How the World's Metrologists Redefined the Kilogram and the SI Base Units Using Fundamental Constants

On November 16, 2018, inside a conference hall in Versailles, France, delegates from 60 nations voted unanimously to sever the world's measurement system from physical objects. When the electronic voting system tallied the final 'yes,' the International Prototype of the Kilogram—a platinum-iridium cylinder forged in 1889 and kept under three nested glass bell jars in a vault outside Paris—was instantly rendered obsolete as the ultimate arbiter of mass.[2][4]

The problem with the artifact, known affectionately as Le Grand K, was that it was slowly changing. Over a century of microscopic contamination and cleaning, its mass had drifted by roughly 50 micrograms compared to its official copies around the world. In a global economy where pharmaceutical dosages and quantum manufacturing rely on extreme precision, a foundational unit that fluctuates by the weight of an eyelash is a critical vulnerability.[2][6]

The marketing language surrounding the "new SI" promised a revolution, but the immediate practical reality was entirely invisible to the public. When the changes officially took effect on May 20, 2019, a kilogram of apples still weighed exactly the same, and commercial scales required no recalibration. The shift was entirely under the hood, moving the anchor of the metric system from a piece of metal to the immutable laws of quantum mechanics.[1][3]

Instead of a physical cylinder, the kilogram is now defined by the Planck constant, denoted as h. The General Conference on Weights and Measures (CGPM) fixed the exact value of the Planck constant at 6.62607015 × 10⁻³⁴ joule-seconds. Because a joule can be broken down into kilograms, meters, and seconds, fixing this constant allowed metrologists to define mass purely through mathematical relationships.[1][6]

To understand how a quantum mechanical constant defines a bag of flour, one must look at the Kibble balance. This multi-million-dollar instrument balances the downward gravitational pull on a physical mass against an upward electromagnetic force. By measuring the electrical current and voltage required to suspend the mass perfectly, scientists can calculate the exact weight using the fixed Planck constant.[1][5]

This electromagnetic force relies on two distinct quantum phenomena: the Josephson effect, which relates to voltage, and the quantum Hall effect, which relates to electrical resistance. Because both of these effects incorporate the Planck constant and the elementary charge, researchers can derive mass without ever needing to reference a physical standard.[5][6]

The kilogram was the last artifact to fall, but it was not the first unit to be abstracted into the cosmos. The meter underwent a similar transformation in 1983. Prior to that, it was defined by a physical metal bar, and later by the wavelength of krypton-86 radiation. Today, the meter is defined strictly by the speed of light in a vacuum, which is fixed at exactly 299,792,458 meters per second.[1][6]

The second, which is the most precisely realized of all the base units, was redefined in 1967. It is based on the behavior of the cesium-133 atom. Specifically, one second is defined as exactly 9,192,631,770 periods of the radiation corresponding to the transition between two hyperfine levels of the unperturbed ground state of the cesium-133 atom.[1][3]

The 2018 vote also overhauled four other units: the ampere, the kelvin, the mole, and the candela. The ampere, the unit of electrical current, was previously defined by an impossible theoretical experiment involving two infinite parallel wires. It is now based on the elementary charge of an electron, fixed at 1.602176634 × 10⁻¹⁹ coulombs.[1][4]

The kelvin, the base unit of temperature, was decoupled from the triple point of water—a specific physical state that was difficult to replicate perfectly across different laboratories. It is now tied to the Boltzmann constant, fixed at 1.380649 × 10⁻²³ joules per kelvin, allowing temperature to be measured by the kinetic energy of particles.[1][6]

The mole, used by chemists to measure the amount of a substance, was previously defined as the number of atoms in 12 grams of carbon-12. This meant the mole was dependent on the mass of the kilogram. It is now a fixed number, the Avogadro constant, exactly 6.02214076 × 10²³ elementary entities, severing its reliance on mass entirely.[1][6]

The candela, measuring luminous intensity, remains tied to the luminous efficacy of monochromatic radiation of frequency 540 × 10¹² hertz, fixed at 683 lumens per watt. Together, these seven constants form a closed, interlocking mathematical framework that describes the physical universe.[1][3]

The result is a system where anyone with sufficient technology can realize the base units anywhere in the universe. "This is the most important decision that the CGPM has made in perhaps 100 years," noted Martin Milton, director of the International Bureau of Weights and Measures (BIPM), during the historic proceedings.[2]

Yet, as metrologists readily admit, this creates a pedagogical challenge. Teaching a child that a kilogram is the mass of a specific metal block in France is intuitive; teaching them that it is derived from the Planck constant via a Kibble balance requires a university-level understanding of quantum physics.[2][4]

The legacy of the Versailles vote is not a change in how much things weigh, but a permanent foundation for future scientific discovery. By anchoring the International System of Units to the fundamental constants of nature, the scientific community ensured that our measurements will never again drift away from the reality they are meant to describe.[2][4][6]