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Journal of Mathematical Physics

Symmetry in Quantum Theory

Gérard G. Emch, C. Piron

Published Apr 1, 1963• 43 citations
Publisher

Abstract

The aim of this note is to give a precise definition of symmetry in quantum theory in order to generalize Wigner's representation theorem, in the framework of lattice theory and projective geometry. We do not require the concept of physical state; the results are valid for any field (division ring) used in the realization of the lattice of propositions. Physical systems with the most general superselection rules are included in the theory.

DOI
10.1063/1.1703978

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•Mar 20, 1997•0 citations
The aim of this note is to give a precise definition of symmetry in quantum theory in order to generalize Wigner's representation theorem, in the framework of lattice theory and projective geometry. We do not require the concept of physical state; the results are valid for any field (division ring) used in the realization of the lattice of propositions. Physical systems with the most general superselection rules are included in the theory.
A generalization of Wigner's representation theorem is presented that may be applied when superselection rules (of a specified character) must be taken into account. At the same time some gaps in existing proofs of Wigner's theorem are eliminated. The connection between Wigner,s theorem and the so called fundamental theorem of projective geometry is pointed out, and a new, short proof of the latter theorem (valid for an arbitrary field of numbers) is presented. The equivalent possibilities of characterizing symmetry transformations as mappings leaving invariant the logical structure of quantum mechanics, as isometric linear or conjugated linear transformations connecting two complex Hilbert spaces, or as mappings leaving invariant the algebraic structure of quantum mechanics (or equivalently the structure of the equations of motion) are discussed and lead to an invariant (coordinate free) characterization of reversal of the direction of motion (time inversion). An analogue of Wigner's theorem for symmetry transformations in the recently discussed quaternion quantum mechanics is proved in an appendix, and it is pointed out how quaternion quantum mechanics, as well as ordinary complex quantum mechanics, may be consistently represented in the framework of real quantum mechanics, which in this sense appears as a super theory. The symmetry transformationsmore » of classical mechanics are discussed in relation to their quantum mechanical analogues in another appendix. They are intimately connected with the canonical formalism. (auth)« less
It is proposed that an unconstrained quantum system is completely specified by its configuration space CCC and its group of symmetries GGG. It is then shown that the theory of projective unitary representations of GGG on a certain Hilbert space determined by CCC and GGG leads to a definitive resolution of supposed ambiguities relating to superselection rules, choice of wave functions, topological properties of GGG and CCC, anomalies, etc.
This paper is concerned with the relationship between the group G of all symmetries of an unconstrained dynamical system and the space of its quantum states. The chief aim is to establish the validity of the claim that "quantizing a system" means deciding what G is and determining all projective unitary representations of G. The mathematical tool suited to this purpose is the theory of central extensions of an arbitrary group G by the circle group T (and the closely related cohomology group H2 (G, T)). The fundamental structural property is that there is a universal state space [Formula: see text] embedding all states, a unitary representation space of the universal central extension [Formula: see text] of G, on which the Pontryagin dual H2 (G, T)⋆ of H2 (G, T) operates as a group of su-perselection rules, so that the superselection sectors are [Formula: see text] and observables are self-adjoint operators Vh → Vh. This leads to a classification of time-evolution operators respecting the symmetries by H2 (G, T) and a general understanding of "anomalies" as arising from a mismatch of sectors to which symmetry transformations and time-evolution refer. For the special case of a Lie group G of symmetries, [Formula: see text] of G coincides with [Formula: see text] the universal covering group, if G is semisimple. More generally, depending on whether or not G is semisimple and/or simply connected, inequivalent quantizations are of topological, algebraic or "mixed" origin; in the algebraic case, the relationship between anomalous conservation laws and invariant dynamics is explicitly given. Illustrative examples worked out include the Euclidean group E (d) and the (connected) Poincaré group P (d, 1) for arbitrary spatial dimension d and the discrete groups (Z/N)2 and Z2 appropriate for planar square crystals — in particular, the absence of "fractional" angular momentum in the plane is established and the difficulties specific to (2,1)-dimensional quantum field theories precisely formulated. The paper concludes by noting that a number of ambiguities generally supposed to affect traditional procedures of quantization resolve themselves satisfactorily in this approach. Mathematically, the paper is self-contained as far as the concepts and results are concerned; occasionally, proofs of well-known facts are reworked when they serve to illustrate the value of the symmetry point of view. The theory of universal central extensions of groups, including important special cases, is treated in detail in an appendix by M. S. Raghunathan, in view of its significance for quantization and the absence of a similar account in the literature.
By the example of electron mesoscopic systems, we show the impossibility of constraints of the quantum principle of superposition imposed by the superselection rule. This rule was introduced by Wick, Wightman, and Wigner in order to avoid the violation of Lorentz invariance due to the absence of physical invariance under rotations by an angle of 2π in states which are a coherent superposition of states with an even and odd number of fermions. We describe a mesoscopic system (a semiconductor double quantum dot at low temperatures) where such superpositions are realized; this is confirmed by experiments. We suggest a new experiment which explicitly demonstrates the absence of physical invariance under rotations by an angle of 2π. We note that an alternative to the superselection rule is the existence (along with x, y, z , and t ) of additional spinor (Grassmann) dimensions of spacetime introduced in quantum field theory for realization of supersymmetry. It is proved that additional dimensions are real; their physical meaning is clarified for nonrelativistic systems of fermions.
Abstract Physical systems often have some kind of obvious symmetry, such as the rotational symmetry of the central force field about a nucleus. Mathemati cally this means that the system is invariant under the action of a symmetry group G. (The main ideas of group theory are reviewed in Appendix Al.) Such a physical symmetry must also be present in quantum theory in a way that is compatible with the other important structures, namely the vector spaces and inner products. The simplest way in which this can happen is for G to act on the inner product space 1i by unitary transformations, since these respect both the linear structure and the inner product.