fifth force, postulated fifth basic force of nature (the four known forces of nature are gravity, electromagnetism, and the strong and weak interactions). Proposed in 1986 to account for gravitational discrepancies observed during some experiments, it was said to result in a repulsive effect about 1,000 times less powerful than gravity, and its strength was said to fall off quickly with distance, having a range of about 700 ft (200 m). The results of some initial experiments supported the possibility that the fifth force might exist, but later investigations, including a large-scale, highly accurate oceanic experiment, provided no evidence of such a force.
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