[Physics] Electromagnetism Bianchi Identity

differential-geometryelectromagnetismmaxwell-equationsspecial-relativity

I am currently studying a book about the Lorentz covariant formulation of electromagnetism. At the point I am at in the book, the author has just introduced the covariant form of Maxwell's equations (with (+—)):

$$ \partial_\beta F^{\alpha \beta} = -\frac{1}{c} j^\alpha $$
and
$$ \partial_\rho F_{\nu \sigma} + \partial_\nu F_{\sigma \rho} + \partial_\sigma F_{\rho \nu} = 0 $$

My question is about this second equation. I understand where this comes from in regards to the electrodynamics of the problem, but when introducing it in the text, the author states the following:

"…for any antisymmetric tensor $F_{\mu \nu}$ satisfies the identity:
$$ \epsilon^{\mu \nu \sigma \rho} \partial_\rho F_{\nu \sigma} = 0$$
"

Where $\epsilon$ is the Levi-Civita symbol here. I've had a little difficulty with this statement because I've seen other people appeal to the same argument regarding the general properties of antisymmetric tensors, but I am having a difficult time proving it myself. I can understand if we have an $F$ of the form:
$$F_{\mu \nu} = \partial_{\mu}A_{\nu} – \partial_{\nu} A_{\mu} $$
That's a rather trivial proof, but it seems that the author (and others that I've seen) appeal to this as a general property of antisymmetric tensors. So, if anyone would want to show me how to prove why this would be a general property of antisymmetric tensors, I would be very grateful.

Best Answer

If I understand it correctly, you want to prove that $ \epsilon^{\mu\nu\sigma\rho} \partial_\rho F_{\nu\sigma} = 0 $ for a general anti-symmetric $F_{\nu\sigma}$.

From Bianchi Identity we have : $$ \epsilon^{\mu\nu\sigma\rho} (\partial_\rho F_{\nu \sigma} + \partial_\nu F_{\sigma \rho} + \partial_\sigma F_{\rho \nu}) = 0 \\ \epsilon^{\mu\nu\sigma\rho}\partial_\rho F_{\nu \sigma} + \epsilon^{\mu\nu\sigma\rho}\partial_\nu F_{\sigma \rho} + \epsilon^{\mu\nu\sigma\rho}\partial_\sigma F_{\rho \nu} = 0 \\ $$

Now realise that in the all the terms the indices are contracted (hence can be replaced by other indices) and write all the terms such that the indices on $\partial$ and F are same in each term so that the $\epsilon$ indices get modified :

$$ \epsilon^{\mu\nu\sigma\rho}\partial_\rho F_{\nu \sigma} + \epsilon^{\mu\rho\nu\sigma}\partial_\rho F_{\nu \sigma} + \epsilon^{\mu\sigma\rho\nu}\partial_\rho F_{\nu \sigma} = 0 \\ (\epsilon^{\mu\nu\sigma\rho}+\epsilon^{\mu\rho\nu\sigma}+\epsilon^{\mu\sigma\rho\nu}) \partial_\rho F_{\nu \sigma} = 0 $$

Now, use the fact that $\epsilon$ is fully anti-symmetric in its indices so that :

$$ \epsilon^{\mu\nu\sigma\rho} = \epsilon^{\mu\rho\nu\sigma} =\epsilon^{\mu\sigma\rho\nu} $$

So, we get :

$$ \epsilon^{\mu\sigma\rho\nu}\partial_\rho F_{\nu \sigma} =0 $$

Also, again exchanging indices we get :

$$ \epsilon^{\mu\nu\sigma\rho}\partial_\rho F_{\nu \sigma} =0 $$

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