Getting the Ricci and metric tensor from T ?

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Discussion Overview

The discussion revolves around the process of deriving the Ricci tensor and metric tensor from the energy-momentum tensor \( T_{ab} \) within the context of General Relativity (GR). Participants explore the relationship between these tensors and the challenges of solving Einstein's field equations, particularly in terms of formulating the necessary partial differential equations (PDEs).

Discussion Character

  • Exploratory
  • Technical explanation
  • Homework-related

Main Points Raised

  • One participant notes the dependence of solutions on the specific form of \( T_{ab} \), indicating that there is no universal solution applicable to all cases.
  • Another participant mentions that a common solution arises when \( T_{ab} \) is static and spherically symmetric, referencing the Schwarzschild metric as an example.
  • There is a request for clarification on how to calculate the Ricci scalar and tensor, indicating a need for foundational understanding.
  • Participants discuss the relationship between the Ricci tensor, Christoffel symbols, and the metric, providing links to external resources for further exploration.
  • One participant expresses surprise at the original poster's inquiry about solutions without understanding the relationship between the Ricci tensor and the metric.

Areas of Agreement / Disagreement

Participants generally agree that the solutions to Einstein's equations depend on the specific form of \( T_{ab} \), but there is no consensus on a singular method for deriving the Ricci tensor and scalar from the metric, as the discussion remains exploratory and unresolved.

Contextual Notes

Limitations include the lack of detailed derivation steps for the Ricci tensor and scalar, as well as the dependence on the specific forms of the tensors involved. The discussion also reflects varying levels of familiarity with the subject matter among participants.

Who May Find This Useful

This discussion may be useful for individuals learning General Relativity, particularly those seeking to understand the relationships between the energy-momentum tensor, Ricci tensor, and metric tensor.

cuallito
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Okay, we have Einstein's field equation:

R_ab + 1/2 R g_ab = 8pi T_ab

Let's say we have T_ab defined for some region of space, and we want to calculate the spacetime from that. How would you calculate R_ab, R and g_ab? Supposedly you can write it as a system of PDEs but I cannot find them anywhere!

I'm just starting to learn GR, if I could see how you'd put it in a computer to solve them it would help me "see" it better. Thanks.
 
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But it all depends on who T_ab looks, there is no "general" solution to ALL.

The easiest solution is obtained by T_ab as static (no timedep.) and spherical symmetric, that solution for the metric is called the Schwarzschild metric.

http://en.wikipedia.org/wiki/Schwarzschild_metric
here is an outline of it's derivation:
http://en.wikipedia.org/wiki/Deriving_the_Schwarzschild_solution

I mean HOW to solve it depends from case to case, you are just after the form of the PDE's? But that is trivial to find, just plug in whatever you have for the Ricci tensor and Ricci scalar in terms of the metric (your multidimensional function which depends on several variables)
 
Okay, how do you calculate the Ricci scaler and tensor in the first place then?
 
Just learning it on my own. I have Wald's GR right now, I'm getting Einstein's populist book and GR A to B soon thru ILL hopefully.
 
cuallito said:
Just learning it on my own. I have Wald's GR right now, I'm getting Einstein's populist book and GR A to B soon thru ILL hopefully.

ok cool!

But how come you asked about solutions to Einsteins equation if you don't know how how ricci tensor and ricci scalar is related to the metric?... I strongly recommend to study in logical order :-)
 

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