Rudolf Haag postulated[1] that the interaction picture does not exist in an interacting, relativistic quantum field theory, something now commonly known as Haag's Theorem. The theorem was subsequently proved by a number of different authors. It is, however, inconvenient as in the canonical development of perturbative quantum field theory  which includes quantum electrodynamics  cited as one of the great successes of modern science  the interaction picture is used throughout. Citing the formulation used by Arageorgis[2]: * If two pure ground states are not equal, then they generate unitarily inequivalent irreducible representations. * If two local quantum fields are unitarily equivalent at any given time, then both fields are free if one of them is free. References 1. ^ Haag, R: On quantum field theories, Matematiskfysiske Meddelelser, 29, 12 (1955). 2. ^ Arageorgis, A.: 1995, Fields, Particles, and Curvature: Foundations and Philosophical Aspects of Quantum Field Theory in Curved Spacetime, Ph.D. Thesis, Univ. of Pittsburgh. Further reading * John Earman, Doreen Fraser, Haag's Theorem and Its Implications for the Foundations of Quantum Field Theory, Erkenntnis 64 (2006): 305344, online at philsciarchive * Doreen Fraser, Haagâ€™s Theorem and the Interpretation of Quantum Field Theories with Interactions, PhD thesis, U. of Pittsburgh, online
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