Slipping standards

As everyone else's New Year obsession with weight wears off, science's is just getting started. Researchers are gathering at the Royal Society on 24 January to focus on science's embarrassing weight problem - or, more precisely, its mass problem.

The internationally agreed standard of mass, the kilogram, is the only unit we have that is not based on hi-tech wizardry and a fundamental, invariant constant of nature. The metre is defined as the distance that light travels through a vacuum in a certain fraction of a second. That second is based on the time it takes for a caesium atom to perform a certain number of oscillations. There are plans afoot to make the second even more accurate by using a different atomic oscillation. The humble kilogram, meanwhile, is derived from a lump of metal kept in a basement in Paris.
The International Prototype Kilogram doesn't look humble.

It's a shiny cylinder of iridium-platinum alloy locked in a vault that can be opened only by turning three keys simultaneously. It looks beautiful, almost iridescent under its double bell-jar covering. It is polished regularly, too, to remove surface pollutants that might affect its mass.

All this pomp and ceremony is to no avail, however. The alloy was chosen because it is the most stable material we have - but it seems it is not stable enough. There are dozens of supposedly exact copies of the Paris kilo, forged in the late 19th century. In the mid-1990s, scientists found disparities between their masses. The difference in mass between them is tiny, equivalent to scattering a couple of grains of salt on a cylinder, but that's enough to upset scientists with a bent for atomic precision. Especially as, for reasons that no one understands, the disparities between the masses are growing.

And so the Paris kilogram has to go. One proposed replacement involves creating a sphere of polished silicon containing a known number of atoms. Because we know the mass of each silicon atom, that will give us a precise standard. Another option is to make use of the interchangeability of mass and energy. You can get a measure of mass on a machine that measures it against the energy in a configured electromagnetic field.

It probably all sounds a bit abstruse, the obsessive-compulsive side of science. But standards matter. That is why every government serious about science invests in a national standards laboratory (ours is in Teddington). Exquisite precision in our dealings with nature gives us the tools to make accurate and reliable measurements, such as those needed to monitor climate change, create the next generation of computers, squeeze more data through mobile-phone networks and keep GPS satellites in sync. And to confirm you really are that little bit heavier than you were this time last year.

Tags:Science