Redefining the SI units

With the progress of science and technology, new and more accurate methods of the determination of the base units became available, and the definitions of most of the base units changed. Increasingly the base units are being defined by invoking physical constants. As these natural constants are very accurately known and always and everywhere have the same value, the definition of the base units (and hence of measurements always and everywhere) becomes grounded in nature itself. For instance, the second is determined by the frequency of light associated with the transition between two specified excitation levels of a caesium atom at rest and the meter is determined by the speed of light in vacuum. By excluding artifacts as the world standards, a different issue was avoided. Since they were stored in Sèvres, the artifacts had been changing over time, but as they were the world standards, it was impossible to determine by how much.

Currently all SI base units are defined via physical constants, but one. The last physical artifact standing is Le Grand K, or the famous piece of metal which has a mass of exactly one kilogram. For the best part of twenty years, metrologists around the globe have been collaborating to develop and agree on a way to link the kilogram to a physical constant. After years of discussion and preparation, on Friday November 16, 2018, in Versailles, France, a group of 60 countries made history. With a unanimous vote, they dramatically transformed the international system that underpins global science and trade by changing the world's definition of the kilogram, the ampere, the kelvin and the mole, for ever.

The decision, made at the General Conference on Weights and Measures in Versailles, France, which is organised by the International Bureau of Weights and Measures (BIPM), means that all SI units will now be defined in terms of constants that describe the natural world. This will assure the future stability of the SI and open the opportunity for the use of new technologies, including quantum technologies, to implement the definitions. The new definitions will come into force on 20 May 2019.

Redefinition

The new definitions impact four of the base units: the kilogram, ampere, kelvin and mole:
 

  • The kilogram – will be defined by the Planck constant (h)
  • The ampere –- will be defined by the elementary charge (e)
  • The kelvin – will be defined by the Boltzmann constant (k)
  • The mole – will be defined by the Avogadro constant (Na)


Expected definitions from May 2019
 

Kilogram (kg)

The kilogram is the SI unit of mass

The kilogram is defined by taking the fixed numerical value of the Planck constant h to be 6.626 070 15 × 10-34 when expressed in the unit J s, which is equal to kg m2 s−1, where the metre and the second are defined in terms of c and ∆ν.

Metre (m)

The metre is the SI unit of length

The metre is defined by taking the fixed numerical value of the speed of light in vacuum c to be 299 792 458 when expressed in the unit m s−1, where the second is defined in terms of the caesium frequency ∆ν.

Second (s)

The second is the SI unit of time

The second is defined by taking the fixed numerical value of the caesium frequency ∆ν, the unperturbed ground-state hyperfine transition frequency of the caesium 133 atom, to be 9 192 631 770 when expressed in the unit Hz, which is equal to s−1.

Ampere (A)

The ampere is the SI unit of electric current

The ampere is defined by taking the fixed numerical value of the elementary charge e to be 1.602 176 634 × 10−19 when expressed in coulombs, which is equal to A s, where the second is defined in terms of ∆ν.

Kelvin (K)

The kelvin is the SI unit of thermodynamic temperature

The kelvin is defined by taking the fixed numerical value of the Boltzmann constant k to be 1.380 649 × 10−23when expressed in the unit J K−1, which is equal to kg m2s−2 K−1, where the kilogram, metre and second are defined in terms of h, c and ∆ν.

Mole (mol)

The mole is the SI unit of amount of substance

One mole contains exactly 6.022 140 76 × 1023 elementary entities. This number is the fixed numerical value of the Avogadro constant, NA, when expressed in the unit mol–1 and is called the Avogadro number.

Candela (cd)

The candela is the SI unit of luminous intensity in a given direction

The candela is defined by taking the fixed numerical value of the luminous efficacy of monochromatic radiation of frequency 540 × 1012 Hz, Kcd, to be 683 when expressed in the unit lm W−1, which is equal to cd sr W−1.