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  • Roundness (and Mass) Redefined

Photonics Spectra
May 2008
Most spheres look round enough to the unaided eye.

But study a marble, a bowling ball or even a distant planet closely enough, and it quickly becomes apparent that its roundness is full of imperfections. The best that can be said is that its shape is generally circular. In fact, perfect mathematical roundness probably never has been achieved by humans.


Researchers are seeking to create a perfect sphere that will be used to refine the definition of a kilogram. Courtesy of the Australian Centre for Precision Optics.

A team of international scientists is seeking to come as close as possible with the fabrication of two spheres that are made with monoisotopic silicon. The material was produced in Russia, made into a near-perfect crystal in Germany, and fabricated into spheres at the Australian Centre for Precision Optics, part of the national research agency CSIRO in Sydney.

The spheres — completed in early April after years of painstaking work — were created through precision optical fabrication techniques of grinding and polishing.

In terms of roundness, their diameters are almost perfectly uniform throughout, with differences measured in nanometers. What’s more, the surface is almost perfectly smooth and free of defects. A series of tests now will be carried out by various national standards groups around the world to measure their mass, volume, density and diameter, which is ∼94 mm. Testing of the spheres is expected to last into 2010.

The larger purpose of the project is to define a kilogram in terms of a fundamental constant of nature. Mass is currently the only international standard unit of measurement that is defined by a physical object: a metal chunk that is stored in France. The investigators are seeking to redefine the standard by determining the number of silicon atoms that are contained within one kilogram.

Once this is completed, the definition of a kilogram could be forever changed, with mass joining the six other base units of science that already are defined by natural constants.

A solid with a structure that exhibits a basically symmetrical and geometrical arrangement. A crystal may already possess this structure, or it may acquire it through mechanical means. More than 50 chemical substances are important to the optical industry in crystal form. Large single crystals often are used because of their transparency in different spectral regions. However, as some single crystals are very brittle and liable to split under strain, attempts have been made to grind them very...
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