Revision of the International Systems of Units: Motivation, Realization and Consequences
From the year 1875, when the International System of Units (SI) was established by the General Conference of Weights and Measures, the SI has been modified several times when better accuracy or stability was needed in science, industry or society. In this year’s revision, put into effect at the 20th of May, a change of paradigm of the basics of the SI occurred. As a consequence, non-realizable idealistic definitions and material artifacts that can change with time were banned and the SI now rests on (mostly fundamental) constants. Thus, as the basis of the SI, the seven former base units of length (m), mass (kg), time (s), electric current (A), temperature (K), luminous efficacy (cd), and the amount of substance (mol) are replaced by seven defining constants namely the speed of light in vacuum (c), the frequency of the hyperfine transition of Cs (), Planck’s constant (h), the elementary charge (e), Avogadro’s number (NA), Boltzmann constant (kB) and the (technical) constant of luminous efficacy Kcd. As a result, in the revised SI each physical quantity can be realized consistently in any desired way with the prospects to increase accuracy and ease of operation without changing the system again in the future. Several examples (and one counterexample) for these future prospects will be given in the talk together with the reasons that led to this essential change of the SI.
Sprecher: Prof. Dr. Fritz Riehle, Physikalisch-Technische Bundesanstalt Braunschweig (PTB)
Kontakt: Prof. Norbert Lindlein