Units Based on Constants: The Redefinition of the International System of Units

Recent decisions of the General Conference on Weights and Measures of the Metre Convention have opened the door to a fundamental change of the International System of Units, the SI, in 2018. The revised SI will be based on fixing the numerical values of seven defining constants corresponding to the...

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Bibliographic Details
Published in:	Annual review of condensed matter physics Vol. 7; no. 1; pp. 35 - 59
Main Authors:	Stenger, J, Ullrich, J.H
Format:	Journal Article
Language:	English
Published:	Annual Reviews 10-03-2016
Subjects:	BIPM defining constants Metre Convention new SI revision of SI
Online Access:	Get full text
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Summary:	Recent decisions of the General Conference on Weights and Measures of the Metre Convention have opened the door to a fundamental change of the International System of Units, the SI, in 2018. The revised SI will be based on fixing the numerical values of seven defining constants corresponding to the seven traditional base units. This will bring about basic modifications in the definition of the kilogram, the ampere, the kelvin, and the mole and will have consequences for units derived from them. In contrast, the second, the meter, and the candela will not be affected. This paper describes the motivation and rationale behind this endeavor, introduces the new definitions, and reviews the requirements for the redefinitions as well as the status of the experiments that are currently being carried out to fulfill them. Benefits and a wide-reaching impact for the realization and dissemination of the units are anticipated because innovative technologies with ever-increasing accuracy will emerge, enabling the direct realization over widespread measurement ranges without the definitions themselves having to be changed.
ISSN:	1947-5454 1947-5462
DOI:	10.1146/annurev-conmatphys-031115-011311