The 2019 SI Redefinition: How Physical Constants Replaced Physical Artifacts

What Changed on May 20, 2019?

On May 20, 2019 — World Metrology Day — the International Bureau of Weights and Measures (BIPM) enacted the most significant revision of the International System of Units (SI) since the metric system was established in the 19th century. Four of the seven SI base units were redefined: the kilogram, the ampere, the kelvin, and the mole.

The principle behind the revision was conceptually clean: instead of defining units by reference to physical objects or arbitrarily chosen phenomena, define them by fixing the numerical values of fundamental physical constants of nature. These constants — Planck's constant, the elementary charge, the Boltzmann constant, and the Avogadro constant — are believed to be invariant throughout the universe. A unit tied to a universal constant cannot drift, degrade, or be destroyed.

The other three base units — the meter, the second, and the candela — were already defined in terms of physical constants and remained unchanged in their underlying definitions, though their formal language was updated for consistency.

The Problem with Artifacts (Le Grand K Was Drifting)

The trigger for the redefinition was a growing crisis of confidence in the International Prototype of the Kilogram (IPK), known informally as "Le Grand K" — a cylinder of platinum-iridium alloy manufactured in 1879, stored in a vault at the BIPM in Sèvres, France, under three nested glass bells.

By definition, the IPK weighed exactly 1 kilogram. Every national kilogram prototype was calibrated against it. But over 130 years of periodic comparisons, a disturbing pattern emerged: the IPK and its official copies were diverging in mass. By the 1990s, measurements showed that the IPK had gained or lost approximately 50 micrograms (50 × 10⁻⁶ g) relative to its copies — a discrepancy of about 5 parts in 100 million.

No one could determine which object was "correct." The IPK was the definition; by circular logic, it always weighed exactly 1 kg. But its mass relative to other nominally identical objects was measurably changing, which meant that the global kilogram standard was quietly drifting. Every measurement of mass anywhere in the world that traced back to the IPK was affected.

The same logic applied to the ampere, which was defined in terms of a hypothetical force between infinite parallel wires — a definition that was practically impossible to realize precisely. The kelvin was defined by the triple point of water, which is sensitive to isotopic composition. The mole was defined by counting atoms in 12 grams of carbon-12, which tied it indirectly back to the drifting kilogram.

The 4 Newly Fixed Constants

Planck Constant h → Kilogram

The kilogram is now defined by fixing the Planck constant h at exactly 6.626 070 15 × 10⁻³⁴ J·s (joule-seconds). Since a joule is kg·m²·s⁻², and the meter and second are already defined independently, fixing h determines the kilogram.

The Planck constant relates the energy of a photon to its frequency: E = hf. It is a cornerstone of quantum mechanics and appears in virtually every quantum mechanical calculation. Its value was measured with extreme precision before 2019 using the Kibble balance (described below), allowing the fixed value to be chosen with confidence.

Elementary Charge e → Ampere

The ampere is now defined by fixing the elementary charge e at exactly 1.602 176 634 × 10⁻¹⁹ C (coulombs). One ampere is defined as the flow of 1/1.602 176 634 × 10⁻¹⁹ elementary charges per second — that is, exactly 6.241 509 074 × 10¹⁸ electron charges per second.

The old definition — a force of 2 × 10⁻⁷ newtons per meter of length between two infinitely long parallel wires carrying the defined current — was theoretically clean but practically unrealizable with high precision, as no infinitely long wires exist.

Boltzmann Constant k → Kelvin

The kelvin is now defined by fixing the Boltzmann constant k at exactly 1.380 649 × 10⁻²³ J/K (joules per kelvin). The Boltzmann constant relates the average kinetic energy of particles in a gas to temperature: E = (3/2)kT for a monatomic ideal gas. Fixing k makes temperature a statement about energy at the molecular scale.

The previous definition — the kelvin is 1/273.16 of the thermodynamic temperature of the triple point of water — was precise but sensitive to isotopic variations in water samples. Cells of pure water at the triple point (273.16 K, 0.01°C, 611.73 Pa) were the working standards, and isotopic purity affected the result at the level of a few millikelvin.

Avogadro Constant Nₐ → Mole

The mole is now defined by fixing the Avogadro constant Nₐ at exactly 6.022 140 76 × 10²³ mol⁻¹. One mole of any substance contains exactly this many elementary entities (atoms, molecules, ions, etc.).

Previously, the mole was defined as the amount of substance that contains as many entities as there are atoms in 12 grams of carbon-12. This tied the mole to both the kilogram (the drifting artifact) and to the mass of a specific isotope. The new definition is purely a counting number, independent of any physical object.

The Kibble Balance: Weighing with Electricity

The practical realization of the new kilogram definition depends on the Kibble balance (formerly called the watt balance), invented by Bryan Kibble at the UK's National Physical Laboratory in 1975.

The Kibble balance compares mechanical power to electrical power with extraordinary precision. In the weighing phase, the gravitational force on a test mass is balanced by the electromagnetic force on a coil carrying a precisely measured current in a known magnetic field. In the velocity phase, the same coil moves through the field at a controlled speed, generating a measurable voltage. By combining these measurements, the mass can be expressed in terms of the Planck constant, the meter, and the second — all now defined independently.

Current Kibble balances achieve measurement uncertainties of approximately 1 × 10⁻⁸ (10 parts per billion) for mass measurements. The National Institute of Standards and Technology (NIST), PTB (Germany), NRC (Canada), and BIPM each operate independent Kibble balances, and their results agree within measurement uncertainty.

What Didn't Change (Meter, Second, Candela)

Three base units retained their pre-2019 physical-constant definitions:

• The meter has been defined since 1983 as the distance light travels in vacuum in exactly 1/299 792 458 of a second. The speed of light in vacuum c is fixed at exactly 299 792 458 m/s.
• The second has been defined since 1967 as the duration of 9 192 631 770 periods of the radiation corresponding to the transition between two hyperfine levels of the ground state of the caesium-133 atom.
• The candela is defined by fixing the luminous efficacy of a specific frequency of monochromatic radiation (540 × 10¹² Hz, approximately 555 nm green light) at exactly 683 lm/W. This frequency was chosen because the human eye is most sensitive to it.

These three definitions already satisfied the criterion of being tied to invariant physical constants and required no revision.

Impact on Science and Industry

For everyday measurement — weighing produce, calibrating bathroom scales, testing electrical circuits — the 2019 redefinition changed nothing detectable. The fixed values of the constants were chosen precisely to match the best measurements of the previous artifact-based definitions, ensuring continuity.

The impact is felt at the top of the measurement hierarchy: national metrology institutes can now realize the kilogram and ampere independently, without reference to any physical artifact held in France. Countries no longer need to travel to Sèvres for calibration. Any sufficiently equipped national laboratory can realize SI units from first principles.

For quantum computing and precision physics research, the fixed Planck constant and elementary charge eliminate a layer of uncertainty that previously propagated through calculations. The new definitions are also future-proof: as measurement techniques improve, realizations of the units will become more precise without requiring any change to the definitions themselves.

Timeline: Major SI Milestones

The 2019 redefinition represents a philosophical shift: measurement is no longer anchored to the physical world through objects, but through the mathematical structure of quantum mechanics and electrodynamics. The universe itself is now the standard. For unit conversions involving weight and temperature, every calculation ultimately traces back to these seven fixed constants.