Heavily guarded in a Paris vault, one piece of metal is the current benchmark for all calculations when it comes to grams and kilograms.
The International Prototype of the Kilogram (IPK) was decided on at the first General Conference on Weight and Measures in Paris in 1889.
But physicists argue this is no longer the way we should define a kilogram – because the IPK itself isn't exactly stable and is gradually losing tiny amounts of weight.
Physicists at the Federal Physical-Technical Institute in Braunschweig (PTB) are working on a new definition for the kilogram - by first working out the mass of a single silicon atom.
No longer fit for purpose
At the end of the 19th century, several IPK candidates were produced, and one chosen to be the definitive kilogram, said PTB spokesperson Jens Simon.
Other IPK candidates have kept a stable weight over the years - but for some inexplicable reason, the IPK itself has lost mass.
It may only have lost 0.000005g in the last century – but the very fact it has changed makes it no longer fit for purpose.
As all scales are calibrated one way or another to the IPK, its loss means that in relative terms, everything has got slightly heavier over the past century.
Knowing this, scientists would have to regularly re-calibrate all scales to the IPK to maintain accuracy over the years.
The International Prototype of the Kilogram is slowly losing mass. Photo: DPA
A new definition
By 2018 at the latest, the IPK will be obsolete, as a new worldwide kilogram definition is adopted.
One thing is for certain: the new definition won't be dependent on a single object, as it has been since 1889.
It's risky to base a worldwide measurement system on a single piece of metal, said PTB physicist Arnold Nicolaus.
"Just imagine if it broke."
Nicolaus has played a key role in early experiments to discover the new definition.
In a heavily controlled environment, physicists produce silicon balls in an extremely precise manner – so much so that the scientists know exactly how many silicon atoms are in each ball.
Through doing this, the physicists can accurately measure the mass of one silicon atom.
Once scientists know the precise mass, they can work out exactly how many silicon atoms make up a kilogram – giving the kilogram a stable and universal mass.
In the coming months, the Braunschweig team will measure two particularly pure silicon balls, in hopes of improving the accuracy of their findings.
Once the team have finished their measurements, any well-equipped laboratory could - in theory - create a universally valid kilogram.