Kilogram Leaves Physical Object Behind for Constant-Based Definition
The scientific community officially redefines the kilogram using the Planck constant instead of a physical metal cylinder. This historic shift also updates the ampere, kelvin, and mole to rely on fundamental constants.
On May 20th, World Metrology Day, the scientific community officially changes the definition of the kilogram. For 130 years, the kilo is defined by a physical cylinder of platinum-iridium alloy, known as Le Grand K and stored in a vault outside of Paris. However, every time scientists handle the cylinder, it loses atoms, shedding an estimated 50 micrograms over its lifetime.
Beginning this Monday, the kilogram is officially measured by a fundamental physical constant known as the Planck constant. The kilo now corresponds to the mass of an exact number of photons of a particular wavelength, meaning it is defined in terms of seconds and the meter. Along with the kilogram, three other units of measurement — the ampere, kelvin, and mole — also receive new definitions based on physical constants.
For most people, the new definitions do not change much, but for the scientific community, this is a historic moment. Researchers gain far more accurate tools with which to make measurements, a shift that could help reexamine the laws of physics. Experts describe this update as the most important decision made by the International Bureau of Weights and Measures in decades.