Hello, If I understand well, since a Lorentz transformation applied on a particle induces a Wigner rotation which depends on the momentum, the spin reduced density matrix that is naively done by tracing out the momentum has no (Lorentz) transformation law. Only the overall system can be Lorentz-transformed correctly. But what I don't understand is the difference between massive and massless particles, i.e. my former statement seems to be actually only a problem for photons and not for massive particles. Why ? Thanks a lot for your help!