  In gauge quantum field theories, which are our best contemporary physical theories of all the forces other than gravity, each theory is associated with a different symmetry group, and the unification of theories was achieved by looking for theoretical structures with the relevant combined symmetry. (For example, the unitary group U(1) for quantum electrodynamics, U(2) for the unified electroweak theory and SU(3)/ Z(3) for the strong interaction.) Lyre argues for OSR in the interpretation of quantum field theory. He argues that “the traditional picture of spatiotemporally fixed object-like entities is undermined by the ontology of gauge theories in various ways and that main problems with traditional scientific realism…can be softened by a commitment to the structural content of gauge theories, in particular to gauge symmetry groups” (2004, 666). He goes on to note that his favoured interpretation of gauge theories (in terms of non-separable holonomies) is one according to which the fundamental objects are ontologically secondary to structure because the objects of a theory are members of equivalence classes under symmetry transformations and no further individuation of objects is possible. Similarly, Kantorovich (2003) argues that the symmetries of the strong force are ontologically prior to the particles that feel that force, namely the hadrons, and likewise for the symmetries of the so-called ‘grand unification’ of particle physics in the standard model. Cao in his book on quantum field theory sometimes sounds like an ontic structural realist, because he denies that the structures postulated by field theories must be “ontologically supported by unobservable entities” (Cao 1997, 5). However, in his (2003a) he explicitly criticises OSR and argues for a version of ESR in the context of a discussion of quantum field theory.