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When is a kilogram not a kilogram?

A kilogram

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

This post was filed under: News and Comment, Writing Elsewhere.

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Comments and responses

Comment from Mort Karman


    12.55, 25/09/2007

At least it is closer then the foot. which was King John’s shoe size. Or the head, which could also mean a potty or a sex act.
I like exact when it comes to time, so I got a so called “atomic clock”.
The problem is it uses an AM radio control from the US time signal in Boulder, Colorado.
As you know when we have electrical storms we have static on the AM radio. This screws up the “atomic clock” and I have to use the old clock to reset it.
So, really, who cares if our measurements are a little off. Most of us are also a little off.


Comment from sjhoward (author of the post)


    18.09, 26/09/2007

Indeed, we are all a little off. My left foot is a size 8, my right a size 10, so I guess it’s a good job that measurements weren’t based on my feet. Thought I thought the myth was that the foot was based on the foot of King Henry I, not King John?


Comment from Mort Karman


    22.16, 27/09/2007

You are right, it was King Henry. King John had a toilet named in his honour. (I think)




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