Fifth force

In physics, there are four conventionally accepted fundamental forces or interactions that form the basis of all known interactions in nature: the gravitational, electromagnetic, strong nuclear, and weak nuclear forces. Some speculative theories have proposed an additional fundamental fifth force to explain various anomalous observations that do not fit existing theories. The characteristics of this fifth force depend on the theory being advanced. 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.

The search for a fifth force has increased in recent decades due to the discovery that most of the mass of the universe is accounted for by an unknown form of matter called dark matter. Many physicists believe that dark matter is some new undiscovered subatomic particle. Some believe this particle could be linked with a fifth force.

A new fundamental force might be difficult to test. Gravity, for example, is such a weak force that the gravitational interaction between two objects is only significant when one of them has a great mass. Therefore, it takes very sensitive equipment to measure gravitational interactions between objects that are small compared to the Earth. A new (or "fifth") fundamental force might similarly be weak and therefore difficult to detect. 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.) who were reanalyzing results of Loránd Eötvös 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.

There are at least three kinds of searches that can be undertaken, which depend on the kind of force being considered, and its range.

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