Veritasium’s excellent YouTube upload helps explain the complexities that be get the confirmation and validation of the metric systems key base measure, the kilogram.
A kilogram isn’t what it used to be, nor is exactly what it should be. And since the kilogram is the only base unit still defined by a physical artifact, the international prototype kilogram or IPK, whose mass has changed over the centuries, then this is perhaps the reason how the world’s roundest object could help resolve the longest running problem in measurement — how to define the kilogram.
The IPK, and it’s 40 similar cylinders all minted at the same time, and dispersed across the globe, has not been able to maintain its mass. There’s no real scientific understanding as to the reasons why. But the upshot is that science has had a goal to eliminate the kilogram’s dependence on a physical object and redefine it. Two main approaches being considered to achieve this end are: the Avogadro Project and the Watt Balance.
The Avogadro project aims to redefine Avogadro’s constant (currently defined by the kilogram — the number of atoms in 12 g of carbon-12) and reverse the relationship so that the kilogram is precisely specified by Avogadro’s constant. This method required creating the most perfect sphere on Earth. It is made out of a single crystal of silicon 28 atoms. By carefully measuring the diameter, the volume can be precisely specified. Since the atom spacing of silicon is well known, the number of atoms in a sphere can be accurately calculated. This allows for a very precise determination of Avogadro’s constant.
Now, where did I put my bloody ruler?