Invarients from the Faraday tensor

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    Faraday Tensor
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Discussion Overview

The discussion revolves around the contraction of the Faraday tensor with itself, specifically exploring the expression F_{\mu\nu}F^{\nu\mu} and its relation to electric and magnetic fields. Participants are examining methods to simplify the calculation involved in deriving this expression, which is relevant to theoretical physics and electromagnetism.

Discussion Character

  • Exploratory, Technical explanation

Main Points Raised

  • One participant presents the result of the contraction as F_{\mu\nu}F^{\nu\mu}=2(E^{2}-c^{2}B^{2}) and describes the tedious process of calculating 16 terms.
  • Another participant suggests that using the antisymmetry of the Faraday tensor reduces the number of terms to calculate to 6.
  • A third participant acknowledges the suggestion, indicating it simplifies the process.
  • A fourth participant notes that recognizing the structure of the tensor can further simplify the calculation by identifying the first row as -{\vec E} and the spatial part as -{\vec B}.

Areas of Agreement / Disagreement

Participants generally agree that there are methods to simplify the calculation of the contraction of the Faraday tensor, but no consensus is reached on a single preferred method.

Contextual Notes

The discussion does not resolve the best approach to calculating the contraction, and assumptions about the properties of the Faraday tensor and the definitions of the electric and magnetic fields are not explicitly stated.

Who May Find This Useful

Readers interested in electromagnetism, tensor calculus, or theoretical physics may find this discussion relevant.

peterjaybee
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Hello,

a full contraction of the faraday tensor with itself can be shown to be

[tex]F_{\mu\nu}F^{\nu\mu}=2(E^{2}-c^{2}B^{2})[/tex]

I have done this by calculating 16 terms in the sum i.e. F11F11 + F12F21, and get this answer, but this is very tedious.

Is there a faster way to show this that I am missing?
 
Last edited:
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Yes, by using the fact that the Faraday tensor is antisymmetric. This way you only have to calculate 6 terms.
 
Last edited:
That is a good point. makes it much easier. Thanks
 
It also gets easier if you recognize that the first row is just [tex]-{\vec E}[/tex], and the 3X3 space-like part is just [tex]-{\vec B}[/tex], a bit mixed up.
 

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