» Redefining the SI

05 November, 2018

On 16th November 2018, measurement scientists from the 60 Member States of the Metre Convention will come together to vote on the redefinition of the International System of Units (SI), changing the world’s definition of the kilogram, the ampere, the kelvin and the mole.

If agreed, these changes will come into force on 20th May 2019. The vote will take place at the General Conference on Weights and Measures (CGPM) at the Palais des Congrès in Versailles, which is organised by BIPM. The decision will mean that all SI units will 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 decision will bring an end to the use of physical objects to define measurement units. The kilogram, for example, is currently defined as being the mass of the International Prototype of the Kilogram, a cylinder made out of a platinum-iridium alloy stored at the BIPM. This object has served science and technology well for nearly 130 years, but its stability during this period could only be confirmed by comparisons with identical copies, which is a difficult process and potentially inaccurate.

The proposed change will see the kilogram defined in terms of the Planck constant – the fundamental constant of quantum physics. It is ready for use everywhere and always.

“The SI redefinition is a landmark moment in scientific progress.” said Martin Milton, Director, BIPM, “Using the fundamental constants we observe in nature as a foundation for important concepts such as mass and time means that we have a stable foundation from which to advance our scientific understanding, develop new technologies and address some of society’s greatest challenges.”

The expected new definitions impact four of the seven base units of the SI: the kilogram, the ampere, the kelvin and the mole, and all units derived from them, such as the volt, ohm and joule.

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

Although the size of these units will not change (a kilogram will still be a kilogram), the four redefined units will join the second, the metre and the candela to ensure that the set of SI base units will continue to be both stable and useful. The revised SI will maintain its relevance by facilitating technical innovations. Just as the redefinition of the second in 1967 provided the basis for technology that has transformed how we communicate across the globe, through GPS and the internet, the new changes will have wide-reaching impact in science, technology, trade, health and the environment, among many other sectors.

View a video about the change produced by the National Institute of Standards and Technology.