Redefining kilogram releases it from its earthly form

Since 1889, Le Grand K, a sleek cylinder of platinumiridium metal, has ruled from its underground vault in Paris. An absolute monarch, it was the very definition of 1 kilogram of mass. Scientists from around the world made pilgrimages to it, bringing along their national kilogram standards to weigh and adjust accordingly.

“The mother ship is never wrong,” said Robert Vocke Jr., a chemist at the National Institute of Standards and Technology in Gaithersburg, Maryland.

No longer. In November, at a conference center steps from the Palace of Versailles, several dozen nations voted to overthrow Le Grand K and to redefine the kilogram and three other standard units of measure: the ampere, for electrical current; the kelvin, for temperature; and the mole, which describes the amount of a chemical substance.

The vote fulfills an 18th century dream. Henceforth, all seven units in the International System of Units, otherwise known as the SI, will no longer be defined by material objects and instead will be defined by abstract constants of nature.

“This arc of history started before the French Revolution and now, I think we’ve finished the journey,” said Stephan Schlamminger, a NIST physicist. The “democratization of the units,” he said, is now complete.

The SI originated at the end of the 18th century with just the meter and the kilogram. The idea was to standardize the basic units of trade and scientific measurement. After all, for 1 kilogram of gold coins to hold universal value, A replica of the International Prototype Kilogram is on display at the International Bureau of Weights and Measures near Paris. The metal cylinder at the heart of the world’s system for measuring mass is heading into retirement now that a new way to measure the kilogram has been developed.

everyone had to agree on the exact definition of 1 kilogram.

By 1875, 17 nations had signed the Treaty of the Meter in Paris, which set international standards for the meter and the kilogram.

But it was soon realized that a physical object could be scratched, chipped or even destroyed. Scientists

began to dream of standard units of measurement that would remain forever constant — standards with definitions built from the fabric of the universe.

In 1990, metrologists discovered that Le Grand K had mysteriously become lighter than its six official copies by some 50 micrograms. The kilogram standard was in trouble, and the mission to redefine it took on a new level of urgency.

Scientists in various laboratories around the world worked for decades to determine and refine constant measurements, eventually coming up with three methods: the Planck constant, the Kibble balance and Avogadro’s constant.

Crucially, Avogadro’s constant and the Planck constant are intertwined in the laws of physics. Having measured Avogadro’s constant, scientists could derive the Planck constant. And with a precise measure of the Planck constant, they could validate the results of Kibble’s work, and vice-versa.

Last month’s vote cements the values of the Planck and Avogadro constants, and releases the kilogram from its earthly form.