The 2019 Revolution: Redefining Reality

May 20, 2019, marked the most fundamental change in measurement science since the metric system itself was created. On that date, the last physical artifacts in the SI system were officially abandoned, completing a transformation that made measurement standards as eternal and universal as the laws of physics themselves.

The centerpiece of this revolution was the redefinition of the kilogram. For 129 years, the kilogram had been defined by a physical object—the International Prototype Kilogram, stored in a vault at the International Bureau of Weights and Measures near Paris. But physical objects change over time. The prototype kilogram was slowly losing mass due to contamination removal during cleaning, and identical copies stored around the world were drifting apart by tiny but measurable amounts.

This created a philosophical crisis: if the standard kilogram changed mass, did this mean that everything else in the universe was changing mass relative to the standard? The absurdity of this situation demanded a solution based on unchanging constants of nature rather than changeable human artifacts.

The solution came through the Planck constant, one of the fundamental constants of quantum mechanics. By precisely measuring the relationship between mechanical force and electromagnetic force using incredibly sophisticated instruments called watt balances, scientists could determine the exact value of the Planck constant in terms of the old kilogram. They then flipped the relationship: instead of using the artifact to define mass, they used the now-known Planck constant to define mass.

The ampere underwent a similar transformation. Previously defined in terms of the force between current-carrying wires, it was redefined by fixing the value of the elementary charge—the charge of a single electron or proton. This connected electrical measurement directly to the fundamental structure of matter rather than to mechanical measurements.

The kelvin's redefinition fixed the value of the Boltzmann constant, which relates temperature to the average kinetic energy of particles. This made temperature measurement as fundamental as energy measurement and eliminated the need to maintain temperature standards based on specific physical systems.

The mole was redefined by fixing Avogadro's number—the number of atoms in one mole of substance. This completed the transition away from physical standards to purely numerical constants.

These redefinitions required extraordinary precision in measuring the relevant constants. The Planck constant had to be determined to within a few parts in 100 million. This precision demanded the most sophisticated instruments ever built, including watt balances accurate to a few parts in a billion and atomic clocks precise to within a second over the age of the universe.

The practical impact of these redefinitions was minimal for most users—a kilogram of coffee still weighs the same as it did before 2019. But the conceptual impact was revolutionary. For the first time in human history, all our fundamental measurements were based on the deepest laws of physics rather than on human-made objects or arbitrary definitions.