Official metal kilogram relinquishes its role to constants, but won’t yet retire — and more changes to scientific units are set to come
Ampere
"use the charge on a single such particle to determine the ampere — replacing a definition that is based on a hypothetical experiment involving two infinitely long wires."
Kelvin
"The kelvin will soon be defined by the Boltzmann constant, which links energy and temperature, rather than in reference to conditions at a specific temperature of water"
Mole
"a measure of substance that has long been equivalent to the number of atoms in 0.012 kilograms of carbon-12 — will soon equal Avogadro’s number"
Kilogram
"In the case of the kilogram, redefinition meant measuring Planck’s constant, a number that defines the size of packets of energy at the quantum scale"
Second
"currently described in relation to the frequency of microwave light absorbed and emitted by caesium-133 atoms. These atoms are now surpassed by ‘optical clocks’, which use different atoms that interact with higher-frequency visible light and seem to be able to keep time with less error"
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