A Weighty Matter

How much does a kilogram actually weigh. It's an interesting question, originally it was defined as 'a volume of water equal to the cube of the hundredth part of the metre, at the temperature of melting ice', which is fine, but not very practical. In order to make it easier a standard reference kilogram was created known as the 'The International Prototype Kilogram', known as the IKP, a 39 mm high platinum-iridium alloy cylinder which is stored in a vault at the International Bureau of Weights and Measures in Sèvres, France. This kilo doesn't just weigh a kilogram, it is the kilogram, its the standard against which all other kilograms are weighed. The problem is, it's getting heavier.

It is estimated that pollutants building up on the standard kilogram may have added tens of micrograms to the weight of the IPK over the years. So, how do they know its getting heavier you may ask, what are they measuring it against? Well there are in fact 40 official IPK replicas around the world, and they all appear to be gaining weight at slightly different rates. This presents a problem as this divergence, even at such small levels, causes them all to be out of sync with each other. Now researchers at the University of Newcastle have tried to remove these pesky pollutants by exposing similar surfaces to UV radiation, it is hoped that this will remove the pollutant contamination and should, at least in theory, return the metal to its original weight and hopefully stop this problem.

Why should we care? Well the thing is so many other units of measurement depend on the kilogram. As of 2013 the kilogram was the only SI unit still defined by an artifact. If the kilogram changes, so must the newton, if the newton changes, so must the joule and so on. That's why there are plans to define the kilogram not by a prototype, but in terms of fundamental physical constants, in particular the plank constant. If this goes ahead then hopefully the problem may be fixed once and for all but for now we need a way to keep the standard kilogram just that. One kilogram.