Representatives from 54 of the BIPM’s member states have voted to overhaul the International System of Units, ending the long reliance on physical objects to define key measurements.
The decision was taken at the 26th General Conference on Weights and Measures in Versailles, France, and changes the definitions of the kilogram, ampere, kelvin and mole. From May 20, 2019, those units will be tied to constants of nature rather than artefacts kept in storage.
The biggest shift is the kilogram. The International Prototype of the Kilogram, a platinum-iridium cylinder held by the BIPM and used for almost 130 years, will be retired. Its replacement is a definition based on the Planck constant, a fixed value in quantum physics.
According to the BIPM, the new approach is meant to give the SI a more stable foundation and make it easier to use emerging technologies, including quantum technologies, in real-world measurement.
Martin Milton, the BIPM director, called the move “a landmark moment in scientific progress.” He said using natural constants for concepts such as mass and time would support scientific work, new technology and efforts to tackle major social challenges.
Barry Inglis, president of the International Committee for Weights and Measures, said the change marked the result of decades of work by measurement scientists. He said the new system would no longer be limited by objects and would provide universally accessible units with greater accuracy.
The revision affects four of the seven SI base units and all units derived from them, including the volt, ohm and joule. The kilogram will be defined by the Planck constant, the ampere by the elementary electrical charge, the kelvin by the Boltzmann constant and the mole by the Avogadro constant.
The size of the units will not change. A kilogram will still be a kilogram, but the revised system will sit alongside the second, the metre and the candela as part of a stable SI framework.
The BIPM said the earlier redefinitions of the second in 1967 and the metre in 1983 helped lay the groundwork for technologies such as GPS and the internet. It expects the latest changes to have wide effects across science, trade, health, the environment and other sectors.
