The SI base unit of mass, here defined via quantum constants rather than a physical artifact.The kilogram is the SI base unit of mass, and since May 2019 it is defined by fixing the numerical value of the Planck constant, a fundamental constant of quantum mechanics. This replaced the previous definition based on a physical artifact, the International Prototype of the Kilogram (a platinum-iridium cylinder), which had been the standard since 1889. The new definition ties the kilogram to the unchanging laws of physics rather than to a single object that could be damaged, lost, or slowly gain mass from surface contamination.
The kilogram was originally defined in 1795 as the mass of one liter of water, but in 1889 it was replaced by the International Prototype of the Kilogram. The move to a quantum-based definition was approved in 2018 and took effect on May 20, 2019, World Metrology Day.
The definition uses the relation between mass and energy via E=mc² and the Planck constant h, which relates a photon's energy to its frequency. In practice, the kilogram is realized through the Kibble balance, an instrument that measures mass by comparing the force of gravity on a test mass to an electromagnetic force. The electromagnetic force is determined by quantum electrical standards (the Josephson effect and the quantum Hall effect), which are themselves defined by the Planck constant and the elementary charge.
The new definition enables precise mass measurements anywhere in the world without needing to compare to a single physical artifact, improving accuracy in science and industry, from pharmaceutical manufacturing to fundamental physics experiments.
The old International Prototype of the Kilogram is still kept in a vault in France, but it is now just a historical object, no longer the official definition of the unit.