Redefinition of the kilogram will not make the kilogram more precise, but it will make it more stable. A physical object can lose or gain atoms over time, or be destroyed, but constants remain the same. And a definition based on constants would, at least in theory, allow the exact kilogram measure to be available to someone anywhere on the planet, rather than just those who can access the safe in France, says Richard Davis, former head of the mass division of the International Bureau of Weights and Measures (BIPM) in Sèvres, France, which hosts the metal kilogram.
In 2011, the CIPM formally agreed to express the kilogram in terms of Planck’s constant, which relates a particle’s energy to its frequency, and, through E = mc2, to its mass. This means first setting the Planck value using experiments based on the current reference kilogram, and then using that value to define the kilogram. The CIPM’s committee on mass recommends that three independent measurements of Planck’s constant agree, and that two of them use different methods.
One method, pioneered by an international team known as the Avogadro Project, involves counting the atoms in two silicon-28 spheres that each weigh the same as the reference kilogram. This allows them to calculate a value for Avogadro’s constant, which the researchers convert into a value for Planck’s constant. Another method uses a device called a watt balance to produce a value for Planck’s constant by weighing a test mass calibrated according to the reference kilogram against an electromagnetic force.
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