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Monday, 1 May, 2000, 06:59 GMT 07:59 UK
Earth loses weight
Earth BBC
American scientists have discovered that the Earth is lighter than previously thought.

Their calculations suggest our planet weighs in at 5.972 sextillion tonnes - that is 5,972 followed by 18 zeros. This new weight is about 10 billion, billion tonnes less than the best previous estimates.

The researchers from the University of Washington arrived at the new value by using a more precise measurement of Isaac Newton's gravitational constant - one of the fundamental quantities in physics.

Affectionately known as Big G, the constant tells us how much gravitational force acts between two masses separated by a known distance.

Like the other fundamental constants such as Planck's constant and the speed of light, Big G is hugely important. It is a necessary ingredient in determining the mass not only of the Earth but also of the Moon, the Sun and the other planets.

Big G crisis

It is absolutely vital to know the numerical values of all the fundamental constants with high accuracy if we are to get a true, quantitative description of the physical universe.

But whereas the values of the other constants have got ever more precise, Big G has experienced a crisis.

"Gravity is the most important large-scale interaction in the Universe, there's no doubt about it," says Jens Gundlach, who with Stephen Merkowitz announced the new mass for the Earth at a meeting of the American Physical Society in Long Beach, California.

"It is largely responsible for the fate of the Universe, yet it is relatively little understood."

Attempts to measure Big G in the 1990s brought results widely different from the previously accepted figure. This prompted the US National Institute of Standards committee which establishes the accepted value to determine that there was actually 12 times more uncertainty about the figure last year than in 1987.

Huge embarrassment

"That is a huge embarrassment for modern physics, where we think we know everything so well and other constants are defined to many, many digits," Gundlach says.

If accepted, the measurement by Gundlach and Merkowitz would reduce the uncertainty by nearly a factor of 100 from the currently accepted figure, making it far more precise than even the 1987 figure.

Gundlach says his numbers could change as additional data are analysed in preparation for submitting the work for peer review.

To make their measurements, the researchers are using a device called a torsion balance. This records nearly imperceptible accelerations from the gravitational effects of four eight-kilogram stainless steel balls on a gold-coated Pyrex plate about the size of a matchbox but just 1.5 millimetres thick.

The device, operating inside an old cyclotron hall in the UW nuclear physics laboratory, is similar to one used 200 years ago to make the first Big G measurement. But it is computer controlled and contains numerous mechanical refinements that make the more precise measurement possible.

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