Dirac equation

How to construct

Starting from the relativistic relation between energy and momentum:

$E^{2}={\mathbf {p}}^{2}c^{2}+m^{2}c^{4}$

or $E=\pm c{\sqrt {{\mathbf {p}}^{2}+m^{2}c^{2}}}$

From this equation we can not directly replace $E,{\mathbf {p}}$ by the corresponding operators since we don't have the definition for the square root of and operator. Therefore, first we need to linear