Problems with a complex stress-energy tensor

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Discussion Overview

The discussion revolves around the properties of a complex stress-energy tensor derived from a complex scalar field, specifically addressing its symmetry and the implications of using different forms of derivatives. The context includes theoretical considerations related to the Klein-Gordon equation and the formulation of stress-energy tensors in field theory.

Discussion Character

  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant presents a stress-energy tensor and expresses concern that it is not symmetric, noting that Ttr is not equal to Trt due to a minus sign in the partial derivatives.
  • The same participant questions whether the issue arises from differentiating the complex scalar field correctly or if the use of covariant derivatives is necessary, given that it is a scalar quantity.
  • Another participant suggests symmetrizing the tensor or using an alternative definition of the stress-energy tensor that is guaranteed to be symmetric, referencing the relationship between the Lagrangian density and the metric tensor.
  • A later reply indicates that the alternative approach led to the discovery of a symmetric tensor using the Klein-Gordon Lagrangian.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the original tensor's symmetry, but there is agreement on the validity of using an alternative definition that ensures symmetry. The discussion reflects differing approaches to resolving the issue.

Contextual Notes

There are unresolved questions regarding the appropriate use of derivatives in the context of the stress-energy tensor and the implications of using covariant versus partial derivatives. The discussion also highlights the dependence on the definitions used in the formulation of the tensor.

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Hi , I am working with the following stress-energy tensor:

T[itex]\mu\nu[/itex]=[itex]\partial[/itex][itex]\mu[/itex][itex]\phi[/itex][itex]\partial[/itex][itex]\nu[/itex][itex]\phi[/itex]* - g[itex]\mu\nu[/itex]([itex]\nabla[/itex][itex]\mu[/itex][itex]\phi[/itex][itex]\nabla[/itex][itex]\mu[/itex][itex]\phi[/itex]* - m2[itex]\phi\phi[/itex]*)

Where [itex]\phi[/itex] is a complex scalar of the form:
[itex]\phi[/itex](r,t) = [itex]\psi[/itex](r)eiwt

that obeys the Klein-gordon equation and [itex]\phi[/itex]* is the complex conjugate, and g the metric tensor.

My problem is that i think this tensor is not symmetric as Ttr /= Trt by a minus sign on the term of the partial derivates.

Thanks a lot for reading.
 
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I think my problem wasn't really clear, this tensor is supposed to be a stress energy tensor and therefore it should be symmetric but I'm finding that it isn't Ttr /= Trt. I'm not sure if I'm differentiating wrong the complex scalar field or I think originally instead of partial derivatives they were covariant derivatives but since it is a scalar quantity I thought it wouldn't matter.

Hope this gives a better idea of the problem at hand.

Thanks a lot.
 
So why don't you just symmetrize it.

Or better yet, forget the "canonical" stress-energy tensor and use the correct and much simpler definition, Tμν = 2δL/δgμν whose calculation typically involves no taking of derivatives and is guaranteed to come out symmetrical. All you have to remember is that the Lagrangian density contains a factor √(-g) which must also be varied, and δ√(-g)/δgμν = 1/2 √(-g) gμν.
 
Thanks Bill_K, with that and the Klein-Gordon Lagrangian I found a Tensor which is indeed symmetric.

Thanks a lot.
 

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