Reaching international consensus on fundamental changes to the International System of Units (SI)

After a decade of international effort, a historic change to the SI received final approval by a unanimous roll-call vote held on November 2018 in Versailles, France. The venue was the 26th General Conference on Weights and Measures (CGPM). The revisions to the SI come into force on May 20, 2019. The world was able to watch the voting in real time on the BIPM’s YouTube channel. What was approved by the CGPM were redefinitions of the kilogram, the ampere, the kelvin, and the mole, four of the seven base units of the SI. All seven base units are now defined by fixed (i.e. exact) numerical values of seven “defining constants.” Since all SI units can be written in terms of the seven base units, all SI units are now defined by these seven invariants. One of the four new constants with fixed values is h, the Planck constant, whose SI unit, the joule-second, is J s = kg m² s^-1. Because the second and the meter were last redefined in 1967 and 1983, evidently a fixed value for h now redefines the kilogram. Previously, one kilogram was defined as the mass of a designated metal cylinder, which was put into service in 1889. Measuring h to parts in 10⁸ with respect to the mass of a 1 kg object, a prerequisite for the redefinition, required a heroic effort, but valiant attempts were also made to explain the significance of h to SI stakeholders unfamiliar with the rudiments of quantum physics. The revised SI has much to recommend it, but eliminating the last artifact definition of the system is what seized the public’s imagination even as they struggled to make sense of it all. So part of reaching a consensus was providing explanations by gifted scientists who are also gifted communicators. I will discuss these and other consensus-building challenges in my talk.