Haag's Theorem and Observer Equivariance: Representation, Locality, and Structural Objectivity in Quantum Field Theory
Gustaf Ullman
This paper interprets Haag’s theorem as more than a technical obstruction to the interaction picture in relativistic quantum field theory. It argues that the theorem reveals a deeper structural limitation of global representation-based ontology: an interacting QFT cannot, under the assumptions of the theorem, be understood as a simple unitary deformation of a free theory within one fixed Fock representation. The paper connects this lesson to algebraic quantum field theory, superselection sectors, locally covariant QFT, Wilsonian renormalization, and effective field theory. It proposes that these developments all point toward a common principle: physical objectivity is not grounded in a privileged representation, but in structural content preserved under admissible transformations between perspectives, representations, sectors, scales, or regions. The paper does not claim to solve Haag’s theorem or construct interacting QFTs. Its aim is interpretive: to show how Haag’s theorem is naturally intelligible within the observer-equivariant framework, where objectivity is understood as invariant or equivariant structure rather than as a view from one absolute representation.
•May 5, 2026•0 citations