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The sphere would not be defining the kilogram: it will be used to count the number of atoms it contains, which would allow to redefine the Avogadro number without reference to the kilogram, which would then define the kilogram as the mass of a fixed number of Si28 atoms. See the video I posted in another comment.

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I see. Indeed, this is the impression that I got from the last paragraph:

> Now that the spheres are done, researchers in different countries will try to discern the exact number of atoms they contain, to get universal agreement on what exactly is the mass of a kilogram.

However, when it is understood this way, I find the previous word choice confusing:

> two softball-sized, near-perfect spheres, made entirely of silicon-28 atoms, … should remain exactly one kilogram in perpetuity

(emphasis mine). If they are made entirely of silicon-28, and the kilogram is defined as the mass of a certain number of silicon-28 atoms, then they will remain one kilogram in perpetuity (by definition), right?

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