polarization vector

— A massive spin 1 particle has three degrees of polarization for the obvious reason that in its rest frame its spin vector can point in three different directions. The three polarization vectors #92;epsilon#95;#92;mu#123;(a)#125; are simply the three unit vectors pointing along the x, y, and z axes, respectively (a = 1, 2, 3): #92;epsilon#95;#92;mu#123;(1)#125;#61;(0,1,0,0), #92;epsilon#95;#92;mu#123;(2)#125;#61;(0,0,1,0), #92;epsilon#95;#92;mu#123;(3)#125;#61;(0,0,0,1). In the rest frame k#92;mu#61;(m,0,0,0) and so k#92;mu#92;epsilon#95;#92;mu#123;(a)#125;#61;0 Since this is a Lorentz invariant equation, it holds for a moving spin 1 particle as well.