# Gravitational Lensing : Eddington Experiment

Discussion in 'Astronomy, Exobiology, & Cosmology' started by The God, Nov 29, 2015.

1. ### hansdaValued Senior Member

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That means there is no physical reality of spacetime in the "weak field approximation".

So, this is a "mental construct" or "mathematical model" which does not have any corresponding physical reality in the "weak field approximation".

Mathematics is OK.

3. ### hansdaValued Senior Member

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I am only trying to know the physical reality of spacetime in the "weak field approximation".

5. ### hansdaValued Senior Member

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"Curvature of spacetime" is associated with complete potential of GR. Thats why in "flat spacetime", "weak field approximation" is carried out.

Who is the small child. As long as one is not convinced with the physical reality of spacetime in the weak field approximation, this issue can rise.

Do you think in one model it is flat spacetime and in another model it is curved spacetime. Is the reality depended on model?

Is there any model combining Newtonian model with GR? Though I think it will be great, if any such model is there.

7. ### originIn a democracy you deserve the leaders you elect.Valued Senior Member

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Here is a video that is shows how curved space on the earth results in what we see as gravity. I think it was paddoboy who first posted it here. Maybe this will help.

8. ### SchmelzerValued Senior Member

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Learn the math about weak field approximation of GR. It is an approximation of the solution, that means, a spacetime metric $g_{\mu\nu}(x)$ which is not exactly a solution of the Einstein equations, but only approximately. But everything else is full GR, the formula for curvature is that of full GR, the geodesic equation, time dilation and so on follows the same formulas for a given metric as full GR.
I don't know who is the small child you have eaten yesterday. This was simply a polemical example of an incorrect question.

Of course, Newtonian theory has a different model of reality as GR. Ether theory has yet another. Realism fixes only the general idea that some observer-independent reality exists, but does not provide a particular model. A model of what really exists is given by particular physical theories, and every theory can, in principle, describe another reality.

Approximations usually do not change the model of reality, they simply give some slightly incorrect values for the related numbers. Interpretations, instead, often change the model of reality.

No. The Newtonian approximation of GR is inside GR, all it uses is a solution of NT (a particular Newtonian potential) to define a particular metric $g_{\mu\nu}(x)$.

9. ### PhysBangValued Senior Member

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I don't know how to interpret that sentence.
When we speak of the "weak field approximation", we generally mean an application where we can forgo most of the machinery of GR in favor of using some sort of (somewhat modified) Newtonian mechanics for the purposes of rough calculation. That's the model.

10. ### PhysBangValued Senior Member

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You are confusing something done for the sake of expediency or efficient with something done for the sake of a proper description.

In every case in gravity, GR is the correct thing to use for the sake of a proper description. But it is not always expedient or efficient to use the full and correct theory.
That model is GR.

11. ### brucepValued Senior Member

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Yeah I shouldn't always be so critical. The actual physical reality is that the local spacetime curvature, gravity, is an infinitesimal, so we can use any coordinate system we choose to model this actual local spacetime geometry. We use the coordinates of special relativity and if we need to account for infinitesimal local spacetime curvature we can use the coordinates of GR or any mathematical model that we can derive from weak field coordinates plus corrections derived from strong field coordinates. These are tools the physicists, like Phybang, use to do research. Keep in mind these weak field models make predictions with respect to natural phenomena which can be empirically falsified. Empiriacally testable. So the models are falsifiable if they don't make accurate predictions. The precision that Physbang mentions is a consequence of the goals of the analysis. For most weak field analysis Newton's model accuracy is good enough. Consider these coordinate choices, made by the observer, as a tool for reaching the goals of the analysis. For instance the near GPS spacetime can be modeled with weak field approximation of the Schwarzschild metric of GR. I'll link this GPS project that uses some weak field approximation in the analysis of the infinitesimal differences in tick rate ratio between the earth based clock and the satellite clock. The prediction is accurate to trillionths of a second compared to a full blown GR analysis. The first approximation is to set the theta coordinate at zero in the schwarzschild metric. Then at [5] they modify [4] to to get the weak field approximation at query 1. Professor Taylor and Professor Wheeler use this choice of coordinates as a pedagogical tool for assisting beginning students in doing the analysis. Still the analysis is within trillionths of a second accuracy compared with of a full blown analysis using GR. Essentially the prediction is identical to within ten decimal places. So the choice of coordinates are a consequence of the precision associated with the analysis.
Choose the project for the GPS.

Last edited: Jan 29, 2016
12. ### The GodValued Senior Member

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No, I am not, you claim some point and failed to provide further information on this point.

This is false, Euclidean straightlines have no meaning (useful purpose) in curved spacetime. You trolled on straightlines in absence of mass / energy. You supported that picture, because the issue of straightline arose from that picture only. Now you are coping out.

When you imply that you are a PHD (PhD) in cosmology, then your words can be taken seriously by many guest visitors on this forum, so you cannot claim that you are not making an argument from a position of authority. Honesty demands that after claiming that you are a PhD, you must state your field of specialization.

Thats bad. First of all you have not acknowledged that conceptually Newtonian Universal Gravity is different from GR in flat spacetime, despite clear explanantion by Schmelzer too. You are still insisting on this inaccuracy.

Secondly approximation comes from mathematical aspect of Physics. If a Physics application requires 2+ 2, then it is exactly 4, without requiring approximation. You are not able to differentiate that approximation in order to make a practical acceptance is something different from approximations made due to our inability to solve exactly. Try solving a multi body problem in GR, try solving Mercury precession when Sun is also moving around its CM, try solving orbital motion. (read about Kerr Solution of EFE, you will learn something more on the subject)

13. ### PhysBangValued Senior Member

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So you say. I'm waiting for your mathematical proof of this.
Indeed, I believe that there will be straight lines in the absence of mass/energy. Though, to be correct, there also has to be the absence of a cosmological constant.
I supported that position because I learned it in classes and read it in every relativity textbook. I don't care about your pictures and your lying will not change that.
Only you care about credentials. I care about the source of your claims. If you produce a mathematical proof to support your claims, I will believe them. Since I know that you are writing garbage, I won't expect one.

Nothing that you or that Holocaust denier write will change the facts that every physics application is an approximation. Nor will it change that GR is designed to reproduce Newtonian mechanics in certain circumstances.

And nothing will change the fact that you are still trying to pathetically claim that one cannot make extrapolations in GR about what would happen in the absence of mass/energy.

I'm done with this clown. The rest of you can read his attempts to make himself look smarter than physicists by denying the basics of geometry.

Last edited: Jan 30, 2016
14. ### The GodValued Senior Member

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And the troll is attempting to run away after abusing, he cannot handle with proper arguments.

Don't push crap again. PPN is not Modified Newtonian Gravity (Universal), this is the approximation of GR.

So, now you have learnt a bit, good. You still have to answer where in the universe you will find the space without mass and energy...this question was asked to you, 1000 posts ago and you are still trolling.

Don't bluff. No relativity textbook talks of Euclidean Straightlines in curved spacetime. I had tried to educate you that a line of lesser curvature than null geodesic has no existence in curved spacetime, but you want to remain ignorant PHD.

No, i don't. I respond to Paddoboy and Brucep also, and both honestly claimed that they have no formal education. You, in my opinion, fraudulently claimed to be a PhD...if you are a PhD in cosmology, why not state the field of specialization.

You are not qualified to question the Physics proposed by him. You pretend to know a bit of GR, thats it.

You have yet to tell where do you find a part of space without mass and energy..You understand that you will realise how stupid your point is.

Abuses after abuses. whom have you not abused ? I am surprised why Mods are letting you post so much abuses and so much bad Physics.....

15. ### originIn a democracy you deserve the leaders you elect.Valued Senior Member

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On the irony.....

16. ### hansdaValued Senior Member

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Just tell me:

1) Whether "spacetime curvature" exists in reality, physically in the "flat spacetime"( or in weak field approximation ) ? ( note: answer to this question should be either YES or NO )

2) If the answer to the above question no 1) is YES; Is this curvature an infinitesimal? What do you mean by "infinitesimal spacetime curvature" ?

3) If the answer to the above question no 1) is NO; that means spacetime curvature in the flat spacetime( or in weak gravity field) is a mathematical model. "so we can use any coordinate system we choose to model this actual local spacetime geometry"(this is your quote).

17. ### brucepValued Senior Member

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It's pretty simple. The theory of general relativity models gravity as local spacetime curvature. With respect to this model gravity IS local spacetime curvature and IS considered to exist for the GR domain of applicability. So a absolute answer that it exists, or doesn't exist, is irrelevant. It exists in GR but it doesn't exist in Newton's Law of Gravitation. Personally I think we directly measured the geodetic effect, spacetime curvature, during the Gravity Probe B experiment. If you want a yes or no answer then ask me whether the predictions derived from EInstein's theoretical model have been empirically confirmed when tested. The answer is yes.
Infinitesimal means very, very, ....... small. So small that when we connect the dots tangent to every point on the spacetime manifold we find out that the local spacetime can be flat over very large areas. Even in the strong field the local spacetime is flat over the segment where you're at. At the very least. The choice of coordinates used in an analysis, what scientists do, is a function of what's appropriate for the goals of the analysis they're working on. It's about modeling natural phenomena not finding absolute answers to questions that can only be confirmed or falsified each time the questions asked.

18. ### hansdaValued Senior Member

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It seems your answer to my question no 1) in the post # 993 is NO.

19. ### The GodValued Senior Member

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Answer to Q1 is 'Yes' from the GR perspective. Just pick up the EFE, ignore CC for time being, try to solve the equation, technically speaking you will get a metric having non zero curvature value...

As i stated earlier somewhere that a realistic measure of spacetime curvature is R(s)/R, so for earth it comes out to be of the order of 10^-9, call it inifinitesimal or whatever. It is pure maths. To understand the locality and weak field approximation we need to bring in a discussion on Equivalence Principle too.

20. ### hansdaValued Senior Member

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My question Q1) in the post # 993, was in the perspective of existence, in the perspective of physical reality of spacetime.

21. ### The GodValued Senior Member

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Sorry, I missed to add, spacetime is not a thing, it is a mathematical geometry, mathematical model.....I must add, on this forum there is a distinct disagreement on this point, many posters feel that spacetime is real (in a sense physically perceptible).

There was a detailed discussion (full of acrimony) on this point in which Prof Geraint Lewis posted a link of his paper clearly confirming that spacetime is not a thing, and the kind of education being given to students with a perspective that it is some kind of thingie is not proper....

22. ### The GodValued Senior Member

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The problem with people like you, is that you want this sub forum, to discuss only superficial aspects. Anyone attempts to get into depth and trolling starts. You are one of the key hindrance to any fruitful detailed discussion on the subject, as you start abuses or start posting pile of bricks....You must realise that goody goody discussions won't yield anything, the true indepth learning comes only when there is a counter argument.....What can be counter argument for potatoes growing on mars, or mars colonization by 2025, or science news based artiles ? just few goody goodies and thats all, they can co exist but they cannot sustain this forum....you want this forum to be a copy paste clone of some popscience journal ?

23. ### originIn a democracy you deserve the leaders you elect.Valued Senior Member

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Incorrect. The problem is unsupported pseudoscience.
Incorrect. The problem is what you call 'depth' is unsupported pseudoscience.
Incorrect. Pseudoscience is not going to lead to fruitful discussions.
That is not true.
?
You have shown that you do not understand the difference between real science and 'popscience'.