Currently, the mass of the kilogram is decided by the mass of le grand K, a 129-year-old platinum–iridium cylinder that sits under several nested bell jars at the International Bureau of Weights and Measures in France: if it changes in any way, the unit changes with it.

Comparing le grand K’s mass to that of its 67 copies has revealed that the copies’ masses have drifted apart over the years, possibly because they have accumulated tiny amounts of air pollutants. This makes it likely that the kilogram artefact itself has also changed – but we’ll never know, since the kilogram will always weigh, by definition, exactly 1kg. The kilogram is therefore being redefined to stop this drift.

To make it easier to understand: When Big K changes, everything else has to adjust. Or even worse: If Big K were stolen, our world’s system of mass measurement would be thrown into chaos.

That is why metrologists decided to base the SI units’ new definitions on the most stable thing in the universe: natural constants. The kilogram will be described by the Planck constant, h (6.62×10−34m2 kg/s), a quantum mechanical quantity that, in a wider sense, relates mass to energy via E = mc2. Planck constant, a fundamental concept in quantum mechanics that can never, ever change — here on Earth or in the deep reaches of the universe.

Yes, the world will still weigh the same (6×1024kg), whether expressed in the old or new kilogram. Only what exactly we mean by 1kg or 1mol will change.

Michael de Podesta, researcher at the UK’s National Physical Laboratory explained it like this: ‘The redefinition is kind of like having work done on the foundation of your house: you spend a lot of money doing it but afterwards, everything looks exactly the same because everything that’s been done is underground.’

But with a better foundation, things are less likely to break down in the future.

Although the Le Grand K will lose its title as the one and only kilogram, it will remain an important standard. But metrologists never rest and are already working on the next overhaul: the redefinition of the second. They hope it will be ready by 2026.