From sphere to eternity

While we can now precisely calculate the length of a metre or the duration of a second, the weight of a kilogram is still tied to an old bit of platinum-iridium sitting in a Paris vault. Now CSIRO Precision Optics is playing a crucial part in the international effort to complete the modernisation of the metric system.

The metric system has a largely deserved reputation for being more ‘scientific’ than its various predecessors and sole extant rival. But its dirty little (open) secret is that one of its foundations has been shaky from the outset, as there is no precise way of reliably determining exactly what a kilogram amounts to.

When the metric system first came into use in 1799 in revolutionary France the kilogram was defined as being the weight of a litre of water at the melting point of ice. For practical purposes an International Prototype platinum-iridium weight was created to embody this and, over time, various copies of that prototype were sent around the world. Albeit by minute amounts, the weight of those officially sanctioned weights has begun to diverge. This is an issue because, unlike the other base units (the metre, second, ampere, kelvin, candela and mole) used in science, commerce and every-day life, which can be related back to a fundamental unchanging constant of nature, the kilogram is still defined by physical object.

An inexact kilogram is a global problem and a global solution – the Avogadro Project – emerged around the start of the new millennium to address it. Briefly, this project aims to redefine the kilogram in terms of the Avogadro constant by determining the number of atoms contained within a known volume.

As might be imagined, constructing an appropriately sized and shaped object and then counting the atoms in it is no simple task, which is where Australia’s Commonwealth Scientific and Industrial Research Organisation’s Precision Optics department comes in.

“The CSIRO’s Precision Optics department has a long history of creating perfectly round spheres that are used in various laboratories around the world as calibration artifacts,” says Katie Green, the materials and fabrication manager at CSIRO Precision Optics and the woman heading up Australia’s contribution to the Avogadro Project.

“To put it in layman’s terms, it was decided that a sphere would be made from a very well-defined, pure material. So monoisotopic silicon was made in Russia, then grown into a near-perfect crystal in Germany, then sent to Australia to be shaped into a sphere.

“In order to measure the number of atoms in a kilogram they needed an object that was as round as possible and with a very high quality surface finish. No-one in the world can achieve the level of accuracy we can here in Australia on a sphere of this type.”

The spheres were successfully manufactured back in 2008, and the reason for the choice of a sphere as the shape of the object becomes clear, as Green observes.

“With a very round sphere, there is essentially only one dimension needed to be characterised very accurately.  Once you know the diameter of the sphere that value is constant in every direction. From this, you can calculate the volume of the sphere and because you know the crystal structure and the spacing of each atom you can calculate the number of atoms.”

(On a more practical level, a spherical object also has no corners and is thus less likely to get damaged.)

While proud of her team’s contribution, Green is keen to emphasise the degree of international co-operation involved in the Avogadro Project, which involves scientists in Germany, Italy, Japan, Belgium, Australia and the US working in the service of the two organisations in charge of maintaining uniform standards of measurement across the globe: France’s Bureau International des Poids et Mesures and Germany’s Physikalisch-Technische Bundesanstalt.

“All the various participants have different expertise – some characterising the material properties of the sphere, some measuring its physical dimensions – and it’s usually the case that each value is characterised or measured in at least two places independently and the results are then compared,” say Green, adding she is keen for the Avogadro Project to complete its mission.

“We handed over the spheres back in 2008. Since then they have been travelling around the world, basically being measured for their various properties. All that information is coming together and I understand the relevant authorities are now in the final stages of determining whether the sphere we helped create is going to play a crucial role in the new definition of the kilogram.”

Complicating matters, a competing method of determining the exact weight of a kilogram, called the Watt Balance, has emerged, though it’s believed this will be complementary to the Avogadro Project approach rather than supplanting it.

It’s now expected that humankind will finally know what a kilogram really weighs sometime this year or next, something that could come to be profoundly important as science advances.

“Is it going to make much difference to buying a kilo of potatoes at the shop?” laughs Green. “No, it’s not but it is useful in measuring things related to mass such as energy and electricity and important in fields such as aerospace and any industry that relies on incredibly precise measurements.”