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Journal of Mathematical Physics

The symmetry group of the harmonic oscillator and its reduction

Toshihiro Iwai

Published Jun 1, 1982• 14 citations
Publisher

Abstract

The symmetry of the harmonic oscillator is dealt with in the Hamiltonian formalism. Unitary operators representing the symmetry are studied from this point of view. Of additional interest is reduction of the symmetry group SU(4) for the four-dimensional harmonic oscillator. Subspaces are determined from the representation spaces for SU(4) so as to give those for SO(4).

DOI
10.1063/1.525472

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The definition of the symmetry group as a group of unitary operators that commute with the hamiltonian is insufficient. Concepts of maximal, minimal, incomplete, and exce ssive symmetry groups are introduced. A procedure for setting up the minimal symmetry group is proposed. For this purpose it is sufficient to choose a group with the required dimensions of irreducible representations. As an illustration the problem posed by Jauch and Hill is solved. The symmetry group of an anisotropic oscillator is determined. (auth)

•Jun 1, 1963•0 citations
The symmetry of the harmonic oscillator is dealt with in the Hamiltonian formalism. Unitary operators representing the symmetry are studied from this point of view. Of additional interest is reduction of the symmetry group SU(4) for the four-dimensional harmonic oscillator. Subspaces are determined from the representation spaces for SU(4) so as to give those for SO(4).
The formalism of non-Abelian quantum kinematics is applied to the Newtonian symmetry group of the harmonic oscillator. Within the regular ray representation of the group, the Schrödinger operator, as well as two other (new) invariant operators, are obtained as Casimir operators of the extended kinematic algebra. Superselection rules are then introduced, which permit the identification (and the explicit calculation) of the physical states of the system. Next, a complementary ray representation, attached to the space-time realization of the group, casts the Schrödinger operator into the familiar time-dependent space-time differential operator of the harmonic oscillator and thus, by means of the superselection rules, one obtains the time-dependent Schrödinger equation of the sytem. Finally, the evaluation of a Hurwitz invariant integral, over the group manifold, affords the well known Feynman space-time propagator 〈t′,x′‖t,x〉 of the simple harmonic oscillator. Everything comes out from the assumed symmetries of the system. The whole approach is group theoretic and ‘‘relativistic.’’ No classical analog is used in this ‘‘quantization’’ scheme.
In classical mechanics, the system of two coupled harmonic oscillators is shown to possess the symmetry of the Lorentz group O(3,3) applicable to a six-dimensional space consisting of three space-like and three time-like coordinates, or SL(4,r) in the four-dimensional phase space consisting of two position and two momentum variables. In quantum mechanics, the symmetry is reduced to that of O(3,2) or Sp(4), which is a subgroup of O(3,3) or SL(4,r) respectively. It is shown that among the six Sp(4)-like subgroups, only one possesses the symmetry which can be translated into the group of unitary transformations in quantum mechanics. In quantum mechanics, there is the lower bound in the size of phase space for each mode determined by the uncertainty principle while there are no restriction on the phase-space size in classical mechanics. This is the reason why the symmetry is smaller in quantum mechanics.
The conventional quantization of the harmonic oscillator in terms of operators Q and P can be implemented with the help of irreducible unitary representations of the Heisenberg-Weyl group which acts transitively and effectively on the simply connected classical phase space S q,p ≊ 2 . In the description of the harmonic oscillator in terms of angle and action variables ϕ and I the associated phase space S ϕ,I corresponds to the multiply connected punctured plane 2 – {0}, on which the 3-dimensional symplectic group Sp(2, ) acts transitively, leaving the origin invariant. As this group contains the compact subgroup U (1) it has infinitely many covering groups. In the here relevant irreducible unitary representations (positive discrete series) the self-adjoint generator K o of U (l) represents the classical action variable I . It has the possible spectra n + k, n = 0,1,...; k > 0, where k depends on the covering group. This implies different possible spectra for the action variable Hamiltonian ℏωK 0 of the harmonic oscillator. On the other hand, expressing the operators Q and P (non-linearly) in terms of the three generators K 0 etc. of Sp (2, ) leads to the usual framework. Possible physical (experimental) implications and generalizations to higher dimensions are discussed briefly.
The definition of the symmetry group as a group of unitary operators that commute with the hamiltonian is insufficient. Concepts of maximal, minimal, incomplete, and exce ssive symmetry groups are introduced. A procedure for setting up the minimal symmetry group is proposed. For this purpose it is sufficient to choose a group with the required dimensions of irreducible representations. As an illustration the problem posed by Jauch and Hill is solved. The symmetry group of an anisotropic oscillator is determined. (auth)