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 [Phys.Rev.B 103, L100409 (2021)] 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 ∫d3x(r×E) for a free electromagnetic
wave, which are conserved quantities during propagation in vacuum.Comment: 8 pages, no figure