Historic vote means unit of measurement will no longer be defined by a piece of metal first conceived in 1889

The weight is finally over. Nearly 130 years after the kilogram was first defined by a lump of metal in a vault in Paris, scientists have voted for change and a new system that redefines the global measure of mass in terms of a fundamental constant of nature.

Following a historic vote on Friday at the General Conference on Weights and Measures, in Versailles, the kilogram will no longer be defined by the international prototype kilogram (IPK), a platinum alloy cylinder fashioned in 1889, but by Planck’s constant, a number that is deeply rooted in the quantum world.

In the balance: scientists vote on first change to kilogram in a century Read more

“The vote went through unanimously. It was very emotional,” said Stephan Schlamminger, a physicist from the US National Institute of Standards and Technology who has worked on redefining the kilogram for years. “They did a roll call of each country. One got a sense of how big metrology is.”

The IPK, or Le Grand K, has served the world well but scientists have long known that even though, by definition, it is precisely one kilogram, its weight changes over time. When in use, the tiniest wear makes it lighter, while pollution in the air binds to the surface and over time makes it ever so slightly heavier.

The new system retires the IPK and instead defines the unit of mass through the electrical force needed to counteract the weight of a kilogram on a machine called a Kibble balance. The electrical force itself is linked to the Planck constant through quantum electric effects described by two Nobel prize winners, Brian Josephson and Klaus von Klitzing.

Along with the kilogram, three other base units will be redefined as a result of the vote. The units for electric current (ampere), temperature (kelvin) and amount of substance (mole) all become linked to constants of nature, namely the electric charge, the Boltzmann constant and the Avogadro constant respectively. In all, the International System of Units has seven base units, including the unit of time (second), the unit of length (metre), and the unit of luminosity (candela). The definitions of these will remain unchanged.

Martin Milton, director of the International Bureau of Weights and Measures, said the redefinition was “a landmark moment in scientific progress”.

“Using the fundamental constants we observe in nature as a foundation for important concepts such as mass and time means that we have a stable foundation from which to advance our scientific understanding, develop new technologies and address some of society’s greatest challenges,” he said.

While the redefinition will have zero impact on the way fruit and vegetables are weighed at the supermarket, it marks the culmination of decades of work to link the basic units that underpin metrology to constants woven into the fabric of the universe. The changes will come into effect on 20 May 2019, world metrology day.