Relativistic Field Theory

The dynamical variables that describe relativistic systems are fields, that is, functions defined in each point of ordinary space. Important examples are the electromagnetic fields, and Dirac and Yukawa fields. The field description of a physical system opens the way to a direct implementation of the principle of covariance, that guarantees the invariance of the equations of motion under changes of reference frame, and of the principle of causality, which is connected to the principle of locality, namely, the independence of variables associated to different points in space at the same time.