# Bargmann–Wigner equations

This article uses the Einstein summation convention for tensor/spinor indices, and uses hats for quantum operators.

In relativistic quantum mechanics and quantum field theory, the Bargmann–Wigner equations describe free particles of arbitrary spin j, an integer for bosons (j = 1, 2, 3 ...) or half-integer for fermions (j = ​12, ​32, ​52 ...). The solutions to the equations are wavefunctions, mathematically in the form of multi-component spinor fields.

They are named after Valentine Bargmann and Eugene Wigner.

## History

Paul Dirac first published the Dirac equation in 1928, and later (1936) extended it to particles of any half-integer spin before Fierz and Pauli subsequently found the same equations in 1939, and about a decade before Bargman, and Wigner.[1] Eugene Wigner wrote a paper in 1937 about unitary representations of the inhomogeneous Lorentz group, or the Poincaré group.[2] Wigner notes Ettore Majorana and Dirac used infinitesimal operators applied to functions. Wigner classifies representations as irreducible, factorial, and unitary.

In 1948 Valentine Bargmann and Wigner published the equations now named after them in a paper on a group theoretical discussion of relativistic wave equations.[3]

## Statement of the equations

For a free particle of spin j without electric charge, the BW equations are a set of 2j coupled linear partial differential equations, each with a similar mathematical form to the Dirac equation. The full set of equations are[1][4][4][5]

{\displaystyle {\begin{aligned}&\left(-\gamma ^{\mu }{\hat {P}}_{\mu }+mc\right)_{\alpha _{1}\alpha _{1}'}\psi _{\alpha '_{1}\alpha _{2}\alpha _{3}\cdots \alpha _{2j}}=0\\&\left(-\gamma ^{\mu }{\hat {P}}_{\mu }+mc\right)_{\alpha _{2}\alpha _{2}'}\psi _{\alpha _{1}\alpha '_{2}\alpha _{3}\cdots \alpha _{2j}}=0\\&\qquad \vdots \\&\left(-\gamma ^{\mu }{\hat {P}}_{\mu }+mc\right)_{\alpha _{2j}\alpha '_{2j}}\psi _{\alpha _{1}\alpha _{2}\alpha _{3}\cdots \alpha '_{2j}}=0\\\end{aligned}}}

 ${\displaystyle \left(-\gamma ^{\mu }{\hat {P}}_{\mu }+mc\right)_{\alpha _{r}\alpha '_{r}}\psi _{\alpha _{1}\cdots \alpha '_{r}\cdots \alpha _{2j}}=0}$

(1)

for r = 1, 2, ... 2j. (Some authors e.g. Loide and Saar[4] use n = 2j to remove factors of 2. Also the spin quantum number is usually denoted by s in quantum mechanics, however in this context j is more typical in the literature). The entire wavefunction ψ = ψ(r, t) has components

${\displaystyle \psi _{\alpha _{1}\alpha _{2}\alpha _{3}\cdots \alpha _{2j}}(\mathbf {r} ,t)}$

and is a rank-2j 4-component spinor field. Each index takes the values 1, 2, 3, or 4, so there are 42j components of the entire spinor field ψ, although a completely symmetric wavefunction reduces the number of independent components to 2(2j + 1). Further, γμ = (γ0, γ) are the gamma matrices, and

${\displaystyle {\hat {P}}_{\mu }=i\hbar \partial _{\mu }}$

is the 4-momentum operator.

The operator constituting each equation, (−γμPμ + mc) = (−γμμ + mc), is a 4 × 4 matrix, because of the γμ matrices, and the mc term scalar-multiplies the 4 × 4 identity matrix (usually not written for simplicity). Explicitly, in the Dirac representation of the gamma matrices:[1]

{\displaystyle {\begin{aligned}-\gamma ^{\mu }{\hat {P}}_{\mu }+mc&=-\gamma ^{0}{\frac {\hat {E}}{c}}-{\boldsymbol {\gamma }}\cdot (-{\hat {\mathbf {p} }})+mc\\[6pt]&=-{\begin{pmatrix}I_{2}&0\\0&-I_{2}\\\end{pmatrix}}{\frac {\hat {E}}{c}}+{\begin{pmatrix}0&{\boldsymbol {\sigma }}\cdot {\hat {\mathbf {p} }}\\-{\boldsymbol {\sigma }}\cdot {\hat {\mathbf {p} }}&0\\\end{pmatrix}}+{\begin{pmatrix}I_{2}&0\\0&I_{2}\\\end{pmatrix}}mc\\[8pt]&={\begin{pmatrix}-{\frac {\hat {E}}{c}}+mc&0&{\hat {p}}_{z}&{\hat {p}}_{x}-i{\hat {p}}_{y}\\0&-{\frac {\hat {E}}{c}}+mc&{\hat {p}}_{x}+i{\hat {p}}_{y}&-{\hat {p}}_{z}\\-{\hat {p}}_{z}&-({\hat {p}}_{x}-i{\hat {p}}_{y})&{\frac {\hat {E}}{c}}+mc&0\\-({\hat {p}}_{x}+i{\hat {p}}_{y})&{\hat {p}}_{z}&0&{\frac {\hat {E}}{c}}+mc\\\end{pmatrix}}\\\end{aligned}}}

where σ = (σ1, σ2, σ3) = (σx, σy, σz) is a vector of the Pauli matrices, E is the energy operator, p = (p1, p2, p3) = (px, py, pz) is the 3-momentum operator, I2 denotes the 2 × 2 identity matrix, the zeros (in the second line) are actually 2 × 2 blocks of zero matrices.

The above matrix operator contracts with one bispinor index of ψ at a time (see matrix multiplication), so some properties of the Dirac equation also apply to the BW equations:

${\displaystyle E^{2}=(pc)^{2}+(mc^{2})^{2}}$

Unlike the Dirac equation, which can incorporate the electromagnetic field via minimal coupling, the B–W formalism comprises intrinsic contradictions and difficulties when the electromagnetic field interaction is incorporated. In other words, it is not possible to make the change PμPμeAμ, where e is the electric charge of the particle and Aμ = (A0, A) is the electromagnetic four-potential.[6][7] An indirect approach to investigate electromagnetic influences of the particle is to derive the electromagnetic four-currents currents and multipole moments for the particle, rather than include the interactions in the wave equations themselves.[8][9]

## Lorentz group structure

The representation of the Lorentz group for the BW equations is[6]

${\displaystyle D^{\mathrm {BW} }=\bigotimes _{r=1}^{2j}\left[D_{r}^{(1/2,0)}\oplus D_{r}^{(0,1/2)}\right]\,.}$

where each Dr is an irreducible representation. This representation does not have definite spin unless j equals 1/2 or 0. One may perform a Clebsch–Gordan decomposition to find the irreducible (A, B) terms and hence the spin content. This redundancy necessitates that a particle of definite spin j that transforms under the DBW representation satisfies field equations.

The representations D(j, 0) and D(0, j) can each separately represent particles of spin j. A state or quantum field in such a representation would satisfy no field equation except the Klein-Gordon equation.

## Formulation in curved spacetime

Following M. Kenmoku,[10] in local Minkowski space, the gamma matrices satisfy the anticommutation relations:

${\displaystyle [\gamma ^{i},\gamma ^{j}]_{+}=2\eta ^{ij}I_{4}}$

where ηij = diag(−1, 1, 1, 1) is the Minkowski metric. For the Latin indices here, i, j = 0, 1, 2, 3. In curved spacetime they are similar:

${\displaystyle [\gamma ^{\mu },\gamma ^{\nu }]_{+}=2g^{\mu \nu }}$

where the spatial gamma matrices are contracted with the vierbein biμ to obtain γμ = biμ γi, and gμν = biμbiν is the metric tensor. For the Greek indices; μ, ν = 0, 1, 2, 3.

A covariant derivative for spinors is given by

${\displaystyle {\mathcal {D}}_{\mu }=\partial _{\mu }+\Omega _{\mu }}$

with the connection Ω given in terms of the spin connection ω by:

${\displaystyle \Omega _{\mu }={\frac {1}{4}}\partial _{\mu }\omega ^{ij}(\gamma _{i}\gamma _{j}-\gamma _{j}\gamma _{i})}$

The covariant derivative transforms like ψ:

${\displaystyle {\mathcal {D}}_{\mu }\psi \rightarrow D(\Lambda ){\mathcal {D}}_{\mu }\psi }$

With this setup, equation (1) becomes:

{\displaystyle {\begin{aligned}&(-i\hbar \gamma ^{\mu }{\mathcal {D}}_{\mu }+mc)_{\alpha _{1}\alpha _{1}'}\psi _{\alpha '_{1}\alpha _{2}\alpha _{3}\cdots \alpha _{2j}}=0\\&(-i\hbar \gamma ^{\mu }{\mathcal {D}}_{\mu }+mc)_{\alpha _{2}\alpha _{2}'}\psi _{\alpha _{1}\alpha '_{2}\alpha _{3}\cdots \alpha _{2j}}=0\\&\qquad \vdots \\&(-i\hbar \gamma ^{\mu }{\mathcal {D}}_{\mu }+mc)_{\alpha _{2j}\alpha '_{2j}}\psi _{\alpha _{1}\alpha _{2}\alpha _{3}\cdots \alpha '_{2j}}=0\,.\\\end{aligned}}}

## References

### Notes

1. ^ a b c E.A. Jeffery (1978). "Component Minimization of the Bargman–Wigner wavefunction" (PDF). Australian Journal of Physics. 31 (2): 137. Bibcode:1978AuJPh..31..137J. doi:10.1071/ph780137.
2. ^ E. Wigner (1937). "On Unitary Representations Of The Inhomogeneous Lorentz Group" (PDF). Annals of Mathematics. 40 (1): 149–204. Bibcode:1939AnMat..40..149W. doi:10.2307/1968551. JSTOR 1968551.
3. ^ Bargmann, V.; Wigner, E. P. (1948). "Group theoretical discussion of relativistic wave equations". Proceedings of the National Academy of Sciences of the United States of America. 34 (5): 211–23. Bibcode:1948PNAS...34..211B. doi:10.1073/pnas.34.5.211. PMC 1079095. PMID 16578292.
4. ^ a b c R.K Loide; I.Ots; R. Saar (2001). "Generalizations of the Dirac equation in covariant and Hamiltonian form". Journal of Physics A. 34 (10): 2031–2039. Bibcode:2001JPhA...34.2031L. doi:10.1088/0305-4470/34/10/307.
5. ^ H. Shi-Zhong; R. Tu-Nan; W. Ning; Z. Zhi-Peng (2002). "Wavefunctions for Particles with Arbitrary Spin". Communications in Theoretical Physics. 37 (1): 63. Bibcode:2002CoTPh..37...63H. doi:10.1088/0253-6102/37/1/63.
6. ^ a b T. Jaroszewicz; P.S Kurzepa (1992). "Geometry of spacetime propagation of spinning particles". Annals of Physics. 216 (2): 226–267. Bibcode:1992AnPhy.216..226J. doi:10.1016/0003-4916(92)90176-M.
7. ^ C.R. Hagen (1970). "The Bargmann–Wigner method in Galilean relativity". Communications in Mathematical Physics. 18 (2). pp. 97–108. Bibcode:1970CMaPh..18...97H. doi:10.1007/BF01646089.
8. ^ Cédric Lorcé (2009). "Electromagnetic Properties for Arbitrary Spin Particles: Part 1 − Electromagnetic Current and Multipole Decomposition". arXiv:0901.4199 [hep-ph].
9. ^ Cédric Lorcé (2009). "Electromagnetic Properties for Arbitrary Spin Particles: Part 2 − Natural Moments and Transverse Charge Densities". Physical Review D. 79 (11): 113011. arXiv:0901.4200. Bibcode:2009PhRvD..79k3011L. doi:10.1103/PhysRevD.79.113011.
10. ^ K. Masakatsu (2012). "Superradiance Problem of Bosons and Fermions for Rotating Black Holes in Bargmann–Wigner Formulation". arXiv:1208.0644 [gr-qc].