Measurement science goes back to nature for landmark change
14 Nov 2018
As expected, more than 60 leading scientists attending the General Conference on Weights and Measures have voted to change four of the International System of Units (SI units).
The kilogram, mole, ampere and kelvin will no longer be measured in relation to physical artefacts after concerns for many years about a lack of consistent reliability.
Kilograms, for example, have until now been defined as equal to the mass of the International Prototype of the Kilogram – a metal object stored in a vault in France.
From World Metrology Day, 20 May 2019, onwards the units will be defined by fundamental constants in nature – a practice already applied to the other three SI base units, the second, metre and candela.
From that point:
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 adjustment
The new system extends to 20 prefixes to the units, used for specifying multiples and fractions of them – as well as 22 derived units.
The UK’s National Physical Laboratory (NPL) in Teddington, responsible for Britain’s measurement capability, explained that the Paris artefact was susceptible to damage and environmental factor. Furthermore, it is compared to its copies only once in every 40 years, “making calibration to it difficult and potentially inaccurate”.
An NPL spokesperson commented the decision will mean that all of the units are expressed in terms of constants that can be observed in the natural world.
“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.”
See here for further detail from the NPL and below for a video explanation of the SI redefinition from the Veritasium website.