SCIENCE

'World's Roundest Object' May Provide New Definition Of Standard Kilogram (VIDEO)

08/23/2013 04:12 pm ET | Updated Jan 12, 2015

In a world where everything strives to be the best and the biggest, scientists behind The Avogadro Project in Australia have sought a surprising superlative: the world's roundest object.

And they aren't just doing it for bragging rights. Instead, the remarkable sphere may provide a solution to what's known as the "kilogram problem."

Unlike other scientific units, which can theoretically be measured anywhere in the world based on natural properties, the kilogram is still based on a physical object: a cylinder of platinum and iridium that dates back to 1889.

So while the "meter" is defined as the distance light travels in a tiny fraction of a second, and the "second" can be counted by an atom’s transition between two energy levels, the kilogram is no more (and no less) than a physical mass that sits in a secured vault at the Bureau International des Poids et Mesures in Paris.

For reasons no one understands -- and despite precautionary measures -- the cylinder's mass keeps changing. In other words, the kilogram, as defined by the cylinder (and compared to 40 exact replicas of the cylinder kept in other countries), doesn't weigh the same as it used to.

To solve that problem, researchers at the Australian Centre for Precision Optics, which is home to The Avogadro Project, are crafting nearly perfect spheres made of a highly pure and very stable form of silicon. By calculating the sphere's volume and weight, scientists should be able to determine the exact number of silicon atoms in the object itself, thereby providing an unchanging definition for the mass of a kilogram.

Per Australia's Commonwealth Scientific and Industrial Research Organisation (CSIRO), scientists settled on a sphere as the standard shape because it "has no edges that might get damaged," and "only one dimension [its diameter] has to be measured in order to calculate its volume."

As for how the world's roundest objects were made, New Scientist reports two spinning rotors ground them for several months. Afterward, computer-guided lasers measured each for slight derivations that were corrected individually.

"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," CSIRO master optician Achim Leistner said of the end result.

A second, competing method to determine a standard measurement for the kilogram is the "watt balance" -- a system tied to Earth's gravitational pull on a kilogram and the force needed to counteract it. This strategy has also earned quite a following.

Despite these advances, the standard kilogram remains a cylinder that's more than 120 years old -- at least for now. And until the world's roundest object proves its mettle, well, we'll just have to roll with it.

CORRECTION: An earlier version of this article stated incorrectly that the duration of a "second" is based on the decay of an atom. The duration of a second is based on the fluctuating energy state of the atom.

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