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Historic vote to redefine the kilogram changes forever the way we measure mass

Scientists around the world hail "measurement revolution."
A replica of the International Prototype Kilogram is pictured at the International Bureau of Weights and Measures in Sevres near Paris
A replica of the International Prototype Kilogram at the International Bureau of Weights and Measures (BIPM) in Sevres near Paris.Benoit Tessier / Reuters

Talk about massive change. After meeting the needs of science, industry and commerce for more than 130 years, the kilogram has just been fundamentally reinvented.

At a meeting Friday in Versailles, France, representatives from the U.S. and 59 other nations adopted a resolution to define the familiar unit of mass in terms of the Planck constant, 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, which hosted the event where the vote was taken, said in a tweet featuring a photo of packed auditorium. "This has been a measurement revolution!"

Previously, the kilogram was defined as the mass of a specific physical object: a shiny metal cylinder that since 1889 has been stored in a vault on the outskirts of Paris. Cast of a platinum-iridium alloy and roughly the size of a votive candle, the International Prototype of the Kilogram (sometimes called "Le Grand K"), represents the mass of one liter of pure water at its freezing point.

The resolution will take effect on May 20, 2019.

Image: The International Prototype of the Kilogram (IPK)
Since the late 1880s, the shiny metal cylinder under these nested bell jars has been used to define the humble kilogram.BIPM / via Reuters

The kilogram — which was and still is roughly equivalent to 2.2 pounds — was redefined in order to streamline scientific research and development involving ultra-precise measurements of mass. It aims to do that by ending the reliance on Le Grand K and official copies of it held by the U.S. and other nations.

These reference kilograms, which periodically are taken from their protective cocoons for measurement and recalibration, are vulnerable to damage, wear and theft despite the care taken to safeguard them — and 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, told NBC News MACH in an email in advance of the vote.

In everyday life, the redefinition is expected to have no immediate consequence.

"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, director of the International Bureau of Weights and Measures, told NBC News MACH in an email.

Thus a kilogram of coffee contains no more or less coffee under the new definition than under the old one.

The newly ratified plan is part of a broader effort to define all seven basic units of the International System of Units (SI) — the modern version of the metric system that arose in 18th Century France — on fixed numerical values from the natural world. Unlike physical objects like Le Grand K and the metal bar that once defined the meter, these constants never change.

In addition, the new plan subtly redefines the ampere (the SI unit of electrical charge); the mole (the unit for the amount of substance in a material); and the kelvin (the unit of temperature). The newly redefined units join three others whose definitions were already based on constants: the second, the meter and the candela, a unit of luminosity.

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, referring to the catchphrase of the developers of the original metric system. "We have now found that we can achieve what was first conceived over 200 years ago."

If Milton is enthusiastic about the redefined kilogram, he's not alone. At the National Institute of Standards and Technology (NIST), the federal agency that watches over the United States' reference kilograms on its Gaithersburg, Maryland, campus, some researchers were so enthusiastic that they got themed tattoos.

“This is really a pivot point for humanity,” Jon Pratt, a researcher who worked on the plan to redefine the kilogram and one of the tattooed NIST scientists, said in a written statement.

Pratt's tattoo bears a long stream of digits representing the Planck constant, which is designated by a lower-case 'h,' and the French words "A tous les temps, a tous les peuples." English translation: "For all times, for all peoples."

Jon Pratt's tattoo based on an 1830's medallion commemorating the adoption of the metric system in France
The design on Jon Pratt's tattoo is based on an 1830's medallion commemorating the adoption of the metric system in France. J. Stoughton / NIST