National Physical Laboratory

CODATA paper outlines changes to four fundamental constants of nature

The Committee on Data for Science and Technology (CODATA) has produced a paper which contains the updated values of four fundamental constants of nature: the Planck constant, the Boltzmann constant, the Avogadro number and the value of the charge of an electron. It is anticipated that the updated values will enable the redefinition of the International System of Units (the SI), removing the need for any physical artefacts in their definition. The prospective change would be an historic moment for measurement science, marking the final change in a campaign lasting many decades.

SI logo with constantsThe SI is the system of units used for all engineering and scientific applications worldwide. The SI defines seven base units upon which all physical measurements are based. It is administered from the International Bureau of Weights and Measures (BIPM) which in Sèvres, near Paris. BIPM operates under the support of the International Committee on Weights and Measures (CIPM) and significant changes must be approved by the diplomatic-level General Conference for Weights and Measures (CGPM) which meets every four years.

Historically, the SI has been based on physical artefacts. Years of experience have shown that, as measurement science has advanced, these artefacts have ended up limiting the achievable uncertainty of measurement.

Four units of the SI are not currently defined using fundamental constants: the kilogram, the kelvin, the mole and the ampere. For example, the kilogram is defined by the international prototype kilogram (IPK), a block of platinum-iridium alloy, kept in a vault in Sèvres. Measurements have shown that even though the mass of the IPK is extremely stable, it appears to be slowly changing but, because its mass defines what we mean by one kilogram, we cannot say exactly by how much, or even whether it is getting heavier or lighter.

Such problems have led measurement scientists to seek to build a system of units which is not based on physical artefacts but on the most stable things known: the natural constants of the physical world. In order to do this, the best possible measurements of the natural constants must be made in terms of the current system of units. These units are then redefined in terms of the natural constants, whose value is then fixed.

NPL, along with the international measurement community, has been working hard to provide increasingly accurate measurements of certain constants of nature for use in this redefinition attempt. These are the Planck constant, the Boltzmann constant, the Avogadro number and the value of elementary (electron) charge. The CODATA paper presents four highly-accurate values of these fundamental constants, obtained through years of scientific research. Now, CIPM has proposed their use in the redefinition of the four SI units whose definitions still rely on physical artefacts or references.

Scientists at the 26th meeting of the CGPM will vote on whether or not to accept the proposed redefinition in November 2018. It is anticipated that this change would represent an important and historic step forward for the measurement community and the industry that depends on it. It will provide a much more stable SI system, which can be accurately scaled from the smallest to largest measurements, from measuring the mass of both micro-gram dosages of medicines to the mass of a jet engine. For the first time all of the SI units will be able to be realised in-situ at laboratories around the world. If the vote is accepted, it will mark a new era in measurement science, with the potential for astonishing progress over the course of the next hundred years and beyond.

Dr Barry Inglis, President of the International Committee for Weights and Measures (CIPM) said "The decision by the CIPM to propose a resolution to the 26th CGPM on the redefinition of base units in the SI brings the vision of an International System of Units based on invariant constants of nature one step closer to realization. In taking this decision, the CIPM acknowledges the outstanding contributions by many metrologists and the commitments of their institutes to undertake the long-term experiments necessary to make this decision possible."

Read and download the paper

Read NPL's Dr Michael de Podesta's blog post: The Past, Present and Future of Measurement

Find BIPM's resources on the expected future revision of the SI

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Last Updated: 30 Oct 2017
Created: 27 Oct 2017

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