Editing C-symmetry (section)

====Geometric interpretation====
The phase factor <math>\ \eta_c\ </math> can be given a geometric interpretation. It has been noted that, for massive Dirac spinors, the "arbitrary" phase factor <math>\ \eta_c\ </math> may depend on both the momentum, and the helicity (but not the chirality).<ref>{{harvp|Itzykson|Zuber|1980|loc=§&nbsp;2-4-2 ''Charge Conjugation'', page&nbsp;86, equation&nbsp;2-100}}</ref> This can be interpreted as saying that this phase may vary along the fiber of the [[spinor bundle]], depending on the local choice of a coordinate frame. Put another way, a spinor field is a local [[section (fiber bundle)|section]] of the spinor bundle, and Lorentz boosts and rotations correspond to movements along the fibers of the corresponding [[frame bundle]] (again, just a choice of local coordinate frame). Examined in this way, this extra phase freedom can be interpreted as the phase arising from the electromagnetic field. For the [[Majorana spinor]]s, the phase would be constrained to not vary under boosts and rotations.