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arXiv:2501.03233

Spin and Representations

Wonmyeong Cho

Published Dec 6, 2024• 0 citations
arXivHTMLPDF

Abstract

We derive the representation theory of $SU(2)$ from the expository theory of Lie groups and Lie algebras. Based on this, the mathematics of non-relativistic quantum mechanics of a spin $\frac{1}{2}$ particle are described from a representation-theoretic perspective, and are extended to many particle systems.

DOI
10.48550/arxiv.2501.03233

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The Lie algebra su(2) of the classical group SU(2) is built from two commuting quon algebras for which the deformation parameter is a common root of unity. This construction leads to (i) a not very well-known polar decomposition of the generators J- and J+ of the SU(2) group, with J+ = J-† = HUr where H is Hermitean and Ur unitary, and (ii) an alternative to the {J2,Jz} quantization scheme, viz., the {J2,Ur} quantization scheme. The representation theory of the SU(2) group can be developed in this nonstandard scheme. The key ideas for developing the Wigner-Racah algebra of the SU(2) group in the {J2,Ur} scheme are given. In particular, some properties of the coupling and recoupling coefficients as well as the Wigner-Eckart theorem in the {J2,Ur} scheme are examined in great detail.
Abstract This book provides an introduction to quantum theory, primarily for mathematics students. It assumes a knowledge of basic algebra and elementary group theory, with little or no familiarity with more advanced topics. Although it takes a traditional approach, the book exploits ideas of linear algebra and points out some of the mathematical subtleties of the theory. It also covers such topics as Bell's inequalities and coherent and squeezed states, and introduces group representation theory, algebraic quantum theory, and quantum statistical mechanics. Later chapters discuss relativistic wave equations and elementary particle symmetries from a group-theoretical standpoint.
Spin is what makes the world go 'round. The Groups SO (3) and SU (2) How does SU(2) act on its Lie algebra? For physical and mathematical motivation for this section (which involves nonrelativistic quantum mechanics) we refer the reader to some remarks of Feynman and of Weyl. Specifically, Feynman, in his section entitled “Degeneracy,” shows that a process involving a specific choice of direction in space requires that the process be described not by a single wave function ψ but rather by a multicomponent column vector of wave functions Ψ = (ψ 1 , …, ψ N ) T . He then indicates, roughly speaking, that since the physics cannot depend on the choice of cartesian coordinates ( x 1 , x 2 , x 3 ) of space, the N -tuples must transform under some representation ρ: SO (3) → U ( N ) of the rotation group SO (3) of space. This is not quite accurate; since e i γ Ψ represents the same wave function (when γ is a constant), ρ is only a “ray” representation, ρ( g )ρ( h ) = e i γ( g,h ) ρ( gh ) for a function γ( g, h ). Weyl shows that this can be made into a genuine representation, except that it is (perhaps) double-valued. We shall show in this section that there is a natural 2 : 1 homomorphism π of the special unitary group SU (2) onto SO (3), thus yielding a (perhaps double valued) representation of SU (2) into U ( N ).