From Nature.com.
Revamped definitions of scientific units are on their way. In the biggest overhaul of the international system of units (SI) since its inception in 1960, a committee is set to redefine four basic units — the ampere, the kilogram, the kelvin and the mole — using relationships to fundamental constants, rather than abstract or arbitrary definitions.
The changes would take effect in May 2019.
The kilogram is currently defined as the mass of a chunk of metal in a vault in Paris. And an imaginary experiment involving the force between two infinite wires defines the ampere, the unit of electrical current. The mole, meanwhile, is the amount of substance in a system with as many elementary entities as there are atoms in 0.012 kilograms of carbon-12, while the kelvin relates to the temperature and pressure at which water, ice and water vapor co-exist in equilibrium, known as the triple point of water. In the future, these units will be calculated in relation to constants — for example, the ampere will be based on the charge of an electron, while the kilogram will be defined with respect to the Planck's constant, a fundamental in quantum mechanics.
Revamped definitions of scientific units are on their way. In the biggest overhaul of the international system of units (SI) since its inception in 1960, a committee is set to redefine four basic units — the ampere, the kilogram, the kelvin and the mole — using relationships to fundamental constants, rather than abstract or arbitrary definitions.
The changes would take effect in May 2019.
The kilogram is currently defined as the mass of a chunk of metal in a vault in Paris. And an imaginary experiment involving the force between two infinite wires defines the ampere, the unit of electrical current. The mole, meanwhile, is the amount of substance in a system with as many elementary entities as there are atoms in 0.012 kilograms of carbon-12, while the kelvin relates to the temperature and pressure at which water, ice and water vapor co-exist in equilibrium, known as the triple point of water. In the future, these units will be calculated in relation to constants — for example, the ampere will be based on the charge of an electron, while the kilogram will be defined with respect to the Planck's constant, a fundamental in quantum mechanics.
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