A new experiment testing the power of gravity has cast more doubt on the existence of the proposed "fifth force" in nature, researchers said Friday.

Scientists at the Joint Institute for Laboratory Astrophysics in Boulder, Colo., failed to find evidence of the existence of the force in what they called the most sensitive gravity experiment ever conducted."We see no evidence for any fifth force whatsoever in our measurements," said James Faller, a physicist who coordinated the work, which was published in the Oct. 15 issue of the scientific journal Physical Review Letters.

Scientists proposed in 1986 that a "fifth force" existed in addition to the four known forces in nature - gravity, electromagnetism, the strong force that binds atomic nuclei, and the weak force that causes radioactive decay.

The fifth force was proposed to act over very short distances - up to only a few miles - and would help explain phenonmenon that could not be accounted for by the four known forces.

Subsequent experiments aimed at validating the existence of the force produced mixed results, Faller said. Some tests initially indicated such a force may exist while other more recent ones found no evidence, he said.