Fig. 2: Time evolution of a Gaussian initial wave packet according to the Dirac equation

The curve in the background (filled with gray) is the amplitude of the position probability density. Moreover, we see a phase colored plots of the upper component and of the lower component (plotted upside down and with lower saturation) of the Dirac spinor. The Dirac spinor has just two components, because the motion takes place in one space dimension.