In this paper, we consider a general twisted-curved space-time hosting Dirac spinors and we take into account the Lorentz covariant polar decomposition of the Dirac spinor field: the corresponding decomposition of the Dirac spinor field equation leads to a set of field equations that are real and where spinorial components have disappeared while still maintaining Lorentz covariance. We will see that the Dirac spinor will contain two real scalar degrees of freedom, the module and the so-called Yvon-Takabayashi angle, and we will display their field equations. This will permit us to study the coupling of curvature and torsion respectively to the module and the YT angle.
I. HISTORYThe Dirac equation is one of the most impressive successes in all of physics (and as far as we can tell, in all of human achievements): conceived from the purely theoretical (or in Dirac's thoughts, aesthetic) reason to be a covariant first-order derivative field equation, it turned out to account for spin and matter/antimatter duality.Such an extensively comprehensive description comes at the cost of a rather complicated formalism: as a start, spinors (in this paper we only consider Dirac spinors) are 4-dimensional columns of complex scalar fields, amounting to 8 real components. Moreover, the spinor formalism does not put in evidence the essence of any of the various components of a spinor field -So is there a way in which to write the spinor formalism so that all components display a clear meaning? Also, can we reduce the variety of the components by proving that some of them are not in fact true degrees of freedom? And if yes, then how many degrees of freedom are actually present in a spinor?These are all legitimate questions that researchers have been trying to answer, though not with the same impetus with which research has been done in more fashionable branches; still, some research has been done, and to our knowledge, the first to work on this problem were Jakobi and Lochak [1], followed after some time, but with a much richer research production, by Hestenes [2-4].The idea they had was to write the Dirac spinor field in the polar form: as a complex scalar can be written as the product of module times unitary phase, similarly the complex spinor should be writable as a column with four components, each of which being the product of a module times a unitary phase; while for scalars this construction is always trivial, spinors are defined in such a way that a spinorial transformation mixes the various components, and care must be exercised if we want the polar form to respect Lorentz symmetries. The works mentioned above do precisely this: they expound the spinor in a form that is polar while displaying a manifest Lorentz symmetry in its structure. As we will discuss later on, the polar form allows us to give a clear interpretation of the components of the spinor field and it shows which ones are artifacts and which ones are real degrees of freedom.On the basis of these results, Hestenes went further to discuss zitterbewegung effects...