Hello! Just a short question: I am currently calculating the momentum density of a free dirac wavefunction under boost. However, I get some complex density - can this happen or will it be a mistake on my side? Thanks for your comments! Blue2script PS: In some forum there was a note that momentum density is derived from the real Lagrangian. What does that mean? I just took the momentum density given from the energy-momentum tensor of the normal Dirac Lagrangian.