SIredefinition

SI redefinition refers to the 2019 overhaul of the International System of Units in which several base units were defined by fixed values of fundamental physical constants rather than physical artifacts. The goal was to ensure long-term stability, universality, and precision by tying unit definitions to invariant properties of nature. The redefinitions were adopted by the General Conference on Weights and Measures and came into effect on 20 May 2019, implemented through advances in quantum metrology and precision measurement.

The changes impacted all seven base units. The second remains defined by the cesium-133 atom’s hyperfine transition, exactly 9,192,631,770 revolutions per second. The metre is defined as the distance light travels in vacuum in 1/299,792,458 of a second, making the speed of light an exact constant. The kilogram is defined by fixing the Planck constant h at 6.62607015×10^-34 J·s. The ampere is defined by fixing the elementary charge e at 1.602176634×10^-19 C, enabling realization through quantum electrical standards. The kelvin is defined by fixing the Boltzmann constant k_B at 1.380649×10^-23 J/K. The mole is defined by fixing the Avogadro constant N_A at 6.02214076×10^23 per mole. The candela is defined via a fixed luminous efficacy of 683 lumens per watt for radiation at frequency 540×10^12 Hz.

Implementation enabled realisation of units through quantum phenomena such as the Josephson and quantum Hall effects, without reliance on physical reference artifacts. The redefinition has been widely accepted as a foundation for future precision science and industry, though it required substantial changes in laboratory metrology and calibration practices.