The Fight Over the Kilogram Enters Quantum Territory

A controversial effort to determine a new standard weight is underway.

Unlike the other six base units of measurement (meter, second, ampere, kelvin, mole and candela), the kilogram is not currently a mathematical constant. Instead, the standard unit of mass is based on the weight of an actual, IRL metal cylinder locked in a vault at the International Bureau of Weights and Measures in Sevres, France.

If that sounds like the setup for the most boring Bond movie ever, it isn’t. But it is a highly contentious compromise that has been in effect since 1889, when the BIPM agreed that the cylinder would be known as the “International Prototype Kilogram.” Fast forward to the present day, and the locked cylinder is actually losing weight, potentially because it’s shedding gas. The kilo shrinkage is making scientists nervous because it undermines the accuracy of their measurement. Thus, there is a push to ditch the cylinder for something conceptual and consistent.

Doing that would mean digging even further into controversial territory. The new definition of the kilo would be based on a combination of Planck’s constant, a reliable quantum physics unit relating to size, and Avogadro’s number, the quantity of molecules and atoms in one mole of substance.

The exact value of Avogadro’s number, which potentially equals a value greater than the number of grains of sand in the entire world, has evaded many over the years, though a study published by scientists at Italy’s National Institute of Metrology Research in Turin in the Journal of Physical and Chemical Reference Data may change that. They’ve determined a close calculation estimate of Avogadro’s number by counting the number of atoms in a silicon sphere weighing a kilogram. This calculation would allow them to determine Planck’s constant and then reset the measurement of the kilogram.

Giovanni Mana, a member of the National Institute of Metrology Research that published the study, told the Independent about the importance of getting the new standard measurement Right. “Prior to redefining the kilogram, we must demonstrate that the new realization is indistinguishable from the present one, to within the accuracy of the world’s best balances, otherwise, when changing from the present definition to the new one, all users in science, industry and commerce must change the mass value of all the existing artifacts.”

If they pull it off and a new standard is set, it could lead to a better understanding of definitive mass, which would help manufacturers, researchers, and extremely detail-oriented drug dealers.

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