G Force

by David Bradley

It will be immediately apparent to those observing the techniques applied by scientists to the determination mechanical constants, such as Newton's constant of gravitation, G, usually referred to affectionately as "Big G" as opposed to those investigating the Planck constant or the mass of the electron that the experiments are still very much rooted in the nineteenth century, explained Dr Clive Speake of the University of Birmingham. Nevertheless, he added, the old tried and tested methods are still the best, although new techniques are on the horizon.

Newton's constant of gravitation played a significant role in classical physics relating the attractive force between two objects due to gravity, is fundamental to Einstein's theory of general relativity, and today finds a place in recent ideas concerned with quantum physics. G is a universal constant but is not correlated to any other fundamental constants but has a special place in modern physics. It is small but cannot be shielded and so has an additive effect, ultimately Newton's apocryphal falling apple will determine the fate of the universe itself. Unfortunately, while we might consider G to be a universal constant we do not yet have a definitive value for it outside the laboratory, we cannot yet tell whether G is the same on earth as it is at the centre of distant galaxies that spiral around gigantic black holes, for instance, nor whether it is changing in time. It is another paradoxical situation for science to be in, in that we have a value that appeals to very old technology but is simultaneously at the forefront of current physics.

Henry Cavendish was the first to measure G and obtained a value with 5% accuracy using a torsion balance. The modern equivalent of this device the torsion strip balance, which uses four weights instead of two, is currently being used to determine G at BIPM by Speake and colleagues. They are attempting to produce a robust value for G using three largely uncorrelated methods. This has not been attempted before. This work will be completed in the next year and, together with other recent measurements from other groups, will provide the most reliable and accurate value for G to date.

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