|MadSci Network: Physics|
Most physicists believe that there was only one single force present in the very early universe, immediately after the big bang. In the course of time, the original symmetry was more and more broken. Finally, we end up with the four forces we see, which you have named: electromagnetic force, strong and weak nuclear force and gravity. They are effective forces, resulting from a unified force by symmetry breaking happening at some specific energy scale. So we want to construct a force unifying all 4 forces such that the resulting theory is well defined and selfconsistent and that we get back all the other forces in certain limits. For more information see e.g. fundamental force concept or forces and unification.
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