It is well known that in gauge theories, the spin (and orbital angular
momentum) of gauge particles is not gauge invariant, although the helicity is;
neither are the canonical momentum and canonical angular momentum of charged
particles. However, the simple appeal of these concepts has motivated repeated
attempts to resurrect them as physical descriptions of gauge systems. In
particular, measurability of the gluon-spin-contribution to the proton helicity
in polarized proton scattering has generated many theoretical efforts in
generalizing it and others as gauge-invariant quantities. In this work, we
analyze the constraints of gauge symmetry, the significance of gluon spin in
the light-cone gauge, and what is possible and natural in QCD parton physics,
emphasizing experimental observability and physical interpretation in the
structure of bound states. We also comment on the measurability of the orbital
angular momentum of the Laguerre-Gaussian laser modes in optics.Comment: 17 pages, latex, 1 figur
