Bargmann–Wigner equations

In relativistic quantum mechanics and quantum field theory, the Bargmann–Wigner equations describe free particles with non-zero mass and arbitrary spin Template:Math, an integer for bosons (Template:Math) or half-integer for fermions (Template:Math). 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. Eugene Wigner wrote a paper in 1937 about unitary representations of the inhomogeneous Lorentz group, or the Poincaré group. 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.

Statement of the equations

For a free particle of spin Template:Math without electric charge, the BW equations are a set of Template:Math coupled linear partial differential equations, each with a similar mathematical form to the Dirac equation. The full set of equations are:Template:NoteTag

$\begin{array}{cc}& {(-{\gamma }^{\mu }{\hat{P}}_{\mu }+mc)}_{{\alpha }_1{{\alpha }_1}^{\prime }}{\psi }_{\alpha {\text{'}}_1{\alpha }_2{\alpha }_3\cdots {\alpha }_{2j}}=0\\ & {(-{\gamma }^{\mu }{\hat{P}}_{\mu }+mc)}_{{\alpha }_2{{\alpha }_2}^{\prime }}{\psi }_{{\alpha }_1\alpha {\text{'}}_2{\alpha }_3\cdots {\alpha }_{2j}}=0\\ & ⋮\\ & {(-{\gamma }^{\mu }{\hat{P}}_{\mu }+mc)}_{{\alpha }_{2j}\alpha {\text{'}}_{2j}}{\psi }_{{\alpha }_1{\alpha }_2{\alpha }_3\cdots \alpha {\text{'}}_{2j}}=0\\ \end{array}$

which follow the pattern;

for Template:Math. (Some authors e.g. Loide and Saar use Template:Math to remove factors of 2. Also the spin quantum number is usually denoted by Template:Math in quantum mechanics, however in this context Template:Math is more typical in the literature). The entire wavefunction Template:Math has components

${\psi }_{{\alpha }_1{\alpha }_2{\alpha }_3\cdots {\alpha }_{2j}}(𝐫,t)$

and is a rank-2j 4-component spinor field. Each index takes the values 1, 2, 3, or 4, so there are Template:Math components of the entire spinor field Template:Math, although a completely symmetric wavefunction reduces the number of independent components to Template:Math. Further, Template:Math are the gamma matrices, and

${\hat{P}}_{\mu }=i\hslash {\partial }_{\mu }$

is the 4-momentum operator.

The operator constituting each equation, Template:Math, is a Template:Nowrap matrix, because of the Template:Math matrices, and the Template:Math term scalar-multiplies the Template:Nowrap identity matrix (usually not written for simplicity). Explicitly, in the Dirac representation of the gamma matrices:

$\begin{array}{cc}-{\gamma }^{\mu }{\hat{P}}_{\mu }+mc& =-{\gamma }^0\frac{\hat{E}}{c}-\mathit{\gamma }\cdot (-\widehat{𝐩})+mc\\ [6pt]& =-\left(\begin{array}{cc}{I}_2& 0\\ 0& -I_2\end{array}\right)\frac{\hat{E}}{c}+\left(\begin{array}{cc}0& \mathit{\sigma }\cdot \widehat{𝐩}\\ -\mathit{\sigma }\cdot \widehat{𝐩}& 0\end{array}\right)+\left(\begin{array}{cc}{I}_2& 0\\ 0& I_2\end{array}\right)mc\\ [8pt]& =\left(\begin{array}{cccc}-\frac{\hat{E}}{c}+mc& 0& {\hat{p}}_z& {\hat{p}}_x-i{\hat{p}}_y\\ 0& -\frac{\hat{E}}{c}+mc& {\hat{p}}_x+i{\hat{p}}_y& -{\hat{p}}_z\\ -{\hat{p}}_z& -({\hat{p}}_x-i{\hat{p}}_y)& \frac{\hat{E}}{c}+mc& 0\\ -({\hat{p}}_x+i{\hat{p}}_y)& {\hat{p}}_z& 0& \frac{\hat{E}}{c}+mc\end{array}\right)\\ \end{array}$

where Template:Math is a vector of the Pauli matrices, E is the energy operator, Template:Math is the 3-momentum operator, Template:Math denotes the Template:Nowrap identity matrix, the zeros (in the second line) are actually Template:Nowrap blocks of zero matrices.

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

• the equations are Lorentz covariant,
• all components of the solutions Template:Math also satisfy the Klein–Gordon equation, and hence fulfill the relativistic energy–momentum relation,

$E^2=(pc{)}^2+(m{c}^2{)}^2$

• second quantization is still possible.

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 Template:Math, where Template:Math is the electric charge of the particle and Template:Math is the electromagnetic four-potential. An indirect approach to investigate electromagnetic influences of the particle is to derive the electromagnetic four-currents and multipole moments for the particle, rather than include the interactions in the wave equations themselves.

Lorentz group structure

The representation of the Lorentz group for the BW equations is

$D^{\mathrm{B}\mathrm{W}}=\underset{r=1}{\overset{2j}{⨂}}[D_r^{(1/2,0)}\oplus D_r^{(0,1/2)}].$

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

The representations Template:Math and Template:Math can each separately represent particles of spin Template:Math. 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, in local Minkowski space, the gamma matrices satisfy the anticommutation relations:

$[{\gamma }^i,{\gamma }^j{]}_{+}=2{\eta }^{ij}{I}_4$

where Template:Math is the Minkowski metric. For the Latin indices here, Template:Math. In curved spacetime they are similar:

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

where the spatial gamma matrices are contracted with the vierbein Template:Math to obtain Template:Math, and Template:Math is the metric tensor. For the Greek indices; Template:Math.

A covariant derivative for spinors is given by

${𝒟}_{\mu }={\partial }_{\mu }+{\mathrm{\Omega }}_{\mu }$

with the connection Template:Math given in terms of the spin connection Template:Math by:

${\mathrm{\Omega }}_{\mu }=\frac{1}{4}{\partial }_{\mu }{\omega }^{ij}({\gamma }_i{\gamma }_j-{\gamma }_j{\gamma }_i)$

The covariant derivative transforms like Template:Math:

${𝒟}_{\mu }\psi \to D(\mathrm{\Lambda }){𝒟}_{\mu }\psi$

With this setup, equation (Template:EquationNote) becomes:

$\begin{array}{cc}& (-i\hslash {\gamma }^{\mu }{𝒟}_{\mu }+mc{)}_{{\alpha }_1{{\alpha }_1}^{\prime }}{\psi }_{\alpha {\text{'}}_1{\alpha }_2{\alpha }_3\cdots {\alpha }_{2j}}=0\\ & (-i\hslash {\gamma }^{\mu }{𝒟}_{\mu }+mc{)}_{{\alpha }_2{{\alpha }_2}^{\prime }}{\psi }_{{\alpha }_1\alpha {\text{'}}_2{\alpha }_3\cdots {\alpha }_{2j}}=0\\ & ⋮\\ & (-i\hslash {\gamma }^{\mu }{𝒟}_{\mu }+mc{)}_{{\alpha }_{2j}\alpha {\text{'}}_{2j}}{\psi }_{{\alpha }_1{\alpha }_2{\alpha }_3\cdots \alpha {\text{'}}_{2j}}=0.\\ \end{array}$

See also

• Two-body Dirac equation
• Generalizations of Pauli matrices
• Wigner D-matrix
• Weyl–Brauer matrices
• Higher-dimensional gamma matrices
• Joos–Weinberg equation, alternative equations which describe free particles of any spin
• Higher-spin theory

Further reading

Books

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Selected papers

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External links

Relativistic wave equations:

• Dirac matrices in higher dimensions, Wolfram Demonstrations Project
• Learning about spin-1 fields, P. Cahill, K. Cahill, University of New MexicoTemplate:Dead link
• Field equations for massless bosons from a Dirac–Weinberg formalism, R.W. Davies, K.T.R. Davies, P. Zory, D.S. Nydick, American Journal of Physics
• Quantum field theory I, Martin Mojžiš Template:Webarchive
• The Bargmann–Wigner Equation: Field equation for arbitrary spin, FarzadQassemi, IPM School and Workshop on Cosmology, IPM, Tehran, Iran

Lorentz groups in relativistic quantum physics:

• Representations of Lorentz Group, indiana.edu
• Appendix C: Lorentz group and the Dirac algebra, mcgill.caTemplate:Dead link
• The Lorentz Group, Relativistic Particles, and Quantum Mechanics, D. E. Soper, University of Oregon, 2011
• Representations of Lorentz and Poincaré groups, J. Maciejko, Stanford University
• Representations of the Symmetry Group of Spacetime, K. Drake, M. Feinberg, D. Guild, E. Turetsky, 2009