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The Kilogram and the Kitchen Sink

Physicists can’t make up their minds how heavy a kilogram should be. Perhaps they should allow a new generation of scientists to help.
May 31, 2010

In 1983, physicists at the 17th General Conference on Weights and Measures decided to redefine the metre. For a century until then, the metre had been the distance between two points on a bar of platinum and iridium measured at the melting point of ice.

The problem with this definition was not just that these conditions were somewhat arbitrary and difficult to standardise during a measurement but that only the person in possession of the bar could do the experiment. Physicists wanted a measurement that more or less anybody could make.

The new definition is the distance light travels in a vacuum during 1/ 299 792 458th of a second. In principle, anybody with a laser pointer, a stopwatch and a few other bits and pieces can determine this distance. At a stroke, the metre became open source.

Now physicists want to do the same for the kilogram, which is currently defined as the mass of a cylinder of platinum and iridium called the International Prototype Kilogram.

That’s a problem because each time it is picked up, a few atoms rub off the cylinder making it imperceptibly lighter. For this reason almost nobody is allowed to measure the mass of the International Prototype Kilogram, which is stored in a vault in Sevres in France. So nobody really knows how much mass the kilogram is losing or indeed, whether it is gaining the weight of a thin layer of dust and impurities which must surely be gathering on its hundred year-old surfaces.

But what to replace it with? The most widely discussed suggestion is to appeal to the equivalency between energy and mass for a definition. For example, one idea is that a kilogram should be the mass of a body whose equivalent energy is equal to that of a number of photons whose frequencies sum to exactly (299792458^2/66260693) × 10^41 hertz.

If that sounds reasonable enough, you probably haven’t thought about it in as much detail as Ronald Fox at the Georgia Institute of Technology and a couple of buddies.

They point out that a kilogram relying on mass-energy equivalence can only be measured using a piece of equipment called a watt-balance. This passes a current through some coils to generate a force capable of supporting a kilogram, allowing it be measured in terms of a current and voltage.

But a watt-balance is an expensive piece of kit that is hard to use, say Fox and co. They point out that the one owned by the National Institute of Standards and Technology is two stories high, cost \$1.5 million to set up and requires a team of up to 5 physicists to run. And even then, the measurements are notoriously susceptible to noise.

That’s hardly a device that scientists the world over will want or be able to play with.

So Fox and co have another suggestion. Why not make the kilogram equal to the mass of a certain number of carbon-12 atoms, specifically 2250× 28148963^3 of them?

Then a kilogram would be a cube of carbon 8.11cm on each side (8.11cm is roughly the length of 368,855,762 carbon atoms laid side by side).

With that definition, almost anybody could make a kilogram in their own kitchen given some carbon and a knife.

“The day we made a kilogram” might even be the kind of fun that could engage and inspire a new generation of scientists, which ought to be a good enough reason on its own on which to decide.

Ref: arxiv.org/abs/1005.5139: A Better Definition of the Kilogram

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Illustration by Rose Wong

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