modern physics describes physical reality in terms of four known fundamental forces. however, since physics has no accepted universal framework, occasionally physicists have postulated the existence of an additional fundamental fifth force. most postulate a force of roughly the strength of gravity (i.e. it is much weaker than electromagnetism or the nuclear forces) and to have a range of anywhere from less than a millimeter to cosmological scales. ==experimental approaches== the idea of an additional fundamental force is difficult to test, because gravity is such a weak force: the gravitational interaction between two objects is only significant when one has a great mass. therefore, it takes very precise equipment to measure gravitational interactions between objects that are small compared to the earth. nonetheless, in the late 1980s a fifth force, operating on municipal scales (i.e. with a range of about 100 meters), was reported by researchers (fischbach et al.)ephraim fischbach, daniel sudarsky, aaron szafer, carrick talmadge, and s. h. aronson, "reanalysis of the eÃ¶tvÃ¶s experiment", physical review letters 56 3 (1986). who were reanalyzing results of lorÃ¡nd eÃ¶tvÃ¶s from earlier in the century. the force was believed to be linked with hypercharge. over a number of years, other experiments have failed to duplicate this result.university of washington eÃ¶t-wash group, the leading group searching for a fifth force. there are at least three kinds of searches that can be undertaken, which depend on the kind of force being considered, and its range. ===equivalence principle=== one way is to search for a fifth force with tests of the strong equivalence principle: this is one of the most powerful tests of einstein's theory of gravity; general relativity. alternative theories of gravity, such as bransâdicke theory, have a fifth force&mdash;possibly with infinite range. this is because gravitational interactions, in theories other than general relativity, have degrees of freedom other than the "metric", which dictates the curvature of space, and different kinds of degrees of freedom produce different effects. for example, a scalar field cannot produce the bending of light rays. the fifth force would manifest itself in an effect on solar system orbits, called the nordtvedt effect. this is tested with lunar laser ranging experimentlunar laser ranging and very long baseline interferometry. ===extra dimensions=== another kind of fifth force, which arises in kaluzaâklein theory, where the universe has extra dimensions, or in supergravity or string theory is the yukawa force, which is transmitted by 
Force Interaction