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Homework Help: Deriving the continuity equation from the Dirac equation (Relativistic Quantum)

  1. Jun 14, 2008 #1
    So I am trying to derive the continuity equation:

    [tex]\frac{\partial}{\partial x^{\mu}}J^{\mu} = 0[/tex]

    From the Dirac equation:

    [tex]i\gamma^{\mu} \frac{\partial}{\partial x^{\mu}}\Psi - \mu\Psi = 0[/tex]

    And its Hermitian adjoint:

    [tex]i\frac{\partial}{\partial x^{\mu}}\overline{\Psi}\gamma^{\mu} - \mu\overline{\Psi} = 0[/tex]


    [tex]\overline{\Psi}=\Psi^{+}\gamma^{0}[/tex] (Dirac conjugate)

    3. The attempt at a solution
    By multiplying the Dirac equation on the right by [tex]\overline{\Psi}[/tex] and the adjoint on the right by [tex]\Psi[/tex] I get:

    [tex]i(\frac{\partial}{\partial x^{\mu}}(\gamma^{\mu}\Psi)\overline{\Psi} + \frac{\partial}{\partial x^{\mu}}(\overline{\Psi})\gamma^{\mu}\Psi) - \mu(\Psi\overline{\Psi} - \overline{\Psi}\Psi)=0[/tex]

    The first term is basically what I am after (except I am not 100% sure I can simply apply the product rule - what is the correct order?) which means I shoudl expect the second term to go to zero:

    [tex]\Psi\overline{\Psi} - \overline{\Psi}\Psi =0[/tex]

    But because [tex]\gamma^{0}[/tex] is a 4x4 matrix, [tex]\Psi[/tex] is a 4x1 and [tex]\overline{\Psi}[/tex] is a 1x4, I should also expect the second term to be multiplied by the 4x4 identity matrix (so that the subtraction makes sense). However the first term is NOT a constant multiplied by the identity so I don't see how this works.

    Any help would be greatly appreciated...
  2. jcsd
  3. Jun 15, 2008 #2


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    Hi toam! :smile:

    Don't you have to multiply one of them on the left? :confused:
  4. Jun 15, 2008 #3
    I tried that and got something else that didn't work. However I will try again because I was surprised that it didn't work so I may have made a mistake or missed something obvious...
  5. Jun 15, 2008 #4
    Ok so it turned out I had multiplied the wrong function on the left. It worked out quite simply when I fixed that. The lecture notes had erroneously shown both functions multiplied on the right.

    Thanks, tiny-tim.
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