Complex Metric Tensor: Meaning, Weak Gravitational Fields & Einstein Eqns

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Discussion Overview

The discussion revolves around the implications and interpretations of a complex metric tensor within the context of weak gravitational fields and the linearized Einstein field equations. Participants explore the physical meaning of a complex metric tensor and whether it can be simplified to its real part.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions the physical meaning of a complex metric tensor and whether it is valid to consider only its real part.
  • Another participant asserts that a metric tensor should consist of real numbers, as tensors are not numbers and must be represented by symmetric matrices with real eigenvalues.
  • A different participant suggests that if the linearized field equations are being used on a Minkowski background, taking the real part of the complex metric could be acceptable.
  • One participant seeks clarification on whether the use of different coordinate systems (dx0=ct vs. dx0=ict) might contribute to the confusion regarding the complex metric.
  • Another participant notes that solving the linearized field equations is akin to solving the wave equation, implying that complex solutions may arise from specific mathematical forms used in the solution process.
  • A request for more details on how the complex metric was derived is made, indicating a need for further clarification on the issue.

Areas of Agreement / Disagreement

Participants express differing views on the validity and implications of a complex metric tensor, with no consensus reached on whether such a tensor can be physically meaningful or if it should be simplified to its real part.

Contextual Notes

Limitations include potential misunderstandings related to the representation of tensors in different coordinate systems and the mathematical forms used in deriving solutions to the linearized field equations.

ngkamsengpeter
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I am working on the weak gravitational field by using linearized einstein field equation. What if the metric tensor, hαβ turn out to be a complex numbers? What is the physical meaning of the complex metric tensor? Can I just take it's real part?

Or there is no such thing as complex metric tensor and my system is not physically make sense?
 
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I really don't know what you mean by a metric tensor turning out to be a "complex number". A tensor is NOT number to begin with. And if your coordinate system uses real numbers to as coordinates then the components of the tensor must be real numbers. Of course, in a given coordinate system, we can think of a tensor as represented by a matrix and so find its eigenvalues. It is possible for the eigenvalues of a general matrix to be complex but the metric tensor should always be represented by a symmetric matrix which must have real eigenvalues.
 
Since you're working with the linearized field equations, on a Minkowski background presumably, yes you can just take the real part.
 
HallsofIvy said:
I really don't know what you mean by a metric tensor turning out to be a "complex number". A tensor is NOT number to begin with. And if your coordinate system uses real numbers to as coordinates then the components of the tensor must be real numbers. Of course, in a given coordinate system, we can think of a tensor as represented by a matrix and so find its eigenvalues. It is possible for the eigenvalues of a general matrix to be complex but the metric tensor should always be represented by a symmetric matrix which must have real eigenvalues.

What I mean here is that the component of the hmn turn out to be a complex numbers. That is h11 is a complex numbers. In linearized einstein field equation, metric tensor is just sum of flat spacetime metric plus the small pertubation hmn. So if component of hmn is a complex number, so I would think that the metric tensor is also complex. Or should I just take the real part as what Bill_K said?

Thanks.
 
Are you using dx0=ct or dx0=ict? This could be part of the difficulty.

Chet
 
Solving the linearized field equations amounts to solving the wave equation, so if your solution turns out to be complex there must be a fairly simple reason for it. Like you used exp(ikx-iwt) instead of sin and cos.
 
You have to give more details on how you arrived at a comples metric.
 

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