linguistic:paraphrasing, stylistic:tone, structural:scholar_edition
In physics, there are four observed fundamental interactions that form the basis of all known interactions in nature: gravitational, electromagnetic, strong nuclear, and weak nuclear forces. Some speculative theories have proposed a fifth force to explain various anomalous observations that do not fit existing theories. The characteristics of this fifth force depend on the hypothesis being advanced. Many postulate a force roughly the strength of gravity with a range of anywhere from less than a millimeter to cosmological scales. Another proposal is a new weak force mediated by W′ and Z′ bosons. Some physicists speculate that a form of dark energy called quintessence could be a fifth force.