2019 redefinition of SI base units

The revision of the International System of Units that took effect on 20 May 2019, defining all SI units through fixed numerical values of seven physical constants.

Planck constant: The fundamental constant relating a photon's energy to its frequency. Its fixed value now defines the kilogram through quantum mechanical measurements.
International System of Units: The internationally agreed system of measurement units used in science, industry, and commerce. The revision changed the foundations of this system without replacing its seven base units.
General Conference on Weights and Measures: The international body that adopts resolutions on the International System of Units. It approved the revision and set its implementation date.
Traceability (metrology): A property of a measurement result linked to a reference through a documented chain of calibrations. Constant-based standards let laboratories establish traceability without access to a unique artifact.
Elementary charge: The magnitude of the electric charge of a proton or electron. Its fixed value anchors the ampere instead of relying on a force between wires.
SI base unit: One of the seven units designated as a foundation of the International System of Units. The revision retained these seven units while redefining their connection to constants.
International Prototype of the Kilogram: A platinum-iridium cylinder that served as the international mass standard from 1889 to 2019. Replacing this artifact ended the kilogram's dependence on one object's mass.
Quantum metrology: The use of quantum phenomena and constants to make precise measurements. The revised definitions connect practical standards more directly to quantum effects.
Boltzmann constant: The constant relating temperature to the average energy of thermal motion. Its fixed value defines the kelvin through the energy scale of temperature.
Physical constant: A measurable quantity in nature whose value is treated as constant under specified conditions. Fixed numerical values of constants became the shared foundation of SI definitions.

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