Fundamental Constants and the International System of Units

Peter J. Mohr

National Institute of Standards and Technology, Gaithersburg, MD 20899-8420, USA

mohr@nist.gov

There is widespread interest in improving the International System of Units (SI) so that measurement standards would no longer depend on the metal kilogram mass that is maintained by the International Bureau of Weights and Measures (BIPM) near Paris. The new system, which is referred to as the Quantum SI, would instead be based on fixed values of certain fundamental constants.

The Committee on Data for Science and Technology (CODATA) Task Group on Fundamental Constants periodically provides recommended values of the fundamental physical constants for international use. The values are determined by a least-squares adjustment of the constants based on all data that are available at a particular time. The values of the constants are given in terms of SI units, which are the units agreed to by an international treaty among most major nations, including China and the US.

This paper will review the nature of and timescale for the changes to the SI that are being discussed and describe the impact that is expected for the fundamental constants and international metrology.

Keywords: Fundamental Constants, International System of Units, SI