Foundation for stable science with SI redefinition

Measurement scientists from around the world have voted to redefine the International System of Measurement (SI) units, changing the definition of the kilogram, the kelvin, the mole and the ampere.

Taking place at the General Conference on Weights and Measures in Versailles, Paris, hosted by the International Bureau of Weights and Measures (BIPM), this decision will mean that all of the units are expressed in terms of constants that can be observed in the natural world, and ensure they will remain reliable into the future.

The National Physical Laboratory (NPL), the UK’s National Measurement Institute, which is responsible for measurement standards across the country, has been an international leader in the global effort to achieve redefinition. “The SI redefinition is a landmark moment in scientific measurement,” said Dr JT Janssen, Director of Research at NPL. “Once implemented, all the SI units will be based on fundamental constants of nature whose value will be fixed forever. This will pave the way for far more accurate measurements and lays a more stable foundation for science.”

The redefinition would bring an end to physical artefacts like the kilogram, which is currently defined as equal to the mass of the International Prototype of the Kilogram (a block of metal stored in a vault in France). This artefact is susceptible to damage and environmental factors, and is compared to its copies only once in every 40 years, making calibration to it difficult and potentially inaccurate.

This decision will ultimately lead to a more practical definition of the SI. It will mean that all of the units are expressed in terms of constants that can be observed in the natural world. Just as redefining the second and the metre helped enable GPS navigation, the redefined SI is expected, over time, to enable new technologies we have yet to even imagine, whilst maintaining continuity for practical users.

The redefinition will come into effect on World Metrology Day, on 20^th May 2019.

The expected new definitions:

• The Kilogram will be defined by the Planck constant
• The Ampere will be defined using the elementary charge, a fixed value
• The Kelvin will be defined using the Boltzmann constant

1 Mole will be 6.022 14X ×10²³ entities of a substance, where X represents additional digits that can be added based on the most recent adjustments. When the mole is used, the elementary entities must be specified and may be atoms, molecules, ions, electrons, other particles, or specified groups of such particles.

Old definitions:

• The Kilogram is the unit of mass; it is equal to the mass of the international prototype of the kilogram
• The Ampere is that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 m apart in vacuum, would produce between these conductors a force equal to 2 x 10–7 newton per metre of length
• The Kelvin is the unit of thermodynamic temperature, which is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water
• The Mole is the amount of substance of a system which contains as many elementary entities as there are atoms in 0.012 kilogram of carbon 12