For over 125 years the world's system of units: now referred to as the SI, has relied for its ultimate standard of mass on a 1 kg cylinder of precious metal held in Paris. Over this time other base units have moved away from a definition involving a physical artefact to a definition which effectively fixes the value of a fundamental constant. For example: the metre was once defined as the distance between two lines on a particular metal bar but is now defined by effectively fixing the value of a fundamental constant: the speed of light in vacuum. This ensures the stability of the unit, allows anyone to have direct access to the unit and enables advances in length measurement to be made without the need to change the definition of the metre.
The prospect of a system of units based wholly on atomic and fundamental constants has been the goal of scientists for well over a century but, up to now, the kilogram has resisted this change. This is because the kilogram artefact is very stable and the production of an accurate way of measuring kilogram-sized masses in terms of a fundamental constant has proved very difficult. But now we are at the point where we have two very different experiments which can relate kilogram-sized masses to a fundamental constant: the Planck constant. There is agreement between these two experiments on the value of this constant, as measured in existing SI units, and so by fixing its value in the new SI we can achieve our goal of an SI based wholly on atomic and fundamental constants with no discontinuity in measurements of mass within the SI.
The two experiments are: the watt balance, which relates virtual electrical and mechanical power, and the X-ray crystal density (XRCD) experiment, which counts the number of atoms in a spherical crystal of almost pure silicon-28.
The talk will describe the two experiments and the changes to the SI which are likely to occur in 2018.