In 2018, we're getting a new kilogram

How do you feel about the kilogram? It's probably not something you think about a lot, but it's one of the seven units of measure that define our world at the most basic level. 

And it's not good enough. At least, that's what metrologists – who define those basic units – say. Our current kilogram is the only unit of measure still defined by a physical object – a four-centimetre cylinder made of platinum and iridium that's been stored in a vault near Paris, under three glass domes, since 1889.

This "original kilogram", as it's known, has lost a little weight over the years. About 50 millionths of a gram, to be precise. But as all of the world's scales refer indirectly to this kilogram, that means they now weigh ever-so-slightly incorrectly.

Universal constant

To fix this problem, a new standard is needed - one which doesn't change over time. And in 2018, at the 26th General Conference on Weights and Measures, that standard will be agreed.

Instead of being based on a lump of metal in a vault, the new kilogram will be based on a universal constant. Specifically, it'll be based on the Planck constant, which links the amount of energy a photon carries with the frequency of its electromagnetic wave.

To turn that into a weight, metrologists will need to create an incredibly precise set of scales that balance a weight on one side with an electromagnetic force on the other. The weight of the new kilogram will balance out a very precise amount of electromagnetic force, which won't change over time. That amount of force will define the new kilogram.

The task of building that set of scales has fallen to the Institute for Process Measurement and Sensor Technology at the National Metrology Institute of Germany. There, they've spent the last decade developing measuring instruments that are regarded as the most precise in the world, and are already used to compare different kilogram prototypes. 

New standard

In 2014, when the subject was last discussed, the International Committee for Weights and Measures said that the data on the accuracy of the new approach wasn't sufficient enough to adopt it as the standard. Since then, however, a lot of work has been done, and it's looking likely that the new kilogram standard will be adopted in 2018.

For physicists that work with ultra-precise measurements, pharmaceutical companies that need to calculate precise dosages of medicine, police forensics departments and food testing laboratories, that's going to mean some changes in how they do things. 

But for now, you probably don't need to buy a new set of bathroom scales.