The Last Object
In a basement vault in Sèvres, a suburb of Paris, there sits a small cylinder of metal about the size of a golf ball. It is made of an alloy of platinum and iridium — ninety percent platinum, ten percent iridium — chosen for its exceptional hardness, its resistance to oxidation, and its extreme density. It weighs approximately one kilogram. The word approximately is doing a great deal of work in that sentence, because from 1889 until 2019, this object did not weigh approximately one kilogram. It weighed exactly one kilogram, by definition, always, regardless of what was happening to it physically. It was the kilogram. The entire global system of mass measurement — every scale, every balance, every instrument in every hospital and laboratory and factory on Earth — was ultimately calibrated against this one cylinder.
It is called the International Prototype of the Kilogram. Scientists gave it a more intimate nickname: Le Grand K.
And it was slowly, mysteriously changing.
This is the story of the most philosophically peculiar object in the history of measurement, the crisis its behaviour created, the half-century scientific race to replace it, and the 2019 decision that ended with humanity anchoring the kilogram not to any object that can be touched or lost or contaminated, but to a number written into the fabric of the universe itself. It is a story about what it means to define something, about the strange position of an object that is simultaneously a physical thing and an abstract standard, and about the remarkable lengths to which scientists will go to make sure that a kilogram today means exactly the same thing as a kilogram tomorrow.
How the Kilogram Got Its Object
The kilogram began not as a cylinder but as a volume of water. In 1795, during the early years of the French metric system, the kilogram was defined as the mass of one cubic decimetre of pure water at the temperature of melting ice — approximately four degrees Celsius, the temperature at which water reaches its maximum density. This was an elegant and appealingly universal definition: water is everywhere, its properties are consistent, and the relationship to the litre (also a cubic decimetre) gave the system a neat internal coherence.
The problem was practical. Measuring mass by filling a container with water and weighing it introduced too many sources of error: the exact temperature of the water, the purity of the water, the precision of the container's volume. For everyday commerce and science, you needed something you could simply put on a balance and compare directly — not a procedure but an object.
In 1799 a platinum cylinder was cast to embody the kilogram definition, and this became the first physical prototype. It was called the Kilogramme des Archives, and it lived up to its name by residing in the French national archives. For most of the 19th century it served well enough, but as international trade and scientific cooperation expanded, the need for a truly global standard became pressing. What did the kilogram mean in Germany, in Britain, in the United States? Each country had its own reference weights calibrated, with varying degrees of precision, against the French prototype. The chain of calibration was long and each link introduced its own uncertainty.
The solution, agreed upon at the first General Conference on Weights and Measures in 1889, was to create a new artefact — more durable, more precisely made — and distribute official copies to every member nation. The new prototype was made of platinum-iridium, chosen because platinum alone is too soft and gradually deforms under handling, while the ten percent iridium addition creates an alloy of exceptional hardness without significantly affecting density. Forty identical copies were manufactured and carefully compared against the original. The original was kept in Paris. The copies went to national metrology laboratories around the world, where they became the kilogram for each country, connected to the Parisian original through a chain of careful comparison measurements.
The ceremony of distribution in 1889 was an act of measurement theatre: the prototype and its copies weighed and compared before witnesses from nations across the world, the numbers recorded in official reports, the copies packed and transported to their new homes in London, Washington, Tokyo, Moscow. From that moment, the mass of every object on Earth that was measured in kilograms was ultimately traceable to the vault in Sèvres.
The Vault and What It Contained
The International Bureau of Weights and Measures — the BIPM, from its French initials — occupies a 4.3-hectare site in Sèvres that was designated international territory by the Metre Convention of 1875, placing it outside the jurisdiction of any single country. The prototype kilogram was stored there under conditions of extraordinary care: enclosed within three nested glass bell jars in a vault whose access required the simultaneous presence of three keyholders, one from the BIPM and two from the French government, as a safeguard against any single person having unchecked access.
The vault was opened only for what metrologists called official comparisons — periodic ceremonies, occurring roughly once every forty years, in which the prototype was removed from its bell jars, cleaned with a chamois leather and steam according to a precise protocol, and weighed against its official copies. These comparisons were some of the most careful weighing operations ever performed. The balances used were enclosed in temperature-controlled chambers to prevent thermal expansion. The air pressure and humidity were measured and corrected for, because buoyancy in air affects a mass measurement at the level of precision required. The weighings were repeated dozens of times and the results averaged. Every precaution that human ingenuity could devise was taken to ensure that what was being measured was mass, not measurement artefact.
The results of these comparisons, over the decades, were deeply unsettling.
The Mystery of the Diverging Copies
The first official comparison after 1889 took place in 1939. The second in 1946, delayed by the Second World War. The third, most comprehensive comparison took place in 1988 and 1989, involving the original prototype, all surviving official copies, and the national prototypes of forty-six member states. The results were analysed carefully and published in 1994. They showed something that nobody could explain and nobody could explain away: the original and its copies were no longer in agreement.
The official copies, on average, appeared to have gained about 50 micrograms relative to the original since 1889. Or the original had lost 50 micrograms. Or some combination of both. The data could not distinguish between these possibilities, because there was no external reference to check either against. This is the philosophical abyss at the heart of a self-referential standard: if the definition is the object, and the object changes, then the definition has changed, and there is no vantage point outside the definition from which to observe the change. The kilogram was, by definition, whatever Le Grand K weighed at any given moment. If Le Grand K had been slowly shedding atoms since 1889, then the kilogram had been slowly shrinking along with it, and every mass measurement made over that century had been slightly wrong in a way that no measurement could detect.
Fifty micrograms sounds like nothing. In everyday terms it is: the mass of a small grain of sand, a single eyelash, a few flakes of dead skin. But in the domain of precision measurement — in pharmaceutical manufacturing where drug doses are measured in micrograms, in semiconductor fabrication where material deposition is measured in nanograms, in fundamental physics experiments where small mass differences determine the outcome of crucial tests — 50 micrograms matters. And more troubling than the number itself was the uncertainty: was the drift continuing? Was it accelerating? Nobody knew, and nobody could know, because the instrument of measurement was also the thing being measured.
The cause of the divergence remained, and still remains, unknown. Hypotheses proliferated. The platinum-iridium surface might be slowly absorbing atmospheric contaminants — mercury vapour, hydrocarbons, water — that added mass over time, which would explain why cleaning sometimes changed the measured mass slightly. Conversely, it might be very slowly losing atoms from its surface through processes not fully understood. The different histories of the original and the copies — different environments, different handling procedures, different numbers of cleanings — might have produced different rates of drift. All of these explanations are plausible. None has been conclusively confirmed.
What the 1994 data established beyond argument was that the physical prototype was not a stable anchor. Something was happening to it, and the system of mass measurement built around it was consequently compromised in ways that could not be quantified.
The Race to Replace a Definition
The response from the international metrology community was, eventually, unanimous: Le Grand K had to go. The kilogram had to be redefined in terms of something that would not change — something fundamental, universal, immune to contamination and handling and the slow passage of time. But what?
The answer, when it came, required connecting the kilogram to quantum physics — specifically to a number called the Planck constant, one of the most fundamental quantities in all of science.
Max Planck had introduced his constant in 1900 while trying to explain a puzzling phenomenon in the physics of radiation. The constant, denoted h, describes the relationship between the energy of a photon and its frequency: E = hf. It is an extraordinarily small number — 6.62607015 × 10⁻³⁴ joule-seconds — but it is precisely the same everywhere in the universe, at all times, under all conditions. The Planck constant is not a property of any particular object or substance. It is, as far as we can determine, a fixed feature of reality itself.
The connection between the Planck constant and mass requires a device called a Kibble balance, named after British physicist Bryan Kibble who invented it at the National Physical Laboratory in 1975. The Kibble balance works by equating two forces: the mechanical force of gravity acting on a test mass, and the electromagnetic force generated by passing a current through a coil of wire in a magnetic field. By measuring the electrical quantities precisely — voltage and current, using quantum electrical standards that are themselves anchored to fundamental constants — and balancing them against the gravitational force on a known weight, the balance allows an extraordinarily precise determination of the ratio between mass and the Planck constant.
This is not conceptually simple, and the practical difficulties were immense. The electrical measurements require standards of voltage and current accurate to parts per billion. The magnetic field must be uniform and precisely characterised. The local value of gravitational acceleration must be measured at the exact location of the balance to many decimal places. The balance must be operated in vacuum to eliminate air buoyancy. Each national metrology laboratory that built a Kibble balance had to independently solve these problems and then compare their results with every other laboratory — because the whole point was to replace an arbitrary physical object with a universally reproducible measurement, and universally reproducible means the same result in Paris and Washington and Tokyo and Beijing.
The measurements took decades. The Kibble balance programme began in earnest in the 1990s. By the 2010s, multiple laboratories around the world had achieved sufficient precision that the values of the Planck constant they derived agreed with each other to within a few parts in 10⁸ — a few ten-thousandths of a percent. This was good enough.
The Vote, and What It Meant
On November 16, 2018, delegates from sixty member states of the General Conference on Weights and Measures gathered in Versailles — a symbolically apt location, minutes from the Sèvres vault — and voted unanimously to redefine the kilogram. The resolution was straightforward in its language: the kilogram would henceforth be defined by fixing the numerical value of the Planck constant to be exactly 6.62607015 × 10⁻³⁴ joule-seconds.
This sentence is worth sitting with for a moment. The Planck constant was not being measured. It was being declared. Just as the metre had been redefined in 1983 by fixing the speed of light to exactly 299,792,458 metres per second — thereby making the speed of light exact by fiat and defining the metre as a consequence — the kilogram was now being defined by making the Planck constant exact and letting the kilogram follow from the arithmetic. Mass became, in the most literal sense, a quantum mechanical quantity.
The new definition came into force on May 20, 2019, World Metrology Day. From that moment, Le Grand K ceased to be the kilogram. It became, instead, a historical object — an extraordinary piece of precision metalwork with a fascinating history, still stored in its vault under its three bell jars, but no longer the thing that mass is measured against. The vault now contains an artefact, not a definition.
The practical consequence, for most people and most scales, was precisely nothing. The new definition was calibrated to preserve continuity: the kilogram of May 20, 2019 was the same mass as the kilogram of May 19, 2019. Nobody's groceries changed weight. Nobody's drug doses needed adjusting. The change was philosophical and technical, not practical — but philosophical and technical changes in the foundations of measurement have a way of mattering enormously over time, even when their immediate effects are imperceptible.
What Was Gained and What Was Lost
The practical gains of the redefinition are significant and will compound over time. The new kilogram is, for the first time, truly universal: any laboratory anywhere in the world that can build a sufficiently precise Kibble balance can realise the kilogram independently, without reference to Paris, without needing access to any artefact, without contributing to a chain of calibration that ultimately bottomed out in one metal cylinder in one basement vault. Mass measurement has been democratised in a way that the prototype system could never achieve.
The new definition also eliminates drift. Le Grand K could change; the Planck constant cannot. The value fixed in 2018 will be the value used in a million years, or until some future physics revolution requires a more fundamental reformulation of the entire SI system. The kilogram will not shrink or grow as the years pass. The definition is, for the first time in the kilogram's history, truly stable.
There is also something subtler gained: honesty. The prototype system required scientists to pretend that a physical object was a definition — to treat the mass of Le Grand K as exactly one kilogram even when they could observe that it was changing relative to its copies. This pretence was necessary but uncomfortable, and the discomfort grew more acute as measurement technology became sensitive enough to detect the divergence. The Planck constant definition is not a pretence. It is a genuine physical fact, chosen because it is genuinely invariant, and it allows the kilogram to be what a unit of measurement ought to be: a fixed and reliable standard against which physical reality can be assessed, rather than a physical reality that is itself the standard.
What was lost is harder to name. There is something philosophically satisfying about a kilogram that you can hold — a thing of definite substance, visible and tangible, connecting the abstract concept of mass to the physical world in a direct and immediate way. The platinum-iridium cylinder had a presence that the Planck constant does not. You could photograph Le Grand K. You could, if you were authorised, touch it. The new kilogram exists only as a mathematical relationship between quantum constants and SI base units; it has no object, no location, no presence. In this sense the 2019 redefinition completed a process that began when the metre was untethered from the Earth's circumference: the units of measurement have become, one by one, fully abstract.
The Object That Remains
Le Grand K is still in Sèvres. It will stay there, almost certainly, for as long as the BIPM exists. The cylinder has too much history and too much symbolic weight to discard, even now that it no longer carries scientific weight. It sits in its vault, under its three bell jars, a former definition that still looks exactly like a kilogram because it was made to be exactly a kilogram and has changed by only fifty micrograms in 135 years.
Scientists still weigh it occasionally, now that they have an independent reference to compare it against. The comparison is interesting. Le Grand K and the current Kibble balance realisation of the kilogram agree to within a few tens of micrograms — consistent with the hypothesis that the cylinder drifted by about 50 micrograms over its lifetime, in line with what the 1994 comparisons suggested. The mystery of why it drifted has not been solved. It may never be solved, because solving it would require access to Le Grand K's full history in a level of detail that was never recorded.
What the comparison does confirm is that the old system was compromised in exactly the way that the metrologists suspected. The kilogram that the world used from 1889 to 2019 was not perfectly stable. It was good — remarkably good, for a physical artefact maintained under such careful conditions — but it was not perfect, and perfection is what a fundamental standard of measurement requires.
The platinum-iridium cylinder that sat at the centre of global mass measurement for 130 years has retired. The standard it once embodied now lives in an equation, in the quantum behaviour of atoms, in a number that is the same at the surface of a neutron star and in the basement of a Swiss laboratory and in the empty space between galaxies. Mass is now measured against the universe. The last object has been replaced by the last abstraction.
The Wider Story: All Seven SI Units Are Now Fundamental Constants
The kilogram's 2019 redefinition completed a decades-long project to anchor every base unit of the SI system to fundamental physical constants rather than to physical artefacts or arbitrary phenomena. Understanding this context reveals just how radical the transformation has been.
The second was the first to make the leap, redefined in 1967 in terms of the caesium atom's hyperfine transition frequency — as described in our post on the history of time. The metre followed in 1983, defined by fixing the speed of light. The ampere (electric current), the kelvin (temperature), the mole (amount of substance), and the candela (luminous intensity) were all redefined alongside the kilogram in 2019, each anchored to a fundamental constant: the elementary charge, the Boltzmann constant, the Avogadro constant, and the luminous efficacy of a specific radiation respectively.
The result is a measurement system in which every base unit is defined by something that does not change. Not an object, not a material, not a terrestrial phenomenon — a constant of nature. If civilisation were destroyed and rebuilt from scratch, a sufficiently advanced future science could reconstruct every SI unit exactly, without any surviving artefact or human record, simply by measuring the fundamental constants of physics and applying the definitions. The system is, for the first time, truly universal and truly permanent.
The kilogram was the last holdout — the last unit still tied to a human-made object — which is why its 2019 redefinition felt like a threshold crossed. The SI system that existed from 2019 onwards is qualitatively different from the one that preceded it: it is no longer a system anchored partly to the physical world and partly to abstraction. It is now fully abstract, fully universal, and — as far as human understanding of physics allows — fully stable.
The vault in Sèvres still holds its contents. But the definition has moved somewhere else entirely.