1. J.D. Louck and L.C. Biedenharn, J. Math. Phys. 11, 2368 (1970).
2. W. Pauli, Springer Tracts in Modern Physics 37, 85 (1965).
3. Indices can be raised or lowered using the metric g
μν = g
μν = diag(+1, −1, −1, −1), μ = 0,1,2,3. ∈μναβ is the completely antisymmetric Levi-Civita symbol with ∈0123 = +1. Repeated indices are summed over.
4. E.P. Wigner, Ann. Math. 40, 149 (1939).
5. The case P
μ light-like, W
μ time-like is incompatible with Eq. (1).