The Kilogram is Being Retired

The Kibble Balance determines Planck's constant, the physical constant which is now the basis of the new kilogram. (Photo: BIPM)
Published May 17, 2019

On May 20, we will have a new definition of what a kilogram is. KTH’s Andreas Archenti discusses why an artifact gets sacked and why this event is bigger than the millennium shift.

In Sèvres, a suburb of Paris, in a vault far below the ground, rests a well-guarded metal cylinder. The metallic clump of platinum and iridium is small enough to fit in the palm of one’s hand. Double glass caps and a carefully controlled atmosphere protect the lump of metal from the effects of the outside world. This artefact is known as international kilogram prototype, or more informally: ‘Le Grand K’, or ‘Big K’.

All this may sound like the prelude to a Dan Brown novel, but Big K really does exist, and is nothing less than the exact measure of the mass of one kilogram. It is the origin of all kilograms – the ‘kilo’ to which the entire world relates.

Andreas Archenti, Professor and researcher in precision and dependability at KTH. (Photo: Jonas Thorén)

However, in recent decades, the kilo has become the subject of growing concern among researchers involved in measuring techniques, or ‘metrology’, as the doctrine is called. Besides the fact that Big K does not actually contain what it promises – its mass has actually changed over time – other problems exist. Andreas Archenti, a Professor and researcher in precision engineering, metrology and dependability at KTH, explains:

– The problem of measuring the kilogram against a physical object is that a mere thumbprint on Big K would change the kilo for all mankind. And if Big K should suddenly weigh 800 g for some reason, it would still be counted as one kilo. And if it were to be stolen – then the world would have no reference to the kilogram at all.

The slightest change in Big K’s accuracy affects areas such as medicine, electronics and technology – fields in which accuracy is crucial. And when the weight of the kilo vacillates, this also effects other units, the determination and measurement of which depend on the kilo.

The international kilogram prototype IPK, or Big K. (Photo: BIPM)

Thus, in November of last year, the International Bureau of Weights and Measures (BIPM) decided to retire the French metal cylinder. On May 20 (on World Metrology Day), a new definition of the kilogram will be revealed to the world. The Kelvin, Ampere, and Mol units are also being updated, for a total of four out of the seven SI units. The goal has been to define all units of measurement according to physical constants, because these remain stable over time.

This does not mean much for the average layperson, but for someone like Andreas Archenti, who has dedicated his life to precision, it is a remarkable step.

– For me, this will be a holy day. Not even the changeover to the new millennium can compare to this event, he laughs.

– It won’t make any difference to a person baking bread at home, but if you're a researcher, it’s significant, he says. The more uncertainty we eliminate from our calculations, the more accurate our description of reality becomes. It is the need for greater precision and accuracy in research and innovation that has driven these changes to the SI system.

So how does one create a standard for mass that does not change over time – and that is also accessible to everyone? Advances in a number of different research fields, including precision engineering and metrology where Andreas is active, made it possible to finally retire Big K. Above all, it is necessary to develop measuring instruments that can produce results that are sufficient enough to justify abandoning the old definition. And now these instruments exist.

Over the past year, numerous experiments have been conducted, including two experiments which (independently of each other) yielded satisfactory results in determining the Planck constant. Starting on May 20, it is this constant that will serve as the basis for defining a kilogram. 

– One test is a physically based experiment conducted using an advanced wave that balances the gravitational forces against electromagnetic forces, thus enabling researchers to determine an extremely accurate value for Planck's constant.

– The second experiment is chemically based, and calculates the number of atoms in a silicon sphere with a known isotope composition.

Most units have been revised – is this the last time we’ll see SI units receive new definitions?

– That’s an interesting question – but it’s impossible to answer. Now that the SI units are defined with physical constants, the lion’s share of the work is completed. But the question is – are they truly constant over a long period of time? And in the future we may have new measuring instruments that provide even lower measurement uncertainty.

And what will happen to Big K now?

– Well, maybe it’ll show up in some exhibition at the Louvre in Paris...

Text: Anna Gullers

Fact about Andreas Archenti

Occupation: Professor and researcher in precision engineering, metrology and dependability at KTH
Family: Wife and two children
Lives in: Örby in Stockholm
Hobbies: Collects old machines and measuring instruments
Favourite unit: They’re each special in their own way, but if I had to choose it would be ‘length’ – although on a day like today I might say ‘mass’.

Page responsible:Ulrika Georgsson
Belongs to: Sustainable Production Development
Last changed: May 17, 2019