Why shedding a kilo is good for science

The world has shed a kilo this Monday. In an historic moment for science, Le Grand K – the lump of metal that's been the standard unit for calibrating scales over the past 130 years – is being scrapped. In its place, a mass of photons will provide the new international definition  of the almighty kilogram.

Also known by its less sexy name IPK (the International Prototype of the Kilogram), Le Grand K is a cylinder of platinum-iridium alloy that's been locked away in a vault near Paris since 1889. It has many copies around the world that have ensured everyone followed one system of measurement.

Of course Le Grand K's retirement doesn't mean it will become scrap metal. It will just become irrelevant.

The change, taking place on World Metrology Day, won't be felt by ordinary folk: it won't affect the way fruit and veggies are weighed at the supermarket, or the number you see at home on your bathroom scales. It is, however, a watershed moment for scientists.

Throughout its lifetime, Le Grand K is estimated to have lost about 50 micrograms – because every time it's handled it sheds atoms and its mass slightly changes. Frustrated by K's imprecision, physicists have long been in the market for a more reliable alternative.

From Monday 20 May, the shiny new kilogram will be perfect, as it will be fixed to the Planck constant – a fundamental concept in quantum mechanics that can never change. Just like the speed of light.

For decades scientists have been using a machine called the Kibble balance to precisely measure the Planck constant to a close enough degree that it can be used to redefine the kilogram.

They voted to make the change at the General Conference on Weights and Measures, held in Versailles in 2018. In doing so, the scientists were honouring the very philosophy of a metric system that was designed to be “for all times, for all people”.

Today it's known as the International System of Units, with metres and kilograms used as the fundamental units of length and mass. For example, 1 US pound is defined as 0.45359237 kilograms.

So even if you live in a country that uses imperial units – such as the US and Britain – your measurements are still derived from the metric system.