London, July 1 : An international group of engineers and craftsmen have built a pair of nearly perfect spheres that are thought to be the roundest objects in the world.
According to a report in New Scientist, the unusual balls were created as an answer to the "kilogram problem".
The kilogram is the only remaining standard of measurement tied to a single physical object: a 120-year-old lump of platinum and iridium that sits in a vault outside of Paris, France.
But the mass of this chunk of metal is slowly changing relative to the 40-odd copies kept by other countries, and no one knows why or by how much.
So, researchers charged with policing units and measures, called metrologists, have come up with several suggestions to redefine the kilogram.
One proposal, pushed by an international team called the Avogadro Project, aims to define the kilogram in terms of a specific number of silicon atoms.
As to just how many atoms there would be, the newly created silicon spheres would help determine that.
Over the next few years, groups in Italy, Belgium, Japan and the US will try to calculate the exact number of atoms in each one.
To determine the volume of each sphere, they will use optical interferometers to measure its width from 60,000 different points on its surface.
Meanwhile, X-ray crystallographers will take pictures of the silicon crystal structure to determine the spacing and density of the atoms.
By multiplying volume by density, each group should produce its own count of how many silicon atoms make up a kilogram.
To make the round objects, the silicon was purified in ex-Soviet centrifuges once used to refine uranium for nuclear weapons.
The centrifuges separated the silicon by isotope, allowing researchers to create a remarkably pure batch of silicon-28.
From Russia, the material travelled to Germany's national metrology institute, PTB, to be grown into a giant crystal by ageing equipment that produced silicon for East Germany decades ago.
After six failed attempts, a pure crystal was made and cut into two 5-kg blocks that were shipped to Australia.
Opticians manipulated two spinning rotors to grind the surface by hand. After months of sanding, the team produced two spheres with diameters of 93.75 millimetres.
The mass of each sphere matches that of the Australian copy of the kilogram. The small-scale roughness of the balls varies by only 0.3 nanometres, and their curvature by 60 to 70 nanometres.
According to optical engineer Achim Leistner, "If you were to blow up our spheres to the size of the Earth, you would see a small ripple in the smoothness of about 12 to 15 mm, and a variation of only 3 to 5 metres in the roundness."