 The inequivalent representations of quantum field theory can be generated by spontaneous symmetry breaking (see the entry on  symmetry and symmetry breaking),  occurring when the ground state (or the vacuum state) of a system is not invariant under the full group of transformations providing the conservation laws for the system. If symmetry breaks down, collective modes are generated (so-called Nambu-Goldstone boson modes), which propagate over the system and introduce long-range correlations in it.