Time to amend Newton's law of gravity?

Newton's law of gravity may need an overhaul. Earlier this week geophysicists reported that the gravitational force between two bodies short distances apart is stronger than predicted by Newton. The results are the most recent in an ongoing debate over the accuracy of Newton's equation.

The latest results seem to contradict earlier experiments that also deviated from the law but found gravity to be weaker over short distances than expected.

According to Newton, gravity should decrease as the square of the distance between two objects increases. Scientists expected the latest experiment to confirm this, but were surprised to find otherwise, says geophysicist Mark Ander of the Los Alamos National Laboratory.

Dr. Ander and colleagues from other institutions collected data over a two-month period from a sensitive gravity meter as it was lowered into a mile-deep hole in Greenland. The researchers observed a force exceeding the expected force of gravity at distances beginning at 1,600 and up to at least 5,500 feet. The additional force is 50 times weaker than gravity.

Ander thinks that his data are ``far more sensitive'' than previous results because the new data were gathered in an easy-to-model environment, consisting mostly of air, ice, and rock.

While some media reports have zeroed in on the possibility of a new ``fifth force'' to explain the results, physicists say there are several possible explanations for the discrepancies between what Newton saw, and what modern researchers have observed with precise modern instruments.

``The variation would have to come from some new component of gravity or a totally new force of nature,'' says Robert Hughes, a theoretical physicist at Los Alamos. The four known forces are gravity, electromagnetism, and the strong and weak forces.

Dr. Hughes says the most compelling explanation is that gravity has one, or even two, components that have gone unaccounted for. Newton's equation could be modified, he says, using the existing Yukawa theory, and no new fifth force of nature would be necessary to explain the phenomenon.

The Yukawa theory says that gravity is transferred between bodies like a baseball is transferred between pitcher and catcher. These ``baseballs'' can take one of three forms: traditional gravity is transferred by what are termed graviton particles, while the hypothetical component forces are transferred by graviphoton and graviscalar particles.

According to this line of reasoning, the graviphoton would at times cause particles to repel each other while under other conditions it would cause them to be attracted. The graviscalar would always be an attractive component. Thus, sometimes the two components would cancel each other out, and the force of gravity predicted by Newton would result. At other times, either additional attraction or repulsion would be observed.

This theory might explain why Frank Stacey of the University of Queensland in Australia got results a few years ago suggesting a repulsive component to gravity, while Ander's data suggest an attractive component.

On the other hand, these sets of data may be overturned upon closer scrutiny or by new experiments that are planned for the coming year.

One thing is certain. In the words of Ander himself, ``A lot more experimentation will have to be done before this thing is settled.''

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