Beyond Measure: The Hidden History of Measurement

Did you know that they redefined the kilogram? It happened just a few years ago, and I’ll reassure you now, it still weighs the same, but its definition – the way scientists describe and verify its mass – was changed in 2019.

Previously, the kilogram’s definition was based on a lump of metal: on the original kilogram, kept under lock and key in an underground vault near Paris. Whatever that lump of metal weighed, the kilogram weighed, no more and no less. But it’s now defined using a constant of nature: one of those intangible values that physicists have discovered written into the fabric of the universe, apparently unchanging throughout time and space. It still weighs a kilogram though.

It was this discovery of the redefinition of the kilogram that first got me interested in the topic of measurement. I’d been sent on assignment to France in my capacity as a journalist to write the story up. I travelled to the headquarters of the Bureau international des poids et mesures (the international organisation that oversees the metric system) and spoke to scientists to find out why and how this was happening.

Patiently, eagerly, they explained to me that the lump of metal used to define the kilogram no longer met society’s demand for precision, and so the unit itself had to be redefined using a constant of nature. In fact, I discovered, they’d already redefined every other metric unit out there using the same process. Length, temperature, time and more – all these units are now measured using things like the spin of atoms and the speed of light.

The meter, for example, used to be defined as the length of a bar of metal. Now it’s equal to the distance traveled by light in 1/299,792,458th of a second. And the second used to just be the length of a solar day divided by 86,400. Now, it’s the duration of 9,192,631,770 radioactive cycles of an atom of caesium­-133. (Think of these radioactive cycles as similar to the atom’s frequency as it flips between different stable energy states.)

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