"Symplectic unitary representations for the Poincar ́ e group are studied. The formalism is based on the noncommutative structure of the star–product, and using group theory approach as a guide, a consistent physical theory in phase space is constructed. The state of a quan- tum mechanics system is described by a quasi-probability amplitude that is in association with the Wigner function. In this context, we derive the Klein–Gordon and Dirac equations. As an application, we study the Dirac equation with electromagnetic interaction in phase space."
Nonlinear Dirac equations (NLDE) are derived through a group N^2 of nonlinear (gauge) transformation acting in the corresponding state space. The construction generalises a construction for nonlinear Schrödinger equations. To relate N^2 with physically motivated principles we assume: locality (i.e. it contains no explicit derivative and no derivatives of the wave function), separability (i.e. it acts on product states componentwise) and Poincaré invariance. Furthermore we want that a positional density is invariant under N^2. Such nonlinear transformations yield NLDE which describe physically equivalent systems. To get 'new' systems, we extend this NLDE (gauge extension) and present a family of NLDE which is a slight nonlinear generalisation of the Dirac equation. We discuss and comment the fact that nonlinear evolutions are not consistent with the usual framework of quantum theory. To develop a corresponding extended framework one needs models for nonlinear evolutions which also indicate possible physical consequences of nonlinearities.
We give a direct link between description of Dirac particles in the abstract framework of unitary representation of the Poincaré group and description with the help of the Dirac equation. In this context we discuss in detail the spin operator for a relativistic Dirac particle. We show also that the spin operator used in quantum field theory for spin s=1/2 corresponds to the Foldy-Woutheysen mean-spin operator.