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The definition of the kilogram just changed. Here's what that means.

Scientists say the newly redefined unit of mass will help advance scientific research, though it won't change things for consumers.
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, France, in 2018.Benoit Tessier / Reuters file

It’s the end of an era. After 130 years of faithful service, the so-called “prototype kilogram” upon which nations base their weight measurements was just replaced by a new standard based on fundamental laws of physics.

The new definition of the kilogram, which was adopted at an international conference held last November in Versailles, France, went into effect Monday. Instead of being based upon a shiny hunk of metal stored in a vault in Sevres, France, on the outskirts of Paris, the kilogram is now based on the Planck constant, a tiny, unvarying number that plays a key role in quantum physics.

“Every time we can reduce uncertainties in measurement, we open up opportunities for people to innovate,” said Andy Henson, a director at the International Bureau of Weights and Measures in Sevres. “The better you can measure, the more things you can do.”

The kilogram was redefined in order to create a precise, unchanging standard for its value, according to Henson. More than a century of cleanings and exposure to air had caused the original French prototype — known as the International Prototype of the Kilogram, or “Le Grand K” — to lose about 50 micrograms. That’s roughly the mass contained within a handful of fingerprints.

Since it’s based not on a physical object but on a mathematical constant, the new kilogram isn’t subject to such changes. And while 50 micrograms doesn’t sound like much, Jon Pratt, an engineer at the National Institute of Standards and Technology (NIST) in Gaithersburg, Maryland, said the more accurate definition would make a big difference for researchers working with tiny quantities of medications, radioactive compounds and other materials requiring nanoscale precision.

What will the new kilogram mean for the rest of us? Experts say not much. “You don’t need to panic,” Henson said, adding that the switch had been designed to go unnoticed by most people. “Nothing is going to change.”

In other words, a kilogram of something at the store or on the scale will continue to be equivalent to about 2.2 pounds.

Le Grand K isn’t the only old definition going the way of the dinosaur. New definitions for the ampere (used to measure electric current), kelvin (used to measure temperature) and mole (used to measure the number of particles in a substance) — other units in what’s known as the International System of Units — also went into effect Monday.

But Le Grand K and copies of it stored in other countries, including the United States, won’t be riding off into the sunset just yet. Pratt said NIST would continue to use its copies as the standard for the kilogram.

“It’s going to be a while before people believe in electrical measurements,” he said, referencing the new definition of the kilogram. “People really like objects, man.”

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