What do metrologists, a lump of platinum in Paris, optical interferometers and crystallographers all have in common right now?

If you answered "How much -- exactly -- does a kilogram weigh?", you are correct!

The kilogram is the only standard weight of measurement that is only defined by one objecft: a lump of platinum in Paris. And for an unknown reason its weight is changing (relative to other copies of itself from around the world). In an effort to officially define a kilogram, a group of engineers has built the world's roundest objects and other scientists are attempting to count the number of atoms in the spheres, which will give an exact measurement of just what a kilogram means going forward.

I'm not exactly sure how they decided how big to make the spheres, but my guess is that they have the same weight as the lump of platinum in Paris, and by counting the number of atoms in the sphere, the lump in Paris will no longer be necessary in case it continues to change its weight or gets lost or stolen, etc.. Find out more here.

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