From Mine Shafts to Cliffs—The ‘Fifth Force’ Remains Elusive
Is there really a “fifth force”? Since Purdue particle theorist Ephraim Fischbach and his colleagues dusted off the 80‐year‐old data of Roland von Eotvos three years ago, pointing out that this classic test of the equivalence principle hinted that all was not well with Newtonian gravity, a broad array of physicists have been investigating the possible existence of a force weaker than gravity, with a finite macroscopic range. A lonely band of geophysicists had been gathering evidence of such a force in Australian mine shafts since 1981. (See PHYSICS TODAY, October 1986, page 17.) But the jury is still out.