Institute of Metrology in Bern and the University of La Rochelle highlight in a recent article the importance of Earth sciences in the origin and realization of those basic units.

Those units are based on 7 constants, of which the speed of light c, the Avogadro constant NA, and the Planck’s constant h. This becomes effective on May 20, 2019. Hence, the kilogram will be officially redefined as a function of the Planck’s constant, used in Einstein’s energy formula: E = mc² = .

This change in definitions is necessary because the use of the International Prototype Kilogram PK, made of platinum and iridium, poses various problems. There is no way to ensure its long-term stability, it might be destroyed or damaged, and it poses logistical problems when it has to be compared with its copies.

Hence, the meter, the kilogram and the second will be defined by the motions and energy of electrons, atoms and photons. However, the numerical values of the constants will be such that at the time the definition is adopted, the continuity is ensured with the former system. However, the new constants will not be completely divorced from their historical ties. Hence, the SI remains rooted to our home planet: originally, the meter was defined as 1/10,000,000 of the length of a quadrant of Earth’s meridian, the second as 1/86,400 of the duration of the mean solar day, and the kilogram as the mass of one cubic decimeter of water.

In an article published on EOS, a magazine of the American Geophysical Union, Michel Van Camp and his co-authors point out that Earth sciences remain necessary in metrology, the science of measurement. Indeed, to practically realize the kilogram, one will use, among others, an experimental device requiring measuring the gravity g. Nowadays, this is measured with a precision of one billionth of g (g = 9.811 116 655 m/s² at the Royal Observatory of Belgium in Uccle). Such accuracy is made possible thanks, among others, to the research performed at the Royal Observatory of Belgium since more than 50 years.

The Observatory remains also a reference for the time unit, the second, with an accuracy better than a billionth of a second. The definition of the kilogram uses indirectly the second, through the speed of light c, the units of which are in meter per second.


The paper: Michel J. Van Camp, Philippe Richard, and Olivier de Viron, “Universal Units Reflect Their Earthly Origins”, EOS, published online on November 14, 2018. https://eos.org/project-updates/universal-units-reflect-their-earthly-origins

Webstream of the General Conference on Weights and Measures: https://www.youtube.com/watch?v=qA67T7FPBME
The website of the Operational Direction “Seismology and Gravimetry”: http://seismology.be/en
The time office: https://betime.be/en/