Electromagnetic dual tensor. General Relativity

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Homework Help Overview

The discussion revolves around the equivalency of different forms of Maxwell's equations within the context of General Relativity, specifically focusing on the electromagnetic tensor and its dual. The original poster expresses confusion regarding the notation and the relationships between various equations presented in their homework.

Discussion Character

  • Exploratory, Conceptual clarification, Mathematical reasoning, Problem interpretation

Approaches and Questions Raised

  • Participants discuss the notation and structure of the equations, with some suggesting the need to expand the notation for clarity. There are attempts to relate the matrix representations of the electromagnetic tensor to differential forms. Questions arise about the implications of tensor ranks and the nature of the equations being compared.

Discussion Status

Some participants provide guidance on how to approach the equivalency of the equations, noting the importance of understanding the tensor ranks and the implications of the Hodge dual. The conversation reflects a mix of interpretations and attempts to clarify the mathematical relationships without reaching a consensus.

Contextual Notes

The original poster indicates a lack of familiarity with tensor notation and concepts, which may affect their understanding of the problem. There is mention of a specific textbook being used, which may contribute to the confusion regarding notation and terminology.

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Homework Statement


I attach a word document with the equations because I don't know how to write them on the post.
My question reads: Show that Maxwell's equations Eq (1) is equivalent to Eq (3).

Homework Equations


The first term of Eq 1 reads: F sub alpha beta comma gamma. That means partial of F sub alpha beta with respect to x super gamma.
The book says that Eq 1 and Eq 2 are equivalents. I am just showing Eq 2 because I know that the book's notation is confusing.
Eq 2 means the gradient of F, which is the EM tensor.
Eq (4) is the components electromagnetic tensor.
And I have calculated the components of the dual tensor *F which are Eq (5)

The Attempt at a Solution


I have written Eq (3) in matrix notation, but I don't think it resembles Eq (1) or Eq (2). Also, I know that Eq 2 is a 3rd rank tensor due to the gradient.
I have no idea how to show the equivalency between 1 and 3.
The book I am using is Mister, Thorne, Wheeler Gravitation, but this is a undergraduate level course in general relativity, and I am very new at this tensor stuff, so please be gently.

Thank you for your help
 

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Here, I type-set them for you...
[tex]F_{\alpha\beta,\gamma} + F_{\beta\gamma,\alpha} + F_{\gamma \alpha,\beta} = 0[/tex]
[tex]\mathbf{dF} = 0[/tex]
[tex]\mathbf{\nabla}\bullet {}^\star \mathbf{F}=0[/tex]
[tex][F_{\alpha\beta}] = \left[ \begin{array}{c c c c}0 & -E_x & -E_y & -E_z\\<br /> E_x & 0 & B_z &-B_y\\<br /> E_y & -B_z & 0 & B_x\\<br /> E_z & B_y & -B_x 0\end{array}\right][/tex]
[tex][{}^\star F_{\alpha\beta}] = \left[ \begin{array}{c c c c}0 & B_x & B_y & B_z\\<br /> -B_x & 0 & E_z &-E_y\\<br /> -B_y & -E_z & 0 & E_x\\<br /> -B_z & E_y & -E_x 0\end{array}\right][/tex]
 
Your post didn't complete the sentence so I'm not clear on what is your question. But if you want to show equivalence of the first three equations, you have to expand the notation.

[tex]\mathbf{d} \omega = \mathbf{dx}^\mu \wedge \partial_\mu \omega[/tex]

To translate the matrix format to differential forms, you contract the matrix with the bivector basis:
[tex]\mathbf{F} =\frac{1}{2} F_{\mu\nu} \mathbf{dx}^\mu\wedge \mathbf{dx}^\nu[/tex]
The factor of 1/2 is not vital since these are homogenous equations but are there to account for the double counting of e.g. [itex]\mathbf{dx}^1\wedge \mathbf{dx}^2=- \mathbf{dx}^2 \wedge \mathbf{dx}^1[/itex]

(to properly sum you should be summing, not over each index, but over the set of unique anti-symmetric pairs... or throw in the factor of 1/2 to get the same thing.)

You now need to parse through the definitions and see what results.
 
A second observation. Note the first is a rank 3 tensor equation (three free indices).
The second is a rank-3 tensor equation (the outer differential of a bi-vector is a tri-vector), and the last is a vector equation (the divergence of a 2-tensor is a 1 tensor).

Given each case will be a totally anti-symmetrized tensor, one can count the dimensions of these tensor spaces. Given totally antisymmetized indices in 4 dimensions you have 4 choose r ways to select distinct indices so the dimensions are:
rank 0 (scalar) dim=1
rank 1 (vector) dim=4
rank 2 (bi-vector) dim=6
rank 3 ("tri-vector") dim = 4
rank 4 (The Levi-Civita tensor) dim = 1

The hodge dual swaps these equal dimension tensor spaces.
So you should find each of your equations is a set of 4 equations, either 4-vector or "tri-vector"=rank 3 antisymmetric tensor.
 
jambaugh said:
[tex]\mathbf{d} \omega = \mathbf{dx}^\mu \wedge \partial_\mu \omega[/tex]

I updated my post.
Eq 2 means the gradient of the tensor F.
Does your equation dw means the same? Because it seems to me as the differential, and w being a scalar...
As I said (in my update) I am very new at this.
Thank you.
 
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