The International System of Units (SI) is defined by the SI Brochure, which is published by the BIPM. https://www.bipm.org/utils/common/pdf/si-brochure/SI-Brochure-9.pdf
In a landmark decision, the BIPM’s Member States voted in November 2018 to revise the SI, changing the world’s definition of the kilogram, the ampere, the kelvin and the mole.
This decision, made at the 26th meeting of the General Conference on Weights and Measures (CGPM), means that from 20 May 2019 (today) all SI units are defined in terms of constants that describe the natural world. This will assure the future stability of the SI and open the opportunity for the use of new technologies, including quantum technologies, to implement the definitions.
Visit the BIPM website for more information: https://www.bipm.org/en/measurement-units/
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