Which Convention Should Be Used to Calculate Electromagnetic Tensor Invariants?

[parton]

Homework Statement

Hi,

I have to calculate the invariant: \tilde{F}^{\mu \nu} \, F_{\mu \nu}
where F is the electromagnetic field tensor and \tilde{F} the dual one.

Homework Equations

First, the contravariant components of the electromagnetic field tensor are given by:
F^{\mu\nu} = \begin{bmatrix} 0 & -E_1 & -E_2 & -E_3 \\ E_1 & 0 & -B_3 & B_2 \\ E_2 & B_3 & 0 & -B_1 \\ E_3 & -B_2 & B_1 & 0 \end{bmatrix}

and the covariant by:
F_{\mu\nu} = \begin{bmatrix} 0 & E_1 & E_2 & E_3 \\ -E_1 & 0 & -B_3 & B_2 \\ -E_2 & B_3 & 0 & -B_1 \\ -E_3 & -B_2 & B_1 & 0 \end{bmatrix}

And last but not least, the contravariant components of the dual elm. field tensor:
\tilde{F}^{\mu\nu} = \begin{bmatrix} 0 &amp; -B_1 &amp; -B_2 &amp; -B_3 \\ B_1 &amp; 0 &amp; E_3 &amp; -E_2 \\ B_2 &amp; -E_3 &amp; 0 &amp; E_1 \\ B_3 &amp; E_2 &amp; -E_1 &amp; 0 \end{bmatrix}<br />

The Attempt at a Solution

First I note:
\tilde{F}^{a0} = B^{a}

\tilde{F}^{ab} = \epsilon^{abi} E_{i}

F_{a0} = - E_{a}

F_{ab} = \epsilon_{abc} B^{c}

Now, if I use these relations I obtain the wrong solution:

\tilde{F}^{\mu \nu} \, F_{\mu \nu} = 2 \tilde{F}^{a0} F_{a0} + \tilde{F}^{ab} F_{ab} = - 2 B^{a} E_{a} + \epsilon^{abi} E_{i} \epsilon_{abc} B^{c} = 0
where I used the relation: \epsilon^{abi} \epsilon_{abc} = 2 \delta^{i}_{c}

Of course, if i simply insert the components explicitly (the "matrix elements") I get the result \tilde{F}^{\mu \nu} \, F_{\mu \nu} = - 4 \vec{B} \cdot \vec{E} and everything is fine.

I used the following convention: \epsilon^{0123} = 1, latin indices: {1,2,3}, greek indices: {0,1,2,3}

Further I calculated: \epsilon_{123} = \eta_{\alpha 1} \, \eta_{\beta 2} \, \eta_{\gamma 3} \, \epsilon^{\alpha \beta \gamma} = - 1 where \eta = diag(1, -1, -1, -1) is the metric tensor.

I think there has to be a wrong sign, but I don't find it. Does anyone have an idea?

 

[gabbagabbahey]

F_{ab} = \epsilon_{abc} B^{c}

I think you might want to double check this one

 

[parton]

hmmm, I don't see anything wrong here !?

If I check it I obtain the right elements, for example: F_{12} = \epsilon_{123} B^{3} = - B^{3}.

 

[gabbagabbahey]

hmmm, I don't see anything wrong here !?

If I check it I obtain the right elements, for example: F_{12} = \epsilon_{123} B^{3} = - B^{3}.

\epsilon_{123}=+1

 

[gabbagabbahey]

Further I calculated: \epsilon_{123} = \eta_{\alpha 1} \, \eta_{\beta 2} \, \eta_{\gamma 3} \, \epsilon^{\alpha \beta \gamma} = - 1

This assumes that \epsilon_{abc} transforms as a tensor; but it doesn't. It is a tensor density with weight -1.

 

[parton]

ok, \epsilon_{ijk} is a pseudotensor, but where is the difference between covariant \epsilon_{ijk} and contravariant \epsilon^{ijk} components?

In the case with 4 indices there is a difference, for example: \epsilon^{0123} \neq \epsilon_{0123}. Why should it be different with 3 components?

 

[gabbagabbahey]

ok, \epsilon_{ijk} is a pseudotensor, but where is the difference between covariant \epsilon_{ijk} and contravariant \epsilon^{ijk} components?

There isn't a difference in the convention that I'm used to seeing.

In the case with 4 indices there is a difference, for example: \epsilon^{0123} \neq \epsilon_{0123}. Why should it be different with 3 components?

I'm assuming that you are using the same convention (the most common convention) http://planetmath.org/encyclopedia/LeviCivitaPermutationSymbol3.html .

In that case, then \epsilon^{0123} = \epsilon_{0123}.

If you are using some other convention, whereby \epsilon_{123}=-1 , then your problem is in the equation \epsilon^{abi} \epsilon_{abc} = 2 \delta^{i}_{c} instead.

 

[parton]

i've found something, have a look at this (page 9): http://www.worldscibooks.com/phy_etextbook/6938/6938_chap01.pdf"

Further I found something in another wiki (but it's not in english). There is a remark that in relativity there you have to differ between co- and contravariant indices. It is just convention whether you use \epsilon^{0123}= +1 or -1.

But all this doesn't explain my problem

 

[gabbagabbahey]

i've found something, have a look at this (page 9): http://www.worldscibooks.com/phy_etextbook/6938/6938_chap01.pdf"

The convention used in the above link is not very common; but if that is the convention you use in your course then you can stick with it.

According to that convention, \epsilon_{ijk}=-e^{ijk} and e_{123}=1. Therefor, \epsilon^{abi} \epsilon_{abc} = -2 \delta^{i}_{c} in this convention. In addition, epsilon is a tensor in this convention.

So the question you need to ask yourself is which convention is used in your course text/notes?