Scientists Change Definition Of A Kilogram In Historic Vote

The change is being hailed as a 'measurement revolution.'

An old iron set of various weights, measured in kilograms.
Tminaz / Shutterstock

The change is being hailed as a 'measurement revolution.'

Since 1889, the definition of a kilogram has been determined by the mass of a particular physical object — a shiny chunk of platinum-iridium roughly the size of a golf ball frequently referred to as “Le Grand K,” meticulously stored under three glass bell jars in a vault in Paris so secure it requires three people with three separate keys and the oversight of the Bureau Internationale des Poids et Mesures to gain access to it.

But after almost 130 years of the shiny object serving as the standard for the unit of measurement, the metal will no longer serve its purpose. According to NBC MACH, representatives from the United States and 59 other nations met today and, with a historic vote, agreed to adopt a resolution that would instead define the unit of mass in terms of the Planck constant, which is an “unvarying and infinitesimal number” at the heart of quantum physics.

“Celebrations and standing ovation as the vote is accepted,” the Bureau of Weights and Standards tweeted, along with a photo of the packed auditorium where the vote took place in Versailles, France. “This has been a measurement revolution!”

The new resolution is set go into effect on May 20, 2019.

The reason for redefining the unit was to “streamline” scientific research and development involving “ultra-precise” mass measurements. According to NBC MACH, Le Grand K and other official copies of it held by nations around the world could no longer be reliable as their masses have shifted by tiny amounts over the years.

“The change brings our best scientific understanding of the natural world directly into our daily lives, and gives us access to a definition of the mass unit which is available to everyone with the will and the skill to perform the experiments,” Canada’s chief metrologist Alan Steele said via email.

The change, however, is not expected to have an immediate effect on everyday life.

“Our community has worked very hard to be sure that ordinary people will not notice any difference at the time the change is made,” Martin Milton said via email. Milton is the director of the International Bureau of Weights and Measures.

Today’s vote was part of a larger effort to define all seven of the basic units of the International System of Units on fixed numerical values from the natural world, NBC MACH noted. Before the historic vote, the kilogram was the last SI unit still defined by a physical object.

“It’s been a challenge for our community since the time of the French Revolution to find a standard for mass that is available ‘for all time for all people,'” Milton said. “We have now found that we can achieve what was first conceived over 200 years ago.”