There is an international effort being currently undertaken to redefine the kilogram. This measure is currently based on a solid cylinder of platinum-iridium alloy locked in a vault at the International Bureau of Weights and Measures in France for over 125 years. This cylinder was then agreed to weigh exactly 1 kilogram, but inexplicably it has recently begun to lose weight, most likely due to the loss of gas locked inside the cylinder when it was made.
Mathematicians are currently working on replacing this solid metal block, known as the International Prototype Kilogram, with a mathematical constant (based on Planck’s constant for those with a mathematical background).
In order to achieve this, however, they need to work out a way of estimating another mathematical constant called Avogadro’s number (the number of discrete particles – molecules or atoms – in a ‘mole’ of substance. A mole of water, for example, is just a few teaspoons but it contains more molecules than there are grains of sand in the world.
The scientists at the heart of this research project have demonstrated their breakthrough by ‘counting’ the number of atoms in a kilogram sphere of pure silicon. They can estimate this because such pure silicon forms crystals of cubic cells each containing 8 atoms of silicon, and in this way, they can calculate the number of atoms in the sphere by calculating the volume occupied by each silicon atom.
Redefining the kilogram in such a fundamental manner will have an enormous impact on us all. It is the only one of the 7 base units on which all other units of measurement in science are derived that is based on a physical object rather than a fundamental physical constant.
Summarised from i, 15th July 2015 by Steve Connor
