It's no longer based on a metal cylinder in France.
In November, scientists from more than 60 nations got together to redefine the kilogram, the base unit of mass in the International System of Units (SI).
Previously, a kilogram was defined by the Le Grand K, a platinum-iridium cylinder that had to be replaced every so often to ensure its accuracy. As of Monday, it’s now defined by the relationship between energy and mass — and because this physics-based approach is inherently stable, experts believe the new definition will improve the quality of science.
The new definition won’t change what a kilogram means to most people. However, the physical cylinder used to define it previously would occasionally lose a few atoms, and that slight deviation could affect scientific measurements in detrimental ways.
But that won’t be a problem now that a kilogram is defined as equal to the mass of 1.4755214 x 10^1040 photons, each with a frequency matching a cesium atomic clock.
“The revised SI future proofs our measurement system so that we are ready for all future technological and scientific advances such as 5G networks, quantum technologies, and other innovations that we are yet to imagine,” Richard Brown, Head of Metrology at the National Physical Laboratory, said in a press release.
Editor’s note: This article misstated the new photon count. It has been updated.
READ MORE: There’s a Brand-New Kilogram, And It’s Based on Quantum Physics [Live Science]
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