The 2019 Redefinition of the SI Base Units:

A New Foundation for Measurement

Article By Industries Needs

Introduction

On 20 May 2019, World Metrology Day, the International System of Units — the SI — underwent the most fundamental transformation in its history. For the first time since the metric system was born out of the French Revolution, every one of the seven SI base units became defined in terms of fixed numerical values of fundamental constants of nature, rather than in terms of physical objects, laboratory prototypes, or specific material properties. The kilogram, which for 130 years had been defined by a single platinum-iridium cylinder locked in a vault outside Paris, was set free from that artifact forever. In its place stood Planck's constant, an unchanging number woven into the fabric of quantum mechanics.

This was not a cosmetic change. It represented the final step in a two-century-long philosophical journey: the shift from measurement systems anchored to physical objects — which can be lost, damaged, or subtly change over time — toward a system anchored to the invariant constants of the universe itself. This article examines why the redefinition was necessary, what exactly changed, how scientists achieved the precision required to make it possible, and what it means for laboratories, industry, and science going forward.

The Old System: Measurement by Artifact

To understand why the 2019 redefinition mattered, it helps to recall what came before it. Prior to 2019, four of the seven SI base units depended, directly or indirectly, on physical artifacts or specific experimental conditions:

- **The kilogram** was defined as the mass of the International Prototype of the Kilogram (IPK), a cylinder of platinum-iridium alloy manufactured in 1889 and stored at the International Bureau of Weights and Measures (BIPM) in Sèvres, France.
- **The ampere** was defined via a hypothetical experiment involving two infinitely long, infinitely thin parallel wires carrying current, one meter apart, producing a specific force per unit length — a definition elegant on paper but essentially impossible to realize with high accuracy in practice.
- **The kelvin** was defined as a fraction of the thermodynamic temperature of the triple point of water, a specific, reproducible physical condition, but one that depends on the isotopic composition of the water sample used.
- **The mole** was defined in terms of the number of atoms in exactly 0.012 kilograms of carbon-12 — again tying a base unit to a specific physical sample.

The problem with the kilogram illustrates the broader issue best. The IPK and its six official copies were compared periodically, and over decades, scientists observed that their masses drifted relative to one another by tens of micrograms — likely due to surface contamination, cleaning procedures, or material outgassing. Since the IPK was, by definition, exactly one kilogram, any drift meant that the kilogram itself was silently changing while the world's most precise scientific and industrial measurements depended on it staying constant. A mass standard that could drift undermined confidence in mass-dependent quantities throughout the SI, including electrical units linked to mass via classical definitions.

Why Redefinition Became Necessary

Three converging forces drove the push toward the 2019 redefinition:

**First, artifact instability.** As noted above, the IPK's mass appeared to be diverging from its official copies. No one could say with certainty which one — if any — retained its original 1889 mass, since there was no external, artifact-independent reference to check against.

**Second, advances in quantum metrology.** By the early 2000s, two independent experimental techniques — the Kibble balance (formerly called the watt balance) and the X-ray crystal density (XRCD) method using silicon spheres — had become precise enough to measure the Planck constant, h, to a relative uncertainty of a few parts in 10^8. This meant scientists could, for the first time, define the kilogram in terms of h with adequate precision to replace the physical prototype without any loss of accuracy for practical users.

**Third, universality and accessibility.** A definition based on a fundamental constant can, in principle, be realized anywhere in the world with the right equipment, rather than requiring comparison against a single object in France. This democratizes access to the highest levels of measurement accuracy and removes single points of failure from the international measurement infrastructure.

The Seven Defining Constants

The 2019 redefinition fixed exact numerical values for seven constants, and every SI base unit is now derived from this set:

1. **The caesium hyperfine transition frequency**, ΔνCs = 9,192,631,770 Hz — defines the **second**
2. **The speed of light in vacuum**, c = 299,792,458 m/s — defines the **meter** (this had already been fixed since 1983)
3. **The Planck constant**, h = 6.62607015 × 10⁻³⁴ J·s — defines the **kilogram**
4. **The elementary charge**, e = 1.602176634 × 10⁻¹⁹ C — defines the **ampere**
5. **The Boltzmann constant**, k = 1.380649 × 10⁻²³ J/K — defines the **kelvin**
6. **The Avogadro constant**, NA = 6.02214076 × 10²³ mol⁻¹ — defines the **mole**
7. **The luminous efficacy of monochromatic radiation of frequency 540 × 10¹² Hz**, Kcd = 683 lm/W — defines the **candela**

Because these seven values are now fixed by definition and carry no uncertainty, every base unit becomes exactly reproducible anywhere the appropriate experiment can be performed, without reference to any single physical object.

Unit by Unit: What Actually Changed

**The kilogram.** This was the headline change. Instead of equaling the mass of the IPK, the kilogram is now defined by fixing the Planck constant. Realizing the kilogram in practice requires a Kibble balance, which balances the weight of a test mass against an electromagnetic force generated by a current-carrying coil in a magnetic field, linking mechanical power to electrical power measured in terms of the Josephson and quantum Hall effects — both of which are themselves governed by h and e. The National Institute of Standards and Technology (NIST) in the United States and Canada's National Research Council were among the labs whose Kibble balance measurements underpinned the final fixed value of h.

**The ampere.** Previously defined through an idealized force-between-wires experiment that was never practically realizable at high precision, the ampere is now defined by fixing the elementary charge, e. In practice, this is realized through single-electron tunneling devices or, more commonly, derived from the fixed values of h and e via the Josephson and quantum Hall effects, giving direct traceability to electrical standards without needing a mechanical current balance at all.

**The kelvin.** Rather than being tied to the triple point of water — a physical realization dependent on isotopic purity — the kelvin is now defined by fixing the Boltzmann constant, k. This allows the kelvin to be realized through several independent primary thermometry techniques, such as acoustic gas thermometry and Johnson noise thermometry, which can, in principle, be performed at any temperature range rather than being anchored to a single fixed point near 0.01°C.

**The mole.** Formerly tied to the mass of carbon-12, the mole is now defined by fixing the Avogadro constant, NA, as an exact number of elementary entities. This decouples the mole conceptually from the kilogram — previously, a change in the mass standard would ripple into the definition of the mole, which chemists found awkward. Now the mole simply means a fixed count of 6.02214076 × 10²³ entities, full stop.

**The second, meter, and candela.** These three units were already defined in terms of fundamental constants before 2019 (the caesium transition frequency since 1967, the speed of light since 1983, and luminous efficacy since 1979), so their definitions were reworded for consistency with the new constants-based framework but did not change in practical value.

The Kibble Balance: The Instrument That Made It Possible

Central to the redefinition's success was the Kibble balance, invented by British physicist Bryan Kibble in the 1970s. The device operates in two modes. In "weighing mode," it balances the gravitational force on a test mass against an electromagnetic force from a coil carrying current in a magnetic field. In "velocity mode," the same coil is moved through the magnetic field at a known velocity, and the induced voltage is measured. By combining measurements from both modes, the mechanical quantity of mass can be related directly to electrical quantities measured using the Josephson effect (relating voltage to frequency via h and e) and the quantum Hall effect (relating resistance to h and e). This chain of reasoning allows mass to be measured in terms of Planck's constant with extraordinary precision, achieving relative uncertainties below 2 parts in 10^8 — precise enough to serve as the practical realization of the redefined kilogram.

Several national metrology institutes around the world, including NIST, Germany's Physikalisch-Technische Bundesanstalt (PTB), and France's Laboratoire national de métrologie et d'essais (LNE), operate Kibble balances or equivalent silicon-sphere experiments to realize the kilogram independently, providing cross-checks that a single artifact never could.

Implications for Science, Industry, and Metrology

For most people, and even for most working scientists and engineers, the 2019 redefinition changed nothing in day-to-day practice. A kilogram of flour still weighs the same, a car's speedometer reads the same, and a laboratory balance calibrated last year remains valid. The redefinition was engineered so that continuity of value was preserved to within the existing measurement uncertainty of the old definitions — no laboratory would suddenly find its calibrated instruments reading differently by any detectable margin.

Where the redefinition matters most is at the frontiers of precision metrology: national measurement institutes, semiconductor fabrication, pharmaceutical dosing standards, precision electronics, and fundamental physics research. These fields benefit because:

- **Traceability no longer requires physical transport of a reference object** to a national lab for comparison against a master standard; any suitably equipped lab can, in principle, realize the unit independently.
- **Stability over time is guaranteed by physics rather than by careful storage**, since fundamental constants do not drift, corrode, or accumulate surface contamination the way a metal cylinder can.
- **Future-proofing** is built in: as measurement techniques improve, the accuracy with which the defining constants can be realized will only get better, without ever requiring another redefinition of the underlying framework.

Conclusion

The 2019 redefinition of the SI base units closed a chapter that began in 1889 with a single cylinder of metal in a vault near Paris and opened a new one grounded entirely in the invariant properties of nature. By fixing exact numerical values for the Planck constant, the elementary charge, the Boltzmann constant, the Avogadro constant, the caesium hyperfine frequency, the speed of light, and luminous efficacy, metrologists achieved something no earlier generation could: a system of measurement units that no longer depends on any physical object, anywhere, ever wearing out, drifting, or being lost. It stands as one of the most significant — and least visible — achievements in the history of modern science, a quiet revolution that touches every measurement made anywhere in the world, from a pharmacist weighing out a dose to a physicist probing the mass of a subatomic particle.