The re-definition of the base units of the SI: using the rules of nature to create the rules of measurement.

Milton M.

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In November 2018, the General Conference on Weights and Measures is expected to agree upon one of the most significant changes to the base units of the International System (the SI) which will base them on a set of definitions each linked to the laws of physics. This historic change towards using the laws of nature in the definitions will eliminate the final link between the SI and definitions based on physical artefacts. Following the revisions, the kilogram will be linked to the exact value of the Planck constant rather than the International Prototype of the Kilogram, as sanctioned by the 1st CGPM in 1889. For over 200 years, a collective ambition for the metric system has been to provide universality of access to the agreed basis for worldwide measurements. The changes to the definitions of the kilogram, the ampere, the kelvin and the mole that are expected to be agreed in November will be a further step towards this goal. They are based on the results of research into new measurement methods that have used quantum phenomena as the basis for standards that are fundamental. Great attention has been paid to ensure that these new definitions will be compatible with the current ones at the time when the change is implemented. The changes will be unnoticeable to all but the most demanding users. Whilst providing the necessary level of continuity for existing users, the changes have the advantage of being able to embrace future improvements in measurement methods to meet the needs of future users because they are based on the laws of physics. The new definitions will use the rules of nature to create the rules of measurement linking measurements at the atomic and quantum scales to those at the macroscopic level.

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