More on Difference between Angular Momentum and Pseudo-angular Momentum
We extend the discussion on the difference between angular momentum and pseudo-angular momentum in field theory.We show that the often quoted expressions in the paper of Streib only apply to a non-linear system,and derive the correct rotation symmetry and the corresponding angular momentum for a linear elastic system governed by Navier-Cauchy equation.By mapping the concepts and methods for the elastic wave into electromagnetic theory,we argue that the renowned canonical and Benlinfante angular momentum of light are actually pseudo-angular momentum.Then,we derive the"Newtonian"momentum ∫ d3xE and angular momentum ∫ d3 x(r × E)for a free electromagnetic wave,which are conserved quantities during propagation in vacuum.
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