In some books I find the dual antisymmetric tensor $$\tilde{H}^{ab}=-\frac{1}{2}\epsilon^{abcd}H_{cd}$$ and other times I find it with no minus sign. How can I tell which to use? Is this like that in some theories or more conventional in some more than the others?
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It's a convention. You may also occasionally find it without the $\frac{1}{2}$ factor.
Observe that $\tilde{H}^{ab} = -\tilde{H}^{ba}$ anyways, so the minus in the definition translates into a sign/index ordering convention in all equations involving it.
The convention is basically related to the question whether you want the Hodge dual $\star H$ to have components $\tilde{H}^{ab}$, $\tilde{H}^{ba}$ or $\frac{1}{2}\tilde{H}^{ab}$, and may also be related to the actual metric you are using for the dual (i.e. its signature).
answered May 24, 2015 at 22:09