Redefinition of SI Base Units

On 16 November 2018, the 26th General Conference on Weights and Measures (CGPM) voted unanimously in favour of revised definitions of the SI [International System of Units] base units, which the International Committee for Weights and Measures (CIPM) had proposed earlier that year. The new definitions will come into force on 20 May 2019.

The metric system was originally conceived as a system of measurement that was derivable from unchanging phenomena, but technical limitations necessitated the use of artifacts (the prototype metre and prototype kilogram) when the metric system was first introduced in France in 1799. Although designed to not degrade or decay over time these prototypes were in fact losing minuscule amounts of mass over time, even in their sealed chambers. The changes in mass, and with them the values the artefacts provided, were so tiny as to be imperceptible without the most sensitive of equipment. However, by that same logic, those sensitive instruments could no longer provide exact measurements, or at least not within an acceptable tolerance level.

In 1960, the metre was redefined in terms of the wavelength of light from a specified source, making it derivable from universal natural phenomena, leaving the prototype kilogram as the only artefact upon which the SI unit definitions depend. With this redefinition, the SI is for the first time wholly derivable from natural phenomena.

The kilogram, ampere, kelvin and mole have been redefined by setting exact numerical values for the Planck constant (h), the elementary electric charge (e), the Boltzmann constant (k), and the Avogadro constant (N_A), respectively. The metre and candela are already defined by physical constants, subject to correction to their present definitions. The new definitions aim to improve the SI without changing the size of any units, thus ensuring continuity with existing measurements.

The previous major change of the metric system was in 1960 when the International System of Units (SI) was formally published as a coherent set of units of measure. SI is structured around seven base units whose definitions are unconstrained by that of any other unit and another twenty-two named units derived from these base units. Although the set of units formed a coherent system, the kilogram remained defined in terms of a physical artefact, and some units were defined based on measurements that are difficult to precisely realise in a laboratory, such as the Kelvin scale's definition in terms of the triple point of water. The new definitions adopted by the CIPM seek to remedy this by using the fundamental quantities of nature as the basis for deriving the base units. The second and the metre are already defined in such a manner. The change will mean, amongst other things, that the prototype kilogram will cease to be used as the definitive replica of the kilogram as of 20 May 2019.

A number of authors have published criticisms of the revised definitions – including that the proposal had failed to address the impact of breaking the link between the definition of the dalton and the definitions of the kilogram, the mole, and the Avogadro constant N_A.

https://en.wikipedia.org/wiki/2019_redefinition_of_SI_base_units