How the SI System Was Redefined in 2019
By Global Instruments
Introduction
For well over a century, the world's measurements rested on a handful of physical objects locked away in vaults. A cylinder of platinum-iridium alloy in a suburb of Paris defined the kilogram. A specific
temperature of water defined the kelvin. These artifacts were the silent authorities behind every scale, thermometer, and laboratory instrument on Earth. On May 20, 2019, that era ended. The International System of Units (SI) underwent its most significant transformation since its creation, abandoning physical objects as standards in favor of fixed values of fundamental constants of nature. This shift — from artifacts to constants — was not merely a technical adjustment but a philosophical reorientation of how humanity anchors its understanding of the physical world.The Problem With Physical Standards
To appreciate why this redefinition mattered, it helps to understand the system it replaced. The SI, established in 1960, was built on seven base units: the metre (length), kilogram (mass), second (time), ampere (electric current), kelvin (temperature), mole (amount of substance), and candela (luminous intensity). Some of these were defined using natural phenomena from the start — the second, for instance, was tied to the vibration frequency of a caesium atom, and the metre was eventually redefined in 1983 in terms of the speed of light. But others remained anchored to physical objects or laboratory-specific conditions.
The most famous of these was the kilogram. Since 1889, the world's mass standard was "Le Grand K," a cylinder of platinum-iridium alloy stored at the International Bureau of Weights and Measures (BIPM) near Paris. Every kilogram on Earth, in principle, traced its legitimacy back to this single object. The trouble was that Le Grand K was not perfectly stable. Periodic comparisons with its official copies, distributed to nations around the world, revealed that its mass appeared to drift over time — by roughly 50 micrograms over a century, based on comparisons with sister copies. Nobody could say with certainty whether Le Grand K itself had lost mass, whether the copies had gained mass through contamination, or some combination of both. Because the kilogram was, by definition, whatever Le Grand K weighed, the object could never be "wrong" — but the fact that the referent itself could drift undermined confidence in the entire mass-measurement system.
The ampere, kelvin, and mole had their own vulnerabilities. The ampere's definition relied on an idealized and practically unrealizable experiment involving two infinitely long, infinitely thin wires. The kelvin was defined using the triple point of a particular isotopic composition of water, which was difficult to reproduce with total consistency across laboratories. The mole was defined relative to the mass of carbon-12, tying an ostensibly independent unit back to the kilogram's uncertainties. In short, four of the seven base units carried measurement fragility baked into their very foundations.
The Push for Change
Scientists and metrologists — the specialists who study measurement itself — had been arguing for decades that the SI needed to break free from physical artifacts entirely. The vision was elegant: if you define units in terms of unchanging constants of nature, such as the speed of light, the charge of an electron, or the energy associated with a photon of a given frequency, then the definitions themselves become permanent and universally accessible. Any sufficiently equipped laboratory anywhere in the universe could, in principle, reconstruct the units from scratch without needing to travel to Paris or borrow a physical reference.
This aspiration had already succeeded for the metre, redefined in 1983 as the distance light travels in a vacuum in 1/299,792,458 of a second, and for the second, defined since 1967 by the frequency of radiation emitted by a caesium-133 atom during a specific transition between energy states. The remaining challenge was mass, electric current, temperature, and amount of substance — and mass was the hardest problem of all, because unlike frequency or the speed of light, mass could not simply be counted or clocked. It needed a different kind of technological bridge.
Building the Bridge: The Kibble Balance
The breakthrough that made a constant-based kilogram possible was an instrument called the Kibble balance (originally the "watt balance"), invented by British physicist Bryan Kibble in the 1970s. The device works by balancing the gravitational force on a mass against an electromagnetic force generated by a current-carrying coil in a magnetic field. Crucially, the balance allows this electromagnetic force to be measured with extraordinary precision by linking it to two quantum-mechanical phenomena: the Josephson effect, which relates voltage to frequency, and the quantum Hall effect, which relates resistance to fundamental constants. Together, these effects let scientists express a mechanical force — and therefore a mass — in terms of the Planck constant, a fundamental quantity from quantum mechanics that relates a photon's energy to its frequency.
In practical terms, the Kibble balance allowed metrologists to measure the numerical value of the Planck constant with respect to the existing artifact-based kilogram to extremely high precision. Once that value was pinned down by international collaboration — with laboratories including the National Institute of Standards and Technology (NIST) in the United States and Germany's Physikalisch-Technische Bundesanstalt (PTB) refining their results over years — the logic could then be flipped. Instead of using the kilogram to measure the Planck constant, scientists could fix the Planck constant at an exact defined value and use it to define the kilogram.
An independent method, the X-ray crystal density (XRCD) method, corroborated these measurements by counting the number of atoms in a nearly perfect sphere of silicon-28, offering a second experimental pathway to pin down Avogadro's number and, through it, cross-check the Kibble balance results.
The Vote and the New Definitions
On November 16, 2018, delegates from 60 member states gathered in Versailles, France, for the 26th General Conference on Weights and Measures (CGPM), the international body responsible for maintaining the SI. There, they voted unanimously to adopt the revised definitions. The changes officially took effect on May 20, 2019 — World Metrology Day, chosen deliberately to commemorate the anniversary of the 1875 Metre Convention, the treaty that first established international cooperation on measurement standards.
Under the new system, four fundamental constants were assigned fixed, exact numerical values, and the base units were then derived from them:
The kilogram is now defined by fixing the Planck constant (h) at exactly 6.62607015 × 10⁻³⁴ joule-seconds. Because energy, frequency, and the speed of light (itself already fixed) are interrelated, this pins down mass through Einstein's mass-energy relationship combined with quantum theory.
The ampere is defined by fixing the elementary charge (e), the charge of a single proton, at exactly 1.602176634 × 10⁻¹⁹ coulombs. Electric current, defined as charge flow per unit time, follows directly.
The kelvin is defined by fixing the Boltzmann constant (k), which relates temperature to kinetic energy at the molecular level, at exactly 1.380649 × 10⁻²³ joules per kelvin.
The mole is defined by fixing the Avogadro constant (Nₐ), the number of elementary entities in one mole of a substance, at exactly 6.02214076 × 10²³ per mole — severing its former dependence on the mass of carbon-12.
Together with the pre-existing constant-based definitions of the second (via the caesium-133 transition frequency) and the metre (via the speed of light), all seven SI base units are now defined by seven fixed constants of nature rather than by physical artifacts or laboratory-specific reference conditions.
What Changed in Practice — and What Didn't
For the vast majority of people, and even for most scientists and engineers, the redefinition was invisible. A kilogram of rice still weighs the same as it did the day before. A cup of coffee at 90 degrees Celsius is still just as hot. The new definitions were deliberately calibrated so that they matched the old ones to within the limits of measurement uncertainty at the time of the transition — there was no sudden jump in value, only a change in the conceptual anchor underlying the value.
What did change was who could access the primary standard and how. Previously, if a national metrology institute needed to verify its kilogram reference against the ultimate authority, it needed a certified copy traceable to Le Grand K in Paris — a slow, resource-intensive, and access-limited process. Under the new system, any laboratory with a sufficiently precise Kibble balance or an XRCD silicon sphere setup can realize the kilogram independently, without physically referencing an object held by another nation. This democratizes precision measurement and removes a single point of failure from the global system.
The change also matters profoundly for scientific fields that depend on extreme precision — semiconductor manufacturing, pharmaceutical dosing, fundamental physics experiments, and international trade of high-value materials, where even microgram-level discrepancies can have significant consequences. A definition immune to physical decay, contamination, or accidental damage offers long-term stability that an heirloom object never could.
A New Philosophy of Measurement
Beyond its technical details, the 2019 redefinition represents something conceptually elegant: the SI system severed its last ties to physical, human-scale objects and became fully rooted in the invariant laws of physics. Le Grand K, after 130 years of service, was retired from active duty — not discarded, but relegated to historical significance, alongside its official copies, as artifacts of an earlier scientific age rather than active standards.
This transition mirrors a broader arc in the history of science: the gradual replacement of contingent, local, and physical reference points with universal, reproducible, and theory-grounded ones. Just as astronomy moved from Earth-centered to universal frames of reference, and biology moved from arbitrary taxonomies to genetic ones, metrology moved from "this specific object in this specific vault" to "these unchanging numbers, true anywhere in the cosmos." In an age of increasingly interconnected and precision-dependent technology — from GPS satellites to quantum computers — that shift from artifacts to constants may prove to be one of the quieter but more consequential scientific achievements of the early twenty-first century.
Conclusion
The 2019 redefinition of the SI units closed a chapter that began with the French Revolution's original push toward a rational, universal system of measurement, and it opened one grounded not in objects but in the unchanging fabric of physical law. The kilogram no longer depends on the fate of a cylinder in Paris; it depends on the Planck constant, a number as fixed as the laws of quantum mechanics themselves. For scientists, engineers, and anyone who depends on precise measurement — which is to say, nearly everyone — this quiet revolution offers something Le Grand K never could: a standard that cannot rust, cannot be scratched, and cannot drift with time.
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