The theoretical background of the fifth force

Theoretical ideas of the fifth force(s) are reviewed, placing emphasis on the issue of how one can derive the two most important properties: the coupling strength roughly comparable with, or somewhat weaker than, gravity and the force-range of a macroscopic distance. Dilaton models, five-dimensional theory, and models of scalar fields induced from pseudoscalar fields as Nambu-Goldstone bosons of internal symmetries via CP violation are reanalyzed critically. All of these theories aim at predicting new types of phenomena to probe physics characterized by mass scales much higher than is accessible by other conventional means. Most of the theoretical predictions are shown to be consistent with effects expected to be discovered below the present experimental upper bounds. We are still waiting for more experiments with better sensitivity.