THE BIG 'G'

IN RECENT years physicists have been disagreeing about the mass of our planet to the tune of some ten billion billion tonnes. This mass depends on the strength of the gravitational force, which depends on a number - a `fundamental constant' denoted G - affectionately called `Big G'. The uncertainty in our knowledge of the Earth's mass reflects the uncertainty in the value of G.
Last month a group from the University of Washington in Seattle told delegates at a meeting of the American Physical Society in Long Beach, California, that they have measured G with unprecedented precision, reducing the uncertainty a hundredfold. The new result implies that the planet is several billion billion tonnes less massive than physicists had thought.
This sounds like a disconcertingly large amount of mass to have `mislaid' - but that is because the total mass of the Earth is so great. A difference in the third decimal place for the value of G translates to a seemingly immense difference in planetary mass.
Isaac Newton defined G in his famous law of gravitational attraction in 1686. This law helped to explain the motions of the Moon around the Earth, and the Earth and other planets around the Sun. But Newton could only estimate the value of G, and thus the strength of gravity, very roughly, by guessing at the density and size of the Earth. Over 100 years later in 1798, the English scientist Henry Cavendish measured G for the first time.
Cavendish used a device called a torsion balance - basically a dumbbell suspended from a fibre. On either end of the dumbbell are lead spheres, which are gravitationally attracted to larger lead spheres placed nearby. This attraction, although tiny compared with the gravitational pull of a huge object like the Earth, turns the suspended dumbbell, twisting the fibre on which it hangs. The twist is measured by bouncing a light beam off a mirror attached to the fibre and measuring its deflection. The value of G is calculated from this deflection.
Many modern determinations of G still rely on the torsion balance, although more precise measurements can now be made. But these measurements haven't improved a great deal on Cavendish's experiment. The problem is that gravity is a weak force compared with the other fundamental forces of nature. So whereas most of the `fundamental constants' of the physical universe are known to great accuracy, G has not been pinned down to more than three decimal places - a source of some embarrassment to the physics community.
Attempts to improve the situation have only made it worse. Careful measurements of G during the 1990s left the `official' value even more uncertain than in the preceding decade. In 1998 the uncertainty was pronounced to be 0.15 per cent.
The Washington group, led by Jens Gundlach and Stephen Merkowitz, now claim to have established the value within a precision of 0.0015 per cent. They used a variant of the torsion balance technique in which the fibre never actually twists, which avoids a major source of imprecision in the measurement.The new result implies that the planet is several billion billion tonnes less massive than physicists had thought.


CAN U JUST IMAGINE!!!!!