Years ago, I spent a university industrial placement at the UK’s National Physical Laboratory (NPL), the ultimate calibrator for measurements in the UK. In everyday life, it was traditionally responsible for keeping traders and shops honest when it came to selling goods by weight or length. It has similar roles when it came to time - indeed it was behind the development of the atomic clock, which is crucial to ensuring that the modern world - including the internet - works on time. In science and engineering, it makes sure that everyone is ‘on the same page’ when it comes to the units used in fundamental research.
Today, one of the NPL’s key tasks is to maintain the International System of Units (SI Units) in the UK as defined by the Bureau International des Poids et Measures (BIPM) in Paris, France. There are seven SI Base Units: kilogram (mass), second (time), metre (length), ampere (electric current), Kelvin (temperature), mole (amount of substance) and candela (luminosity).
Each SI Base Unit had previously been defined before the SI system was introduced in 1960. However, they were often defined differently by different users. This meant that a ‘second,’ ‘gram,’ or ‘centimetre’ - as defined by one discipline - may have been slightly different from a ‘second,’ ‘gram’ or ‘centimetre’ defined by another, making coordination very difficult. In an increasingly connected world, it was clear that such disjointed systems would soon be untenable.
Indeed, each of the units has had multiple definitions over the years. Ask a child what a second is and they will tell you that it is a 60th of a minute, which is a 60th of an hour, which is a 24th of a day - the time it takes for the Earth to rotate on its own axis. This was once the formal definition - and it still works well for everyday use. However, the Earth does not rotate as steadily as we think - and it is also slowing with time. In 2019, this led BIPM to redefine the second as the duration of 9,192,631,770 changes between two hyperfine energy states of the cesium-133 atom.
Similarly, the metre began life as one ten millionth of the distance betweeen the North Pole and the Equator. This was later replaced by a metal bar of ‘exactly’ one metre. But what if the metre got too hot? It would expand marginally, of course, changing the length of all other ‘metres.’ The solution - in 1983 - was to redefine the metre as the distance travelled by light in a vacuum during 1/c seconds, where c is the speed of light in a vacuum (approximately 299,792,458m/s). In 2019, the metre was again redefined as... *checks notes*... the distance travelled by light in a vacuum during 9,192,631,770/c fluctuations of the cesium-133 atom. I hope that’s clear!
Then we have the kilogram, perhaps the most pertinent SI Base Unit for the global cement sector. The International Prototype of the Kilogram (IPK), affectionately known by its French moniker ‘Le Grand K’ still sits in an air-conditioned, temperature-controlled room at BIPM under three air-tight containers. Two separate keys are required to enter the vault.
Between its casting in London in 1879, and 2019, 1kg was defined as the weight of whatever the IPK weighed. It was said that, if BIPM burned to the ground, there would be metrological chaos! Even without a catastrophe, weight changes were inevitable, even with the best of care. Comparisons with the IPKs’ six sister copies (all at BIPM) and several more international copies - one of which I had the honour of visiting during my time at NPL - found that the IPK had lost 0.0000005g over the years.
So... since 2019, the kilogram has been defined using a Kibble balance, which compares the gravitational force experienced by an object with an electromagnetic force. The upshot is that the kilogram, like the second and the metre, is now defined using physical constants that will not change with time.
The ampere, Kelvin, mole and candela have all been redefined in similar ways, such that there are no longer any SI Units based on physical objects. For now, this means absolute uniformity among measurements and no way for SI Base Units - and the many others derived from them - to drift with time, which is great for scientists.
And in the future... who knows? A future civilisation, operating on a fundamentally different Earth - or even on a different planet entirely - will be able to recreate our units from the ground up and understand the ‘scientific language’ of our era. This is the power of the
SI system!
