Das rundeste Objekt der Welt

Wer schonmal versucht hat, einen perfekten Kreis zu zeichnen, weiß genau, wie schwer das ist. Jetzt hat man zwei nahezu perfekte Kugeln aus Silizium hergestellt, die, wenn man sie auf Erdballgröße skalierte, grade mal Abweichungen von 3 bis 5 Metern von der perfekten Kugel hätten und die Oberfläche hätte lediglich Unebenheiten zwischen 12 bis 15 Milimetern. Und ich liebe die Geschichte dahinter: das Urkilogramm in Paris verliert aus unbekannten Gründen an Masse und mit Hilfe dieser (fast) perfekten Silizium-Kugeln will man nun die genaue Menge an Silizium-Atomen ausrechnen, die ein Kilogramm ergeben.

The unusual balls, discussed last week at the SPIE Astronomical Telescopes and Instrumentation conference in France, 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.

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.

It is no easy task. 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. The important thing is for those numbers to agree with each other.

To shape the spheres, the Australian Center for Precision Optics pulled optical engineer Achim Leistner out of retirement. Leistner, who has been creating precision spheres for decades, considers these final two to be his masterpieces.

The ACPO team used techniques similar to the way Isaac Newton ground lenses for his telescopes 300 years ago. 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.

"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," Leistner told New Scientist.

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