The Real Fifth Force, and More?
In the Standard Model, there is at least one additional type of interaction beyond the four known forces (weak, strong, electromagnetic, and gravitational). This force is needed to explain how all the fundamental particle masses are generated. This part of the theory is the least tested experimentally, so there are a number of different competing ideas on how it may work.
The simplest version introduced one more force--the Higgs force--and one more particle type--the Higgs particle--related to this force. Searches for this particle and efforts to learn more about how particle masses occur are one active area of particle studies. Other models introduce more complicated explanations for particle masses.
In addition, there are many speculations about physics beyond the Standard Model that introduce additional types of extremely-weak interactions. These interactions can only be observed if they mediate a process, such as proton decay, that is otherwise totally forbidden. So far, no experimental evidence for such processes has been found. However physicists like this idea, since such additional processes are predicted when we try to unify the strong, weak and electromagnetic interactions into a single "Grand Unified Theory." Such unification is suggested by the similarities of the underlying mathematical theories for these very different interactions.
Unification of all four force types, including gravity, is also a goal for particle physics. Gravity has a different mathematical structure, and so far no complete quantum theory of gravity has been developed. String Theory suggests possible answers, but much work remains to be done.
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