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Discussion Overview

The discussion revolves around the Einstein Field Equations, specifically focusing on the (0,0) component and its relationship to the stress-energy tensor and gravitational potential. Participants explore the implications of various mathematical formulations and seek clarifications on the connections between different tensors and equations in general relativity.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant equates the (0,0) component of the Einstein Tensor to the mass density, suggesting it aligns with Poisson's equation.
  • Another participant provides a mathematical formulation involving the Ricci tensor and challenges the interpretation of indices in the equations presented.
  • There is a request for a proof relating the Ricci tensor to the gravitational potential, indicating a desire for clarity on the mathematical relationships.
  • Some participants express confusion regarding the equivalence of certain equations and the conditions under which they hold true, particularly in the context of weak gravitational fields.
  • A later reply emphasizes that the relationship between the Ricci tensor and the Newtonian potential is not universally applicable and depends on specific conditions.
  • Participants discuss the implications of setting constants like c and G to 1, with varying interpretations of their significance in the equations.
  • There are repeated requests for clarification on the use of LaTeX for mathematical expressions, indicating a need for better communication of complex ideas.
  • One participant asserts a specific value for the energy density of the Earth, while another questions the generality of such a claim.

Areas of Agreement / Disagreement

Participants express differing views on the relationships between the Einstein Tensor, Ricci tensor, and gravitational potential. There is no consensus on the validity of certain equations or the assumptions underlying them, leading to an ongoing debate.

Contextual Notes

Some statements rely on specific coordinate systems or conditions, such as the weak field limit, which may not apply universally. The discussion also highlights the complexities of tensor notation and the need for careful interpretation of mathematical expressions.

Who May Find This Useful

This discussion may be of interest to students and researchers in general relativity, particularly those exploring the mathematical foundations of the Einstein Field Equations and their applications in gravitational physics.

  • #121
As far as I can tell 109 isn't even close. However, it is unnecessarily cluttered with all of the G and c and M and R terms. You should just stick with A and B.
 
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  • #122
^
OK, I feel that there is something wrong with it. Can you check it with your Mathematica?

Also, there other non-zero Gammas which I haven't mentioned.
 
  • #123
GRstudent said:
Also, there other non-zero Gammas which I haven't mentioned.
Well, there are the obvious symmetries:
\Gamma^{\theta}_{r \theta}=\Gamma^{\theta}_{\theta r} and \Gamma^{\phi}_{r \phi}=\Gamma^{\phi}_{\phi r} and \Gamma^{\phi}_{\theta \phi}=\Gamma^{\phi}_{\phi \theta}

Beyond those the following terms are also non-zero:
\Gamma^{t}_{r t} = \Gamma^{t}_{t r}
\Gamma^{r}_{t t}
\Gamma^{r}_{\phi \phi}
 
  • #124
PeterDonis said:
Also, I'm not sure that the Gammas have to match at the boundary. The metric components themselves do, but I'm not sure the Gammas do. I'll have to check further on that.

You'll definitely see jumps in the christoffel symbols, and in the metric, if you have pressurized shells - for instance, if if you have a ball or radiation trapped in a box. They'll be sudden jumps only in the limit of very thin shells IIRC.

I suspect you won't see any jumps matching interior to exterior solutions lacking such pressure differences, but it's worth checking to be sure.

It'd be wrong to try and match an interior solution to an exterior solution where the interior pressure wasn't equal to the exterior pressure (i.e. zero for a vacuum exterior) without adding some sort of shell to acount for the pressure difference.
 
  • #125
pervect said:
It'd be wrong to try and match an interior solution to an exterior solution where the interior pressure wasn't equal to the exterior pressure (i.e. zero for a vacuum exterior) without adding some sort of shell to acount for the pressure difference.

In the particular solution in question (constant density spherical massive body surrounded by vacuum--it's discussed, for example, in MTW), the pressure is continuous at the boundary (the surface of the body)--at least, I'm pretty sure that's right--but the density is not; it jumps from its interior value to zero at the boundary. The jump in density causes a jump in the radial derivative of g_rr (g_rr itself is continuous), which shows up as a jump in \Gamma^{r}_{rr}. As far as I can tell, that is the only Christoffel symbol that is affected.
 
  • #126
Others so far:

\Gamma^{r}_{\phi\phi}=\sin^2\theta r (Br^2-1)

\Gamma^{r}_{tt}=\dfrac{0.5r(\sqrt{1-r^2B}-3 \sqrt{1-A})B(1-Br^2)}{2 \sqrt{1-r^2B}}

Please check them!
 
Last edited:
  • #127
GRstudent said:
Please check them!
Those are correct.
 
  • #128
List of Gammas:

\Gamma^{r}_{rr}=\dfrac{-rB}{(Br^2 -1)}

\Gamma^{\theta}_{\theta r }=\dfrac{1}{r}

\Gamma^{\phi}_{\phi r}=\dfrac{1}{r}

\Gamma^{\phi}_{\phi \theta} = \dfrac{1}{\tan \theta}

\Gamma^{r}_{\theta \theta}=-r(Br^2-1)

\Gamma^{\theta}_{\phi\phi}=-\sin\theta \cos\theta

\Gamma^{r}_{\phi\phi}=\sin^2\theta r (Br^2-1)

\Gamma^{r}_{tt}=\dfrac{0.5r(\sqrt{1-r^2B}-3 \sqrt{1-A})B(1-Br^2)}{2 \sqrt{1-r^2B}}
 
  • #129
I think those are correct except for \Gamma^r_{tt}
{\Gamma^r}_{tt}=-\frac{r\,B\,\sqrt{1-{r}^{2}\,B}\,\left( \sqrt{1-{r}^{2}\,B}-3\,\sqrt{A}\right) }{4}

and {\Gamma^t}_{rt} is not in the list. Calculated by Maxima ctensor package.
 
  • #130
^
Can you calculate Einstein Tensor of my Gammas?
 
  • #131
Mentz114 said:
I think those are correct except for \Gamma^r_{tt}
{\Gamma^r}_{tt}=-\frac{r\,B\,\sqrt{1-{r}^{2}\,B}\,\left( \sqrt{1-{r}^{2}\,B}-3\,\sqrt{A}\right) }{4}

and {\Gamma^t}_{rt} is not in the list. Calculated by Maxima ctensor package.
I think those are the same. The expression that Mathematica gives seems to be about halfway in-between both your expression and GRstudent's:
{\Gamma^r}_{tt}=\frac{1}{4} B r \left(3 \sqrt{1-A} \sqrt{1-B r^2}+B r^2-1\right)
 

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