The spin operator s = (ħ/2) σ in the Pauli equation fulfills the commutation relation of the angular momentum and leads to half-integer eigenvalues of the eigenfunctions for s. If one tries to express s by canonically conjugated operators Φ and π = (ħ/i) ∂/∂Φ the formal angular momentum term s = Φ X π fails because it leads only to whole-integer eigenvalues. However, the modification of this term in the form s = 1/2 {π + Φ(Φ π) + Φ X π} leads to the required result. The eigenfunction system belonging to this differential operator s(Φ π) consists of (2s + 1) spin eigenfunctions ξm (Φ) which are given explicitly. They form a basis for the wave functions of a particle of spin s. Applying this formalism to particles with s = 1/2, agreement is reached with Pauli’s spin theory. The function s(Φ π) follows from the theory of rotating rigid bodies. The continuous spinvariable Φ = ((Φx , Φy, Φz) can be interpreted classically as a “turning vector” which defines the orientation in space of a rigid body. Φ is the positioning coordinate of the rigid body or the spin coordinate of the particle in analogy to the cartesian coordinate x. The spin s is a vector fixed to the body.
The spin operator s = (ħ/2) σ in the Pauli equation fulfills the commutation relation of the angular momentum and leads to half-integer eigenvalues of the eigenfunctions for s. If one tries to express s by canonically conjugated operators Φ and π = (ħ/i) ∂/∂Φ the formal angular momentum term s = Φ X π fails because it leads only to whole-integer eigenvalues. However, the modification of this term in the form s = 1/2 {π + Φ(Φ π) + Φ X π} leads to the required result. The eigenfunction system belonging to this differential operator s(Φ π) consists of (2s + 1) spin eigenfunctions ξm (Φ) which are given explicitly. They form a basis for the wave functions of a particle of spin s. Applying this formalism to particles with s = 1/2, agreement is reached with Pauli’s spin theory. The function s(Φ π) follows from the theory of rotating rigid bodies. The continuous spinvariable Φ = ((Φx , Φy, Φz) can be interpreted classically as a “turning vector” which defines the orientation in space of a rigid body. Φ is the positioning coordinate of the rigid body or the spin coordinate of the particle in analogy to the cartesian coordinate x. The spin s is a vector fixed to the body.
Abstract Within a nonrelativistic framework, spin is generally included as an intrinsic angular momentum. It is proposed here that consistent results can be obtained with a spatial wavefunction of an oriented complex exponential e ∓ i a ˆ φ , where the bivector i a ˆ is written in terms of the pseudoscalar i = e x e y e z and the axis of rotation given by the unit vector a ˆ , which is generally an (effective) magnetic field direction (this can also be written as e ∓ i σ · a ˆ φ in terms of the Pauli vector σ ). The wavefunction is a multivector and not a complex scalar. The signs in the exponential correspond to the two directions of rotation around the axis. The transformation properties of these wavefunctions are given by the Pauli spinors. Spin can be viewed as a zero-point rotation arising from the noncommutativity of the momentum and the vector potential.
Abstract A novel representation of spin 1/2 combines in a single geometric object the standard Pauli spin operator and spin state. Under the spin-position decoupling approximation it consists of the sum of three orthogonal vectors comprising a gauge phase. In the one-spin case the representation: (1) is Hermitian; (2) endowed with handedness; (3) yields all standard results, including the total spin angular momentum; (4) relates basis spins by proper rotations, thus preserving handedness; (5) allows formalizing irreversibility in spin measurement. In the bipartite case: (1) entangled spins have precisely related gauge phases and opposite handedness; (2) maximally entangled spins relate by one of the four improper rotations in 3D: plane-reflections (triplets) and inversion (singlet); (3) the full spin expressions yield the standard total two-spin angular momentum; (4) all standard expected values for bipartite observations follow. Depending on whether spin operations act one – or two – sided, the formalism appears in two complementary forms, the ‘spinor’ or the ‘vector’ form, respectively. The proposed scheme provides a clear geometric picture of spin transformations and correlations in the 3D physical orientation space.
A novel representation of spin 1/2 combines in a single geometric object the standard Pauli spin operator and spin state. Under the spin-position decoupling approximation it consists of the sum of three orthogonal vectors comprising a gauge phase. In the one-spin case the representation: (1) is Hermitian; (2) endowed with handedness; (3) yields all standard results, including the total spin angular momentum S=(√3 ℏ)⁄2; (4) relates basis spins by proper rotations, thus preserving handedness; (5) allows formalizing irreversibility in spin measurement. In the bipartite case: (1) entangled spins have precisely related gauge phases and opposite handedness; (2) maximally entangled spins relate by one of the four improper rotations in 3D: plane-reflections (triplets) and inversion (singlet); (3) the full spin expressions yield the standard total two-spin angular momentum; (4) all standard expected values for bipartite observations follow. Depending on whether spin operations act one – or two – sided, the formalism appears in two complementary forms, the ‘spinor’ or the ‘vector’ form, respectively. The proposed scheme provides a clear geometric picture of spin transformations and correlations in the 3D physical orientation space.
Classical dynamics of rigid bodies The difficulties in formulating the theory of spin ½ systems outlined at the end of the last chapter may be avoided if we start from a classical theory of rotation in which the translational and orientational coordinates enter on a symmetrical footing, and pass to the quantum theory via the standard canonical quantization procedure. The result is not simply the Pauli theory written in a different matrix representation but a more general physical theory which explicitly incorporates in the wave equation the extended character of the bodies considered, through moment-of-inertia-dependent terms. As expected, the spin ½ character of the wave is manifested through the dependence of the body motion on the total quantum state via quantum potential and torque terms. Spin is not usually approached from this angle in basic quantum mechanics texts so it is worthwhile to build up the theory from scratch (for a more rigorous treatment see Normand (1980) and for further details: Bopp & Haag (1950), Rosen (1951, 1984), van Winter (1954), Dankel (1970), Holland (1988a, b)). We begin by reviewing some basic results in the classical canonical theory of rigid body motion. Our treatment is confined to a rigid homogeneous sphere with moment of inertia I about any axis passing through its centre, where the centre of mass is located. The sphere carries a rigid charge and a magnetic moment. By Chasle's theorem an arbitrary displacement of a rigid body is composed of a translation together with a rotation about a line fixed in the body.
In Part 0 (Parts I and II deal with spin) of this note, we argued that one may begin with two flow equations: d/dx (partial) A(x,t) = p ((1a)) and d/dt (partial) A(x,t) = -E ((1b)) and find that A(x,t) is the classical action for a free particle i.e A(x,t)= L(v) t where L is the classical Lagranian. In particular, after taking the partial derivatives in ((1a)) and ((1b)) one may set x/t=v. Alternatively, one may link ((1a)) and ((1b)) to eigenvalue equations i.e. id/dt exp(i A) = E exp(iA) ((2a)) and -id/dx exp(iA) = p exp(iA) ((2b)). Thus operators appear, in particular one has the momentum operator -id/dx associated with px etc.. These two equations may be incorporated in EE=pp+mm or E= pp/2m (although E and p have different meanings) to obtain the Klein-Gordon and Schrodinger equations. At this point there is no spin. We argued in Part I that spin may be brought in by considering p in ((1a)) as flowing in 3 directions px, py and pz. In such a case one would normally have pi ei where ei is a basis vector, but with the operator equations being used ((2a)) ((2b)), ei is replaced by a basis operator ai (a matrix) such that when pi ai is squared, ai disappears and one obtains pipi = -d/dx d/dx - d/dy d/dy-d/dz d/dz As P. Dirac has shown this leads to 4x4 matrices containing the 2x2 Pauli matrices. As a result, the independent px,py,pz flow is only linked to spin 1/2 . In part II we discussed other spins. Other spin matrices follow different constraints. In this note, we wish to link the idea of basis operators ai with the idea of angular momentum i.e. rxp. In (1), the spin of a fermion is calculated using current from the Dirac equation J (vector) together with r x J i.e. the classical form of angular momentum. In this note, we wish to show the link between this approach and the basis operator one, which does not on the surface, bring in any idea of angular momentum, are the same.
The recently established existence of spherical harmonic functions, Yℓm(θ,ϕ) for half-odd-integer values of ℓ and m, allows for the introduction into quantum chemistry of explicit electron spin-coordinates; i.e. spherical polar angles θs,ϕs, that specify the orientation of the spin angular momentum vector in space. In this coordinate representation the spin angular momentum operators, S2,Sz, are represented by the usual differential operators in spherical polar coordinates (commonly used for L2,Lz), and their electron-spin eigenfunctions are sinθsexp(±ϕs/2). This eigenfunction representation has the pedagogical advantage over the abstract spin eigenfunctions, α,β, that ``integration over spin coordinates'' is a true integration (over the angles θs,ϕs). In addition they facilitate construction of many electron wavefunctions in which the electron spins are neither parallel nor antiparallel, but inclined at an intermediate angle. In particular this may have application to the description of EPR correlation experiments.