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##J^{\mu\nu}## generates rotations and measures the spin angular momentum along the ##i##-direction of a particle when it acts on the ##i##-th direction spin angular momentum eigenstate of a particle.

The Pauli-Lubanski psuedo-vector is given by ##W_{\mu}=\frac{1}{2}\epsilon_{\mu\sigma\rho\tau}J^{\sigma\rho}P^{\tau}## such that ##W_{\mu}|P,j,j_{z}\rangle = -mJ_{i}|P,j,j_{z}\rangle,## where ##|P,j,j_{z}\rangle## is a momentum space eigenstate representing a particle of spin ##j## at rest with ##P^{\mu}=(m,0,0,0)## and ##m\neq 0##.

**What is the physical interpretation of ##W_{\mu}##?**

To prove that ##[J_{\mu\nu},W^{2}]=0##, an explicit form of ##[J_{\mu\nu},W_{\rho}]## is necessary. One way to obtain ##[J_{\mu\nu},W_{\rho}]## is to define ##I=\frac{i} {8}\epsilon_{\alpha\beta\gamma\delta}J^{\alpha\beta}J^{\gamma\delta}## and show that ##W_{\rho}=[I,P_{\rho}]## and ##[J_{\mu\nu},I]=0##.

**What is an easy way to show that ##[J_{\mu\nu},I]=0## using the epsilon symbol in ##I##?**

**##I## is a scalar as all the indices are ##0##, so why can not say that ##[J_{\mu\nu},W^{2}]=0## trivially?**