OPINION

Change makes kilogram worth the weight

Sunday was shopping day for me. You know, a kilogram of flour, two kilograms of apples and so on. But since Sunday, something has changed.

That something is the kilogram.

We all learnt in school that a kilogram is equal to 1000 grams, and it would seem that it can't change. But it turns out that the kilogram has been shrinking.

I promise that I'm not crazy - let me explain.

A kilogram was originally defined as the mass of 1000 cubic centimetres of water. Because this is not the most convenient in terms of everyday use, a physical prototype of a kilogram was created - a cylinder made of platinum and iridium.

A "kilogram" is equal to the mass of this cylinder.

This small chunk of metal is so precious that it's locked in a vault in Paris. There are a number of replicas, or working copies, created to standardise weights.

It's been discovered that, over time, these working standards have become lighter than the original kilogram.

Each time they're moved, these standards leave behind a few atoms.

Over decades, all of these tiny particles being shed from the copies have left them ever so slightly smaller than the original.

To overcome this issue, it has been decided that instead of relying on a physical object as a standard unit of measure, it would be better if we had something non-physical - something that couldn't change over time.

At the 2018 General Conference on Weights and Measures (yes, it is actually a thing), it was determined that the kilogram would be redefined. And this definition officially came into use yesterday, May 20.

So what is the new definition of a kilogram? Well, it's a lot more confusing than the weight of a metal cylinder.

A kilogram will now be defined with reference to something called Planck's constant.

This very small number helps to describe the relationship between weight and electrical current, and it never changes.

By using this constant, scientists can now define a kilogram in a way that won't shrink over time.

Is this new kilogram really going to change anything in our day to day lives? You won't even notice. A kilogram bag of flour is still going to look and feel the same.

But knowing this bit of trivia might help you win a pub quiz one day. And anyway, it's a nice distraction from the election results...

Dr Mary McMillan is a lecturer at the School of Science and Technology, University of New England