The evolution of the International System of Units



On 16 November 2018, the General Conference on Weights and Measures concluded a fundamental revision of the International System of Units (SI). A set of seven constants with determined values now completely codifies the system and forms the basis for the definition of the units.

Among other things, the revision will result in the retirement of the prototype kilogram, the last remaining artefact in the SI, after more than 100 years of use and its replacement by a unit of mass based on natural constants. The kelvin, ampere and mol will now also be defined by means of constants. This means that, in future, it will no longer be necessary to make a distinction between basic and derived units.

The system itself has not come about according to strict scientific criteria, but as a construct of practical considerations and historical framework conditions. Its units, however, are realised by means of sophisticated technical standards and in accordance with precise physical laws.

The revised SI ensures backward compatibility. Moreover, the changes mean that, in future, it will be possible to achieve more consistent, more reliable and more precise measurement results, thereby enabling new scientific discoveries and innovations. At the same time, the change will have no direct consequences for everyday life. The revised SI is also formulated in such a way that improved realisations of the units will be enabled gradually, without this being explicitly specified by the system. The SI is thus on a sound long-term footing and remains the fundament used worldwide for measurements made with the precision demanded by society, commerce and the scientific community.

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