The International System of Units, abbreviated as “SI” (French: Système international d’unités), is the global system for defining physical variables. The SI is a metric system of units, which means one of the base units is the meter. After more than two centuries of constant struggle, the redefinitions of the ampere, kelvin, and mole have come into force since the 2019 General Conference on Weights and Measures (CGPM). The meter and candela had already been defined by physical constants.
The last condition to be met was that the previous definition of the mass unit based on the kilogram artifact also could be replaced. The prototype kilogram, approved by the international CGPM in 1889, was the worldwide reference for the kilogram. It is stored in the form of a cylinder of platinum-iridium and well-protected under three glass bells in the vault of the basement of the International Bureau of Weights and Measures (BIPM) in Sèvres near Paris. The problem with a definition such as "The kilogram is equal to the mass of the international prototype kilogram" is that the weight of this reference kilogram has changed over the years. Even if the drifts are very small, their causes and magnitude are not readily comprehensible as the prototype kilogram, by definition, always weighs exactly one kilogram – no matter how heavy it actually is. With the continuous development of science and technology, maintaining the status quo was not viable. It was time to replace this artifact and derive the kilogram from physical constants. Therefore, the definition of the International System of Units (SI) is based exclusively on the fixed numerical values of seven defining constants. This new, "rock-solid base" will fundamentally change metrological possibilities. At the same time, new standards and guidelines (such as DIN EN ISO/IEC 17025; IATF 16949) ensure that industrial processes are more closely linked to them.
The SI redefinition allows even more precise measuring equipment and, consequently, new future applications. At the same time, industrial development and the resultant higher accuracy requirements were also reasons why metrologists considered a new SI to be indispensable. Presently, everything has come full circle, and metrology and industry can achieve a new level of "quality".
Written by: Dr. André Schäfer
The historical context can be found in the specified source:
"Nature is universal, so that discoveries made in London are equally valid in Paris, in Washington, or on the moon" Terry Quinn
Terry Quinn, Retired Emeritus Director of the Bureau International des Poids et Mesures (BIPM).
Mr. Quinn was Director of BIMP from 1998 to 2003. He played a leading role in the establishment of the global Mutual Recognition Arrangement in measurement, which enabled international recognition of national measurement capabilities in support of innovation and trade.
Terry J Quinn "From Artefacts to Atoms - The BIPM and the Search for Ultimate Measurement Standards", published by Oxford University Press, New York 2012, ISBN: 9780195307863