Gamma Matrices - Expressing The Dirac Equation

Expressing The Dirac Equation

In natural units, the Dirac equation may be written as

where ψ is a Dirac spinor. Here, if were an ordinary 4-vector, then it would pick out a preferred direction in spacetime, and the Dirac equation would not be Lorentz invariant.

Switching to Feynman notation, the Dirac equation is

Applying to both sides yields

which is the Klein–Gordon equation. Thus, as the notation suggests, the Dirac particle has mass m.

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