Nearly 130 years after the mass of the kilogram was first defined, scientists have voted to implement a new system to redefine the measurement.

In a vault southwest of Paris, 40 feet underground, requiring three keys to open, you will find the original kilogram, or the International Prototype of the Kilogram (IPK). Even then, you would not be able to touch it. The oil from your fingers could add more weight and potentially make the kilogram heavier.

Around the world, there are six copies of the original kilogram. Every 40 years or so they are measured up against the original to check if the weight is still the same. However, in recent measurements, scientists have discovered that the copies are beginning to weigh differently than the IPK.

The change may only be a millionth of a gram, but because the IPK may also change in weight, the kilogram is no longer accurate. It may take a moment to understand, but because the IPK is The Kilogram, it means that that is how much a kilogram weighs, no matter if the original loses or gains mass over time.

Industries that rely on tiny measurements, such as pharmaceutical companies, need the kilogram to be accurate and unchanging, and its variability has caused scientists many difficulties when trying to work out what to do about the variation.

‘At the moment, the kilogram is defined in terms of the mass of a particular thing,’ says Ian Robinson of the National Physical Laboratory (NPL) in South London. ‘And if that thing is destroyed or changed or whatever, it’s awkward.’

However, a solution has been found: scientists are going to redefine the kilogram in terms of a natural, universal constant. The kilogram is the last remaining unit defined by an artefact – the meter, for example, is defined at the speed of light in a vacuum in 1/299,792,485th of a second. So this change, you could say, is overdue.

Planck’s constant, 6.62607015 x 10-34, will be the number to which the kilogram is now defined.

To put that into more simple terms, we need to return to the history. Max Planck was working with a group of scientists who were trying to model atomic vibrations, but kept getting it wrong. They thought that the atoms could vibrate at any frequency, but their experiments looked different to reality. Planck looked at the problem from a new perspective: he wondered if there was a baseline, which he called h, and that the atoms could only vibrate in whole number multiples from the base frequency; eg 2h or 5h but not 4.4h. He was correct.

This bizarre but effective assumption is how scientists are going to define the new kilogram, using Planck’s Constant – or that h. Essentially, this is quantum mechanics.

Defining the kilogram by physics of the universe instead of a human mechanism sounds mind-blowing, but ultimately it works out best for the world, because the kilogram will always be the same and (relatively) accessible for all.

‘For all times, for all people,’ is the motto of the metric system, and it seems like that vision is finally being achieved.


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