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Taking a closer look at “big G”

18 Dec 1998

Of all the fundamental constants in physics, the gravitational constant, "big G", is the least well known. Now a team of physicists from the National Institute of Standards and Technology and the University of Colorado in the US have made the most accurate measurement of G ever. The team used laser interferometry to make the measurement (Science 282 2230).

The gravitational constant was first measured by Lord Cavendish in 1798, who used a torsion-balance to measure the force between a pair of lead spheres. Cavendish measured G to be 6.754 x 10-11 metres cubed per kilogram per second squared. Although this technique has been refined many times, it is still subject to a number of systematic errors. Moreover, recent measurements of G have differed by almost 40 times their individual error estimates.

The US team dropped a test mass through a distance of 20 centimetres in a vibration-damped vacuum chamber. The test mass contained a retroreflector which defined one arm of a Michelson-type interferometer. This allowed the position of the test mass to be recorded every 320 microns. The vacuum chamber ensured that there were no errors resulting from temperature, density or pressure changes. The experiments were carried out with a 500 kilogram source mass that was alternatively placed above and below the vacuum chamber. Changing the position of the source mass altered the local gravitational field.

When the source mass was above the dropping region, it caused the falling test mass to slow down slightly, and when it was below, the test mass speeded up fractionally. The team find G to have a value of 6.6873 x 10-11 with an error of less than 0.14%.

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