Four units, four constants
The claim is narrow and specific: that four of the seven SI base units — the kilogram, ampere, kelvin and mole — could be, and as of 20 May 2019 were, redefined by fixing the exact numerical value of a fundamental constant, rather than by a physical object or an idealised experimental setup. The kilogram now follows from the Planck constant, the ampere from the elementary electric charge, the kelvin from the Boltzmann constant, and the mole from the Avogadro constant. The second, metre and candela had already been defined this way in earlier decades. The stated aim was to make the base units as stable as the laws of physics rather than as stable as a manufactured artefact, while keeping every unit’s actual size unchanged for continuity.
An eleven-year approval process
Getting there took over a decade of formal process. The General Conference on Weights and Measures mandated the investigation in 2007; by 2010 proposals existed but the International Committee for Weights and Measures judged the underlying measurements not yet good enough, and the 2011 and 2014 conferences accepted the principle while deferring the vote for the same reason. The threshold was explicit: acceptance required at least three independent experiments producing values for the Planck constant with a relative uncertainty of no more than five parts in a hundred million. That bar was cleared by 2016, CODATA published agreed recommended values for the constants in October 2017, and the General Conference voted unanimously to proceed on 16 November 2018, with the change taking effect the following May.
Why the kilogram had to go
What holds up is the reason for acting in the first place. The kilogram’s definition rested on a single platinum-iridium cylinder held at the International Bureau of Weights and Measures, and comparisons against its official copies found mass drifting by as much as twenty micrograms a year in some of them — a small figure, but one with no floor, since nothing about a physical cylinder guarantees it stops moving. The ampere’s old definition, based on the force between two infinitely long parallel wires carrying current, could never be realised in an actual laboratory. The kelvin’s reliance on the triple point of water broke down at very low and very high temperatures. Fixing constants instead removes each of these specific weaknesses at once.
The balances that made it possible
The technical achievement that made the vote possible was measurement, not policy. Kibble balances — instruments that balance a weight electromagnetically and were formerly known as watt balances — and the Avogadro project, which counts atoms in a nearly perfect silicon sphere, gave independent routes to the Planck constant that converged closely enough to satisfy the CGPM’s threshold. Multiple groups working by different physical methods reaching compatible values was the specific evidence the committee had asked for, and it is what separates this redefinition from a purely administrative decision: the constants were pinned down experimentally before they were fixed by decree, not the other way round.
The seam the fix left behind
The change did not come free. Because the mole and the kilogram were previously linked through the dalton and the Avogadro constant in a way that made certain conversions exact, fixing the Avogadro constant on its own broke that exactness — the relationship now holds only approximately, and the molar mass constant is no longer precisely one gram per mole, though the resulting discrepancy is reported as extremely small. Fixing the elementary charge similarly means that the vacuum permeability and permittivity, previously exact by definition, now carry a small measured uncertainty tied to the fine-structure constant. Critics have also questioned, on philosophical rather than practical grounds, whether the mole and candela were ever base physical units in the same sense as the others.
Worth understanding, not worth dread
For a general reader this is worth the time mainly as a case study in how a standards body actually changes its mind: not on a single measurement, but on convergent evidence from different instruments crossing a threshold fixed years in advance. It will not change how a kitchen scale or a lab balance behaves, and the paper trail of committee votes and CODATA values is dry by design. But for anyone who wants to see the gap between a physical constant being ‘discovered’ and being formally adopted as the basis of a unit, and who is willing to sit with a few broken exact relationships as the price of the change, the account rewards the attention. It is not a story built for drama, and does not need to be.