Modeling the Earth and Sun (2 body orbits) using general relativity?

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Discussion Overview

The discussion revolves around modeling the gravitational interaction between the Earth and the Sun using general relativity (GR) as opposed to classical Newtonian mechanics. Participants explore the complexities and challenges of applying GR to a two-body problem, particularly focusing on the time evolution of their orbits without extreme conditions like relativistic speeds or black holes.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • Some participants note that while modeling the two-body problem using Newton's law is straightforward, applying GR introduces significant complexity, making it a hard problem.
  • One participant mentions that GR provides a small correction to the Newtonian equations of motion, suggesting a modified equation that includes terms for GR effects.
  • Another participant questions whether superimposing two Schwarzschild metrics would allow for a test particle to follow an orbit influenced by both the Sun and Earth, but another participant counters that the non-linear nature of the Einstein Field Equations prevents such superposition.
  • There is a discussion about the lack of a known solution for the two-body problem in GR, with some suggesting that iterative methods starting from a reasonable guess for the spatial metric may be necessary.
  • Participants mention that numerical simulations of the two-body problem in relativity require significant computational resources and have historically faced convergence issues.
  • One participant raises a philosophical question about how changes in the metric due to the motion of bodies propagate through space, while another clarifies that changes do not occur instantaneously but propagate within light cones.
  • There is mention of the Einstein-Infeld-Hoffman equations as a way to obtain approximate solutions under certain conditions, though it is noted that this approach can be tedious with modern computational methods.
  • Some participants express skepticism about the practicality of achieving accurate simulations due to the small differences between Newtonian and GR solutions, suggesting that adjustments to Newtonian solutions might be more feasible.
  • One participant reflects on the clarity of the problem statement in GR, despite the difficulty of finding solutions, and contrasts this with the sufficiency of Newtonian gravity for everyday applications.

Areas of Agreement / Disagreement

Participants generally agree that modeling the two-body problem in GR is complex and lacks a straightforward solution. Multiple competing views exist regarding the methods and feasibility of achieving accurate simulations, and the discussion remains unresolved on several technical points.

Contextual Notes

Limitations include the dependence on initial assumptions for the metric, the challenges of non-linear interactions in GR, and the computational demands of numerical simulations. The discussion highlights the unresolved nature of how gravitational interactions evolve in a two-body system under GR.

  • #61
1. Random numerical ##g_{ij}## will generate valid ##T_{ij}##, with matter everywhere:

rand01.png


Thus, the difficulty is in generating the black vacuum (zeros), instead of the (white) matter.

2. Combinatorially, we consider a 2x2x2 universe grid, with only 2 matter states (matter ON, and matter OF):

334563456.png


The major difficulty is in generating any (black) vacuum at all in any subsquare. But supposing we have found a solution that turns off any subsquare:

w23452345.png


By symmetry of the cartesian dimensional axis, we can deduce the correct ##g_{ij}## codes to turn off any other single square configuration on the grid. Thus, knowing just 1 single-black-square solution can generate all other single-black-square solutions.

Similarly, know 2 distinct solutions that turns off 2 areas of the grid:

234523454345.png
and
243523452.png


allows us by symmetry to deduce the ##g_{ij}## numbers that will turn off any 2-black-squares on the grid.

Further, knowing 2 distinct solutions that turns off 3 areas of the grid:

2523432.png
and
43523452.png


enables us by symmetry to deduce the ##g_{ij}## numbers that will turn off any 3-black-squares on the grid.

Then, knowing 4 distinct solutions that turns off 4 areas of the grid:
65436543.png
23452342.png
(file limit reached)

allows us by symmetry to turn off any 4-black-squares on the grid.

There are 2 distinct solutions for 5-black-subsquares (ie. 3-white-subsquares).
There are 2 distinct solutions for 6-black-subsquares (ie. 2-white-subsquares).
There is 1 distinct solution for 7-black-subsquares (ie. 1-white-subsquares).
There is 1 distinct trivial solution for 8-black-subsquares (zero ##g_{ij}##).

Not counting the trivial solutions for all-white and all-black configurations, there are ##2+2+4+2+2=10## configuration solutions that must be discovered before we are masters of this universe, ie. are able to generate any of the ##2^8=256## possible matter states, and to evolve any sequence of matter states on this grid, eg:

63456354.gif


3. The unfortunate case of the Kerr solution is that it generates 1 single-white-square at the center of a 3x3 grid, for which no other solution can be deduced by symmetry.
 
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  • #62
James1238765 said:
1. Random numerical ##g_{ij}## will generate valid ##T_{ij}##, with matter everywhere
In the sense that you can write down any metric you like, compute its Einstein tensor, divide by ##8 \pi##, and call that the "stress-energy tensor", yes. But whether the resulting SET describes anything that could physically exist is a very different question.

James1238765 said:
the difficulty is in generating the black vacuum (zeros), instead of the (white) matter.
On the contrary, solving the EFE for the case of vacuum is considerably easier.

The rest of your post is just handwaving and is getting considerably more off topic for this forum, since personal speculations are off limits.
 
  • #63
The OP question has been answered as well as it can be. Thread closed.
 

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