Numerical GR

  • Thread starter edpell
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  • #26
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I know someone (online) who is a numerical relativist working on the 2-body problem at The API in Jena (Germany), but if he's on this forum I don't know what his nickname is. He's a recent PhD so I'd say that would work... maybe I can ask him to come here, or I can relay a question to him if you like?
If he'd be willing to come talk basics about his research, that would be a lot of fun. I've always been curious how they do numerical GR.

If you can only relay questions, I guess what I wrote in https://www.physicsforums.com/showpost.php?p=2646415&postcount=9" is along the lines of what I'm curious about. I have a feeling my ignorance of the field would require some translating before those are useful questions though.

Probably the most approachable question is this:
Naively, when I look at Einstein's equations, it only gives information for the Ricci curvature ... so how do you determine the Weyl curvature evolution in numerical GR?

My understanding is that in analytic solutions they use symmetry arguments and boundary conditions at infinity to constrain the form of the metric, which effectively puts in the Weyl terms. Maybe that is not correct, but even if it is along the right track, in dynamic situations you don't have those luxuries. Naively it looks like the Weyl curvature can just evolve however it wants (I assume that is wrong for some reason though).
 
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  • #27
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My understanding is that in analytic solutions they use symmetry arguments and boundary conditions at infinity to constrain the form of the metric, which effectively puts in the Weyl terms. Maybe that is not correct, but even if it is along the right track, in dynamic situations you don't have those luxuries. Naively it looks like the Weyl curvature can just evolve however it wants (I assume that is wrong for some reason though).
I also have a very limited understanding of the issue for GR simulation. But it does seem like two of the issues are initial conditions and coordinate systems.

I would hope that there is some way to give an initial placement and velocity to N-bodies and then through some sort of relaxation technique take an initial guessed (known wrong) space-time condition and relax it to the correct space-time condition.

On the coordinates the paper offered in an earlier post covers some of this. They use two systems of coordinates one fixed and one deformable. The issue is in GR you do not have a fixed cartesian (sic) grid the space-time deforms! So what do you simulate? Give me a lever and a place to stand and I will move the Earth, in this case give me a place to stand and I will simulate GR.
 
  • #28
Stingray
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Gravitational radiation? Can you support that statement with some reference where I can see exact equations used in such simulations, or if you can just write the equation down here, please?
Einstein's equation: [itex]R_{ab} - \frac{1}{2} g_{ab} R = 8\pi T_{ab}[/itex]. This is a nonlinear PDE. Unlike in Newtonian gravity, the field has its own dynamical degrees of freedom in GR. You can not code up anything to solve this in 5 hours. You need a great deal of theoretical knowledge even to turn it into something you might try to write into a program. It is still extremely difficult and time consuming to get high quality simulations. The problem is VASTLY more complicated than solving a problem in Newtonian gravity.

That said, something like the solar system does not need full GR. It is adequate to use what is called the post-Newtonian approximation of it. This assume weak fields and slow speeds to analytically simplify the equations. The result can be simulated without much effort. The very lowest order corrections have a similar effect to making the gravitational field of the Sun look like it is coming from a somewhat more oblate object. This kind of thing is included in modern simulations of the solar system.
 
  • #29
atyy
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I also have a very limited understanding of the issue for GR simulation. But it does seem like two of the issues are initial conditions and coordinate systems.
http://relativity.livingreviews.org/Articles/lrr-2000-5/ [Broken]

http://www.astrobiology.ucla.edu/OTHER/SSO/ [Broken]
 
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  • #30
Stingray
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Nabeshin,
How do people even do numerical simulations?
Naively, when I look at Einstein's equations, it only gives information for the Ricci curvature ... so what determines the Weyl curvature evolution?
You split the spacetime up into a stack of spacelike hypersurfaces representing surfaces of constant time. It is usually assumed that each of these hypersurfaces has the same topology, so the manifold looks like [itex]M = \Sigma \times \mathbb{R}[/itex]. You now specify initial data on one of the hypersurfaces and evolve it forward.

This requires writing Einstein's equation in terms of geometric objects intrinsic to the hypersurfaces. They all have an intrinsic (3D) metric as well as an extrinsic curvature. These things can be related to the 4D curvature (and therefore Einstein's equation) using the Gauss-Codazzi equations. You also have to worry about how the hypersurfaces stack on top of each other. This is parameterized by a lapse function ("relative time separation between leaves") and a shift vector ("shearing between leaves"). The lapse and shift are not constrained by Einstein's equation, and must be specified. Bad choices quickly lead to singular coordinate systems (and crashed computers).

In terms of these variables, Einstein's equation reduces to a pair of constraint equations and a pair of evolution equations for 3-metric and extrinsic curvature. The constraints are analogous to Laplace equations, and do not involve time derivatives. If they are satisfied on one hypersurface, one can show that use of the evolution equations alone guarantees that they are satisfied on all other hypersurfaces.

The problem of specifying initial data is very difficult. So is the boundary problem. There are also a lot of subtleties with precisely which form of the Einstein equation to use, which variables are most efficient (the ones I outlined above aren't the best for numerical stability), lapse and shift choices, etc.

Although I'm not working in numerical relativity, I have a fair bit of knowledge about it if you have more specific questions.
 
  • #31
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Stingray said:
Einstein's equation: [tex]R_{ab} - \\frac{1}{2} g_{ab} R = 8\\pi T_{ab}[/tex] . This is a nonlinear PDE. Unlike in Newtonian gravity, the field has its own dynamical degrees of freedom in GR. You can not code up anything to solve this in 5 hours. You need a great deal of theoretical knowledge even to turn it into something you might try to write into a program. It is still extremely difficult and time consuming to get high quality simulations. The problem is VASTLY more complicated than solving a problem in Newtonian gravity.
Complexity does not scare me, but I acknowledge you're haste to underestimate a complete stranger. The question is rather if you can write down the meanings of the terms in those equations and their relation to physical properties so we know how to input real numerical values and use it in practical case scenario.


Code:
           r
M1-------------------M2     


         r'
M1->-----------<-M2
At time t0 relative velocity between mass M1 and M2 is zero, the distance between them is r, what is their velocity and distance at time t0+10 seconds? - Can you show me how the terms in GR equation relate to this and what would be their numerical value here? That's all I need to know.


That said, something like the solar system does not need full GR. It is adequate to use what is called the post-Newtonian approximation of it. This assume weak fields and slow speeds to analytically simplify the equations. The result can be simulated without much effort. The very lowest order corrections have a similar effect to making the gravitational field of the Sun look like it is coming from a somewhat more oblate object. This kind of thing is included in modern simulations of the solar system.
Can you write down this "correction equation" you are talking about so I can see the physical and mathematical meaning of that correction? - I do not believe any corrections of any kind are included in any simulation of any solar system, can you point any such software?
 
  • #32
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@Dragger: My angry friend, with slight amusement I acknowledge your emotional distress, but I do not recall to have been talking to you before, so can you just tell me what is it we are arguing about and what did I say to make you cry? -- You are confusing analytical and dynamical "solution", former is 'exact' and later is 'approximation', by definition. You surely do not mean to deny that classical modeling of the sun, planets and all their moons - with high degree of precision and absolute stability through millions of years of simulated time - would be impossible, do you?


hamster143 said:
It is not necessary to use numerical GR to model solar system, because solar system can be modelled analytically to a high degree of accuracy.
Solar system can not be modeled analytically, but it can be approximated dynamically... with high degree of precision and absolute stability through millions of years of simulated time. -- This is not the question about necessity, but about ability. Let's make it really, really simple, let's forget all the moons and all the other planets, so lets just model Sun and Mercury, can anyone print down GR equation which can model this?


GR corrections to Newton's law are in good agreement with experiment.
What experiment? What correction?
 
  • #33
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@Dragger: My angry friend, with slight amusement I acknowledge your emotional distress, but I do not recall to have been talking to you before, so can you just tell me what is it we are arguing about and what did I say to make you cry?
Ah, Dunnis, that was the first time I've ever "spoken" to you. What you've interpreted as anger (and crying?) is just a global frusteration with the bulk of your posts. If you want to play the psychologist, let me help... I suppose there was some referred anger which I generally have aimed towards all people who talk, and talk, but choose not to listen to, or understand the responses given.

I think that's enough playing footsie, don't you? The point of the 2-body problem, beyond the actual model, is that when we talk about it we mean the EXACT solution. Again, obviously, when I say that 3+ is unsolvable, I am again speaking of EXACT solutions. You seem to be a code-jockey with delusions "above your station" so to speak. You were told quite a while ago that you were pursuing a dead-end, a "waste of computer time", and instead of accepting that as the state of affairs, you've reformulated your original pointless query into a new one.

The answer is the same, and it's the one Nabeshin, Justin Levy, and Hamster gave you. Like it or not. You're acting like a moderately well educated (I say moderate given "you're" instead of "your"... always a giveaway) brat. If what you're asking for is so easy, why not hit the old internet and find that equation? Better yet, find something simpler and see if you really CAN do any of what you claim.

Btw, this whole post is absolutely SOAKED in my angry tears. :rofl:





--------------------------------------------------------------------------------
@JustinLevy: I've invited him to join us here, or if he'd prefer not to, to relay your question. I suspect he might come over here, if he isn't already however.
 
  • #34
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JustinLevy said:
It would be a waste of computer time because the gravity is so weak in the solar system...

...it is a waste of computer time because you don't seem to understand how massive these calculations are.
Gravity is sooo weak.. I dooon't understand.. calculations are maaassive..
..it's waaaaste of time... it's sooo unnecessary. Awww, you had me at "hello".


Since you clearly do not know this field...
:rofl:

[edit: someone actually edited my post and removed the part where I said how amused I was with this comment, so I'll just follow Dragger's example, put 'rotating head' and call this person delusional, as that seem to the a proper and allowed way to communicate around here. Hilarious indeed.]


Let's make this very clear right now.
Let's do that, but let's also not forget that previously you said "video shows decaying orbits due to gravitational radiation", so again - can you support that statement with some reference?


Are you denying that Newton's gravity cannot explain the precession of mercury (already mentioned to you previously)? Are you actually claiming these must be error in measurements since it disagrees with Newton?
No, but I would like to know how was that conclusion made, when and by whom. What I can tell you though, is that I can model precession of Mercury in many different ways with Newton's gravity, since once all the planets and moons are in the simulation, all interacting simultaneously with each other, and once approximation is upgraded from point masses to volumes and densities, then everything works just fine.


If you are here to promote the Newtonian view over Relativity, I am not interested in having this discussion any further.
No, I'm here to learn how GR does numerical modeling, what is the meaning of the terms in those equations and how to apply it on the simple case scenario with only two planets.
 
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  • #35
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@Dunnis: "The simple case scenario with only two planets" :Rofl: ... Well, it's been nice knowing you on this site Dunnis, but I think the rest of us will be wishing you Adieu fairly soon. Insulting other users (one of which, not me) was trying to help you, is a quick way to feel the wintery freshness of BAN.

Oh, and... "No, but I would like to know how was that conclusion made, when and by whom."... ok... try Google, or a textbook, or school! :smile: You want to be spoon-fed GR... not going to happen, for you, or anyone.
 
  • #36
Stingray
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Complexity does not scare me, but I acknowledge you're haste to underestimate a complete stranger. The question is rather if you can write down the meanings of the terms in those equations and their relation to physical properties so we know how to input real numerical values and use it in practical case scenario.
Just getting numerical simulations of Einstein's equation that didn't crash almost instantaneously was a major research area for years. This has only recently been solved. The issues are very different from any other field of simulation I'm aware of. It is not a stretch to assume to that you do not have the background to figure this out. You clearly do not understand the physics at all. You cannot simulate this system without that background. It is not (to repeat again) just solving a bunch of ODEs.

At time t0 relative velocity between mass M1 and M2 is zero, the distance between them is r, what is their velocity and distance at time t0+10 seconds? - Can you show me how the terms in GR equation relate to this and what would be their numerical value here? That's all I need to know.
No. That's an extremely complicated question. There is no direct translation of Newtonian concepts in full GR. If you're really interested, find a review paper and try to understand the field that way. http://relativity.livingreviews.org/" [Broken] is a good place to start.

Can you write down this "correction equation" you are talking about so I can see the physical and mathematical meaning of that correction? - I do not believe any corrections of any kind are included in any simulation of any solar system, can you point any such software?
The basics of low-order post-Newtonian simulations are reasonably straightforward and could be explained to someone who is not an expert. I really don't have time to do this here. I can't point to any downloadable software for you (unless you just want a bunch of test bodies moving in the field of the Sun with no mutual interaction). As examples that these things are done, take a look at:

http://arxiv4.library.cornell.edu/abs/0802.3371" [Broken]: This paper shows that the solar system is significantly more stable over very long time scales with post-Newtonian corrections.

"trs-new.jpl.nasa.gov/dspace/bitstream/2014/8903/1/02-1476.pdf"[/URL]: This states that PN effects have been included in ephemeris calculations needed for spacecraft navigation since the 1960's.

[PLAIN]http://arxiv.org/abs/astro-ph/0701612" [Broken]: PN N-body simulation.

There are (and have been) experiments of many kinds in the solar system that accurately test the PN equations and look for deviations that might signal a problem with GR.
 
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  • #37
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Just getting numerical simulations of Einstein's equation that didn't crash almost instantaneously was a major research area for years. This has only recently been solved.
40 years spent just on that, right? I would also add, @Dunnis: Of course you're not afraid; to be afraid of something requires a basic appreciation of it, and you're literally clueless. You can't be afraid of a "threat" that is beyond your perception. Stop cluttering the thread.
 
  • #38
Stingray
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40 years spent just on that, right?
Probably closer to 30, but yeah, it was a really long time.
 
  • #39
Ben Niehoff
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Code:
           r
M1-------------------M2     


         r'
M1->-----------<-M2
At time t0 relative velocity between mass M1 and M2 is zero, the distance between them is r, what is their velocity and distance at time t0+10 seconds? - Can you show me how the terms in GR equation relate to this and what would be their numerical value here? That's all I need to know.
It's been pointed out that this question is much more complex than you expect. To elucidate some of the complexity, consider:

1. In GR, you have to solve a minimization problem and do an integral just in order to find the distance between two points! And the answer is not necessarily unique!

2. In GR, gravity is not a force at all. A free-falling body is in a local inertial frame of reference; that is, a free-falling body experiences no acceleration!

3. In GR, the amount of gravitation (i.e. spacetime curvature) between two bodies is not simply a function of distance (such as 1/r^2, etc.). Spacetime itself is a dynamical continuum, and needs to be approximated by some set of finite elements. This is akin to trying to simulate the electric field between N bodies, including radiation effects...except GR is more complicated because the evolution equations for spacetime are highly nonlinear.

4. In GR, gravitation responds not only to mass, but also to energy, pressure, and stress. The mass-energy density itself is not well-defined, but is an observer-dependent quantity. Instead, one needs the entire stress-energy tensor.

This is just the beginning. As others mention, there are also issues with boundary conditions, choices of coordinates, etc.
 
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  • #41
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Gravitational radiation? Can you support that statement with some reference where I can see exact equations used in such simulations, or if you can just write the equation down here, please?
Hulse and Taylor measured the orbital decay of a binary pulsar which was in agreement with the predictions of GR (and not Newton) and their work earned them a Nobel prize so it must have been reasonably rigorous. See http://www.cv.nrao.edu/course/astr534/PulsarTiming.html

This gravity simulator http://gravit.slowchop.com/ [Broken] unfortunately does not include PN effects but I could not resist linking to it, because it does produce very beautiful multiple particle gravity simulations that run fast and smooth on an average PC in 3D! Enjoy.
 
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Frame Dragger said:
Again, obviously, when I say that 3+ is unsolvable, I am again speaking of EXACT solutions.
Exact, eh? At first I thought it was ridiculous to even talk about dynamics and numerical modeling in terms of EXACT solutions, and then I realized your nonsensical objection is self-refuting.



1.) 3+ body problem is unsolvable EXACTLY

2.) Planetary distances, velocities, masses and densities are unmeasurable EXACTLY
--------------------------------------------------------------------------------------

* During 17th to 19th century, some people, different people, somewhere, somehow measured the rate of precession of the perihelion of Mercury's orbit, which led to the final conclusion this rate to be around 5599 arc seconds per century.

* Urbain Le Verrier reported that the slow precession of Mercury’s orbit around the Sun could not be completely explained by Newtonian mechanics and perturbations by the KNOWN planets and masses in the Solar system and according to KNOWN calculation.

These calculations were obviously never supposed to be exact, but what is worse you were not there to tell them it was IMPOSSIBLE, plus they used some analytical and not dynamical methods as there were no computers and numerical modeling back then. Their "calculations" were far from being exact, even appropriate, and their measurements of distances, planetary mass and densities were not EXACTLY what we know today, as they were oblivious to many moons and many other masses we later discovered in the Solar system and measured with much better accuracy. Nevertheless, they got some pretty numbers...


5025.6 - due to precession of the equinoxes - EXACT?
+531.4 - due to gravitational tug of other planets - EXACT?
------------------------------------------------------------------
5557.0(calculated) - 5599.7(observed - EXACT?)

= 42.7 arc SECONDS per CENTURY... give or take 900", eh?


First they use Newtons law of gravity to obtain some result they presume to be vey accurate, just to later use it against itself and prove it was never correct to start with?! - How in the world could anyone think they were able to exactly solve the precession of the equinoxes and gravitational tug of other planets?!! Analytically?? Without computers and with inaccurate and incomplete measurements that have much less accuracy than the final result?


Let me tell you, if you take MODERN measurements and dynamically integrate all the masses via volumes and densities, then you still can play around *within measurement error* and obtain wide range of conditions that will satisfy Mercury’s precession. For example, you can tweak and shift mass distribution of overall system, within measurement error, and get the precession due to equinoxes to be, say 1207 arc seconds, and due to gravitational tug of other planets 4393 arc seconds per century, which would agree with observation.

Yes, dynamical approximations can be far more "exact" than any of those measurements, otherwise we could not obtain stable orbits for millions of simulated years that underwent billions of perturbations and interaction with all the other planets and still match "exact" position at "exact" time of each planet or moon, where "exact" means that the difference between solution and observation is no more than the "error" in measurements themselves, and that is "exact" as you can get.
 
  • #43
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It's been pointed out that this question is much more complex than you expect. To elucidate some of the complexity, consider:
Don't worry about me, I like complexity. What you telling us is your own perception, and if you find it so complex that you can not solve the simplest problem, then maybe you should not be attempting to give advice about it at all.

You go ahead and pick any problem, example or equation from classical physics or dynamical modeling and computer simulation, and I will precisely tell you all the numerical values of all the constants and explain how all the terms relate to physical properties, and this is all I'm asking to know about GR equations - how to apply them on the simplest case scenario, NO MATTER HOW COMPLEX IT APPEARS TO ANYONE, it is either solvable or not.


1. In GR, you have to solve a minimization problem and do an integral just in order to find the distance between two points! And the answer is not necessarily unique!
You are not making sense, you just said the problem can not be solved.

Please provide reference since you seem to be misinterpreting something.


2. In GR, gravity is not a force at all. A free-falling body is in a local inertial frame of reference; that is, a free-falling body experiences no acceleration!
I do not care what you name your terms, I just want to see how they relate to practical case scenario, then I'll figure out the rest and what is what.


3. In GR, the amount of gravitation (i.e. spacetime curvature) between two bodies is not simply a function of distance (such as 1/r^2, etc.). Spacetime itself is a dynamical continuum, and needs to be approximated by some set of finite elements. This is akin to trying to simulate the electric field between N bodies, including radiation effects...except GR is more complicated because the evolution equations for spacetime are highly nonlinear.
Ok. Now, you either can solve that problem or not. If you can, than you should be able to print those equations down and show how to use them in this simple case scenario.


4. In GR, gravitation responds not only to mass, but also to energy, pressure, and stress. The mass-energy density itself is not well-defined, but is an observer-dependent quantity. Instead, one needs the entire stress-energy tensor. This is just the beginning. As others mention, there are also issues with boundary conditions, choices of coordinates, etc.
Sounds interesting. So, do you know how to apply those equations to this example, or not?
 
  • #44
atyy
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Let me tell you, if you take MODERN measurements and dynamically integrate all the masses via volumes and densities, then you still can play around *within measurement error* and obtain wide range of conditions that will satisfy Mercury’s precession. For example, you can tweak and shift mass distribution of overall system, within measurement error, and get the precession due to equinoxes to be, say 1207 arc seconds, and due to gravitational tug of other planets 4393 arc seconds per century, which would agree with observation.
See section 3.5 of http://relativity.livingreviews.org/Articles/lrr-2006-3/ [Broken]
 
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  • #45
atyy
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Furthermore, if you don't know the global topology ahead of time, and instead only know the "topology" of a spacelike slice ... how can you run the equations forward at all? Einstein's equations are local evolution rules, so how can local evolution dictate global topology (whether a spatial point like singularity or ring singularity, or causal horizon, etc appears)? For example the people doing numerical simulations looking at whether naked singularities can form. How can they do it without putting in the topology ahead of time? In a really fun case, how could you "solve" to see if a wormhole appears ... since it seems you'd have to put the topology in ahead of time, which would mean putting in the answer ahead of time?
Maybe you can find some references from http://relativity.livingreviews.org/Articles/lrr-2000-5/ [Broken] Section 3:

"With no matter to support the gravitational field, we find that we must usually use a spacetime with a non-trivial topology, although a black hole can be supported by a compact gravitational wave [2]. It is certainly possible to construct black-hole solutions supported by matter [90, 91, 5], but it is often desirable to avoid the complications of matter sources.

This raises a point about solutions of Einstein’s equations which we have not yet mentioned. When constructing solutions of Einstein’s initial-value equations, we are free to specify the topology of the initial-data hypersurface. Einstein’s equations of general relativity place no constraints on the topology of the spacetime they describe or of spacelike hypersurfaces that foliate it. For astrophysical black holes (i.e., black holes in an asymptotically flat spacetime), the freedom in the choice of the topology has relatively minor consequences. The primary effects of different topology choices are hidden within the black hole’s event horizon."
 
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  • #46
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atyy said:
See section 3.5 of http://relativity.livingreviews.org/...es/lrr-2006-3/ [Broken]

-"Now, the measured perihelion shift of Mercury is known accurately: After the perturbing effects of the other planets have been accounted for, the excess shift is known to about 0.1 percent from radar observations of Mercury between 1966 and 1990."
These "perturbing effects" are calculated with classical methods, so in essence that sentence claims the great deal of accuracy comes from some "dubious" calculations, where the error nonetheless is only a couple of percents, it's contradicting.

We have three different values here, and all three of them come with their own method of acquisition and error built within it, where the second two values require even more measurements that all bring their own errors with them.


1. "NUMBER A", perihelion measurements - observation error ?%


2. "NUMBER "CX", precession of the equinoxes - calculation error ?%
+ liner and spin velocity, density, mass distribution - observation error ?%

3. "NUMBER "CY", gravity tug of all the other planets -calculation error ?%
+ velocities and mass distribution of all the other objects - observation error ?%
-----------------------------------------------------------------------------


So the error of 43 per 5600 arc SECONDS per CENTURY proves very, very, very good accuracy of Newton's gravity actually. See if GR can account for the perturbation and gravity tug of all the other planets and get the accuracy withing 43 second per century.


So, first we say 3-body problem is IMPOSSIBLE to solve and when it nevertheless gets solved with 99% accuracy with Newton's gravity, then we say it is wrong because it was not 100% accurate, right? - Try to find the error for each of those numbers in calculations and measurements and you will see that 43 SECONDS per CENTURY is far, far less than what would be the expected error, or indeed far less than the error that comes along with any of those measurements.
 
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  • #47
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Exact, eh? At first I thought it was ridiculous to even talk about dynamics and numerical modeling in terms of EXACT solutions, and then I realized your nonsensical objection is self-refuting.
You don't even know what is meant by "exact" do you? Lord you're completely out of your depth. http://en.wikipedia.org/wiki/Exact_differential_equation
 
  • #48
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This is the closest thing to a simulation of an orbit in GR available on the internet http://www.fourmilab.ch/gravitation/orbits/ for the very limited case of a low mass test particle orbiting a massive body that has a mass that is many orders of magnitude larger than the mass of the test particle. The source of the applet is downloadable for anyone who wants to see the equations used. It does not seem to include orbital decay due to gravitational radiation but that is probably reasonable for a test particle of insignificant mass. I think the program uses exact GR equations which are possible for this very limited case, rather than numerical aproximations, but it might still be of interest.
 
  • #49
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On a note unrelated to Dunnis...

@Justin Levy:

From my friend working at API in Germany as a PhD in NR. He is working on the 2-body problem.

Grig said:
Hi FD,

This question is not simple to answer. First, though, I would point out that the Weyl tensor is not a primary variable of relativity. All information about a spacetime is contained in the metric, from which the Ricci tensor and Weyl tensor are computed. Just like any mathematical theory, Einstein's equations alone are not enough to determine the solution. We must make choices, such as the inclusion of black holes, or matter, along with spins, angular momentum and masses, not to mention gauge. Indeed, choices must be made about gravitational wave content. Those choices have an impact on the metric. Given proper choices, one can have enough data to use Einstein's equations to uniquely determine the spacetime metric.

Here's the way to look at it. General Relativity is a theory that admits all metrics which conform to Einstein's equations. That is, Einstein's equations give us a set of rules which determine whether a metric is allowable. The physicist must then see to it to choose a spacetime representing the desired physical solution which is allowable.

In numerical relativity, these choices are larely made in the initial data. While I don't choose the gravitational wave content directly, choices I make, such as conformal flatness, has a direct effect on the gravitational wave content. We then use Einstein's equations to evolve the metric. We're not evolving the Weyl Tensor. We simply calculate the Weyl tensor using the information we possess.

cheers,

grigjd3
I hope this is in some way helpful!
 
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  • #50
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Stingray said:
Just getting numerical simulations of Einstein's equation that didn't crash almost instantaneously was a major research area for years. This has only recently been solved.
Perhaps they should first learn how to write code, handle exceptions and debug before attempting to integrate anything, but I would not blame anyone for such funny failure if the function to be implemented is not temporally continuous, causal and sequentially deterministic.


The issues are very different from any other field of simulation I'm aware of. It is not a stretch to assume to that you do not have the background to figure this out. You clearly do not understand the physics at all. You cannot simulate this system without that background. It is not (to repeat again) just solving a bunch of ODEs.
I find your lack of faith disturbing. As I said, I happen to be a master of the dynamical systems, numerical modeling, animation and integration algorithms. We do not really need to talk about me anymore, so relax and concentrate on the discussion.

I did not mention any ODEs, solve the problem any way you can, if you can.


No. That's an extremely complicated question. There is no direct translation of Newtonian concepts in full GR.
Code:
M1= 950kg; M2= 730kg

          r= 25m
M1-------------------M2     


At time t0 relative velocity between mass M1 and M2 is zero.
Q: What is their velocity and distance at time t0+10 seconds?
Are you saying it would take you more than 10-15min to solve this by hand? I was not asking you to translate anything into anything, but simply to assign those variables to the terms in that GR equation. Ok, newer mind that, but can you then print down that other equation, easy one, the one you call "PN"?


The basics of low-order post-Newtonian simulations are reasonably straightforward and could be explained to someone who is not an expert. I really don't have time to do this here. I can't point to any downloadable software for you (unless you just want a bunch of test bodies moving in the field of the Sun with no mutual interaction). As examples that these things are done, take a look at:

http://arxiv4.library.cornell.edu/abs/0802.3371: [Broken] This paper shows that the solar system is significantly more stable over very long time scales with post-Newtonian corrections.

http://arxiv.org/abs/astro-ph/0701612: [Broken] PN N-body simulation.
Don't be too proud of this technological terror you've constructed. The ability to destroy a planet is insignificant next to the power of the Force... gravity force and vector calculus that is. These two papers, that's not really about any relativistic corrections as long as they not first learn how to make correct integration algorithm - they have unstable orbits, and I automatically know what rubbish integration methods they use, amateurs.


trs-new.jpl.nasa.gov/dspace/bitstream/2014/8903/1/02-1476.pdf: This states that PN effects have been included in ephemeris calculations needed for spacecraft navigation since the 1960's.
This is something I would like to see, but the link does not work.
 
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