I Equations of motion from Born-Infeld Lagrangian

We can write the Born-Infeld Lagrangian as:

[tex]L_{BI}=1 - \sqrt{ 1+\frac{1}{2}F_{\mu\nu }F^{\mu\nu}-\frac{1}{16}\left(F_{\mu\nu}\widetilde{F}^{\mu\nu} \right)^{2}}[/tex]

with [itex]G^{\mu\nu}=\frac{\partial L}{\partial F_{\mu\nu}}[/itex] how can we show that in empty space the equations of motion take the form [itex]\partial_{\mu}G^{\mu\nu}=0[/itex]
We should start with an Euler-Lagrange equation, but how can i write it for this Lagrangian?


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The EL equation for this case is
$$\partial_{\nu}\frac{\partial L}{\partial A_{\mu,\nu}}=0$$
where ##A_{\mu,\nu}=\partial_{\nu}A_{\mu}##. Using ##F_{\mu\nu}=A_{\nu,\mu}-A_{\mu,\nu}##, the rest should be straightforward. See also Jackson to see how covariant Maxwell equations are derived for ordinary ##F_{\mu\nu}F^{\mu\nu}## action. For other details about Born Infeld see Zwiebach - A First Course in String Theory.
Last edited:
Thank you, Demystifier.
I have never seen [itex]F_{\mu\nu}[/itex] written like that, but using that:

[itex] \frac{\partial L}{\partial A_{\mu\nu}} = \frac{-\frac{1}{4}({-A^{\nu\mu}+A^{\mu\nu}})+\frac{1}{16}A_{\mu\nu}F_{\mu\nu}(\widetilde{F}^{\mu\nu})^{2}}{\sqrt{ 1+\frac{1}{2}F_{\mu\nu }F^{\mu\nu}-\frac{1}{16}\left(F_{\mu\nu}\widetilde{F}^{\mu\nu} \right)^{2}}}[/itex]
What do i do with the [itex]\partial_{\nu}[/itex] now?
using that
You left out the commas. Look closely at what Demystifier posted; there are commas, so it's ##F_{\mu \nu} = A_{\nu , \mu} - A_{\mu , \nu}##. The commas are partial derivatives, so what he wrote is the same as ##F_{\mu \nu} = \partial_\mu A_\nu - \partial_\nu A_\mu##.

As Demystifier said, you need a good background in ordinary electrodynamics for the topic under discussion.

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