When is a kilogram not a kilogram?

Many physics students will be familiar with the fact that the prototype kilogram is kept in a high-security vault in Paris. What I didn’t realise was that the mass of this specimen is changing, albeit very slightly.

So here’s the question; if this is the one and only true version of the kilogram, and it loses mass, doesn’t it still stay a kilogram?

And doesn’t that mean that other copies, which would have been correct originally, are now wrong?
(I think in fact that they may all be losing mass slightly.)

Something very unscientific about all this . . ., no wonder physicists are embarassed about it.

Seems a bit like when kids make up the rules of a game, and when it turns out that these don’t suit the leader of the gang, he just changes them.

Which seems as good an excuse as I am likely to get to show Eddie Izzard – Do you have a flag?



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