A kilogram © Robert Rathe - www.robertrathe.com

The lump of metal by which we define a kilogram has lost weight: It’s now 50μg lighter than it was when it was created, 118 years ago. We know this because other cylinders created at the same time are now heavier than it – but they are not the designated ‘official’ kilograms, so I guess, in a round about way, it’s now them that’s wrong.

This is one of those delicious stories which messes with my brain. It’s scientific, philosophical, and incredibly accessible. ‘Cool!’

So, if the official measure of a kilogram is now lighter, is it lighter, or is everything else heavier? And how heavy is it? Despite having lost 50μg, its mass must surely still be 1kg, as it is by definition 1kg. Indeed, if it had lost half it’s weight, or gained ten stone, it would still weigh 1kg.

And, even more intriguingly, this is a kilogram that’s been kept in a triple-locked safe – so how can it possibly have lost weight?

Richard Davis, who’s the bloke in charge of the lump of metal, says that nothing will change: A kilogram will still be a kilogram. But what does that mean? A kilogram is the mass of this lump of metal, which has changed, so how can the kilogram philosophically stay the same? Scientifically, we can say that a kilogram is the weight of the lump of metal plus 50μg, but that’s not very satisfying, because if the lump has fluctuated already, who’s to say it won’t again?

The sensible solution is to define a kilogram using some more scientific measure – a popular option is to define it by a number of atoms of a particular type, which would never fluctuate. Except that it might, as our understanding of physics increases.

It’s all a bit reminiscent of the problem of the 2p coin from last May, but maybe that’s just because I like this kind of story.

Anyway, I hope it makes you think.

Originally posted on Gazette Live