Gravitational Lensing : Eddington Experiment

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

  1. The God Valued Senior Member

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    What is this altered version of Newtonian Physics, just the PHD bluff ?....I am sure MOND is certainly not what you are talking about here..
     
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  3. PhysBang Valued Senior Member

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    I'm glad that you admit that there is something that you don't know instead of pretending that you do.

    Read more of the posts and you will find the explanation.
     
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  5. hansda Valued Senior Member

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    Perhaps the "full machinery of GR" can not be applied in the "weak gravitational field". Thats why weak field approximation is made in GR to make it applicable to "weak gravitational field". As you say, it is a wrong but good approximation. I think Newton's model is a better choice than GR in the "weak field approximation" or "weak gravitational field".
     
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  7. Schmelzer Valued Senior Member

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    No, the weak field approximation uses a metric. Even the most trivial linearized gravity https://en.wikipedia.org/wiki/Linearized_gravity already uses a metric, and the whole Parametrized Post-Newtonian formalism https://en.wikipedia.org/wiki/Parameterized_post-Newtonian_formalism uses a metric too. It usually also uses a Minkowski background metric, so that \(g_{mn}(x) = \eta_{mn} + h_{mn}(x)\), but this does not mean that this background metric plays any role except a mathematical one. Once the resulting metric is non-trivial (non-Minkowski) it is (almost certainly) not flat.
    Even the simplest Newtonian approximation, the metric \(ds^2 = (1+\frac{2\Phi(x)}{c^2}) c^2 dt^2 - (1-\frac{2\Phi(x)}{c^2})(dx^2+dy^2+dz^2)\) which directly uses the Newtonian potential already describes a nontrivially curved metric.

    Of course, even if a flat spacetime would somehow be used in an approximation, which would be completely irrelevant. This is also irrelevant for the interpretation of the equations itself - an ether interpretation of GR of course uses a "curved" solution on a flat background, and this "curvature" would be as irrelevant for the interpretation as the fact that the metric measured by usual temperature-dependent rulers in a configuration with inhomogeneous temperature would be formally curved too.
     
  8. Schmelzer Valued Senior Member

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    Nonsense. Of course one can apply the full machinery. One prefers a simplified variant once it gives sufficient accuracy.
     
  9. hansda Valued Senior Member

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    How can spacetime curve in the "flat spacetime" ?
     
  10. PhysBang Valued Senior Member

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    GR was specifically designed to act in a certain way in a weak field: it was designed to work almost exactly like Newtonian mechanics.

    Newtonian mechanics is amazingly accurate, up to a point. Past that point, one needs to use GR.

    One can break down the different ways that GR adds to Newtonian mechanics and approximate them as add-ons to Newtonian mechanics that work in specific, limited circumstances. (This is what the PPN is all about.) The evidence for these add-ons lies in the full power of GR (or alternatives) rather than the add-ons themselves--it is only in the wider theoretical framework that they have the best evidence.
     
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  11. hansda Valued Senior Member

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    Let us talk about reality. The reality of spacetime. The reality of spacetime will not change, whether it is Newtonian Model(weak field approximation) or GR Model(strong gravitationa field). OR, do you think that the reality of spacetime will change in Newton's model and GR model ?
     
  12. PhysBang Valued Senior Member

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    Newtonian physics models a Galilean-relative, flat spacetime in all circumstances. GR doesn't. GR fits the available evidence, including all the evidence for Newtonian physics, better than Newtonian physics.

    Spacetime in the universe as we see does not seem to be flat, except on some scales. We can mimic the way many physical systems behave by pretending to have a flat spacetime and adding corrections based on our experience with using a non-flat spacetime.
     
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  13. paddoboy Valued Senior Member

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    Ignoring the nonsense at the beginning of your post and onto the really crazy part....

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    Let me count the ways:
    [1] Photons/light is massless, although it does have momentum, and it also follows geodesics in curved spacetime.
    [2] The star at the periphery of the galaxy is also following a geodesic in curved spacetime, but they follow different geodesics due to differences in other properties such as mass, direction, inertia, speed etc
     
  14. PhysBang Valued Senior Member

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    I think the proper response about nothing supposedly happening to the stars at the edge of the galaxy is: "It's called an orbit."
     
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  15. The God Valued Senior Member

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    Incorrect Physics ?? No science educated person will call approximation, which fits and gives satisfactory results, as incorrect..
     
  16. The God Valued Senior Member

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    good attempt, why don't you put up some mathematics and show the orbital speed required by the periphery star for a scratching photon deflection angle phi.....then you will realise the import of this.
     
  17. paddoboy Valued Senior Member

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    And you would normally accept the fact that anyone professing to know all there is to know in science, would also know what an orbit is.

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  18. paddoboy Valued Senior Member

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    [1] Photons/light is massless, although it does have momentum, and it also follows geodesics in curved spacetime.
    [2] The star at the periphery of the galaxy is also following a geodesic in curved spacetime, but they follow different geodesics due to differences in other properties such as mass, direction, inertia, speed etc
    or as PhysBang says, an orbit.
    So far all we see from you is the usual prancing about.
     
  19. The God Valued Senior Member

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    Lets see, where you are......you say..following different geodesic in curved spacetime.....so the spacetime in principle is not flat, because stars orbits (so called your different geodesics) are quite curved, are'nt they ? Try responding you will get the answer, take help.
     
  20. paddoboy Valued Senior Member

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    More relentless prancing about from our divine friend.
    You've been given two answers, both correct and both reflecting on your ignorance and fraudulent behaviour.
    In other words, as per gravitational lensing, no problem at all.
     
  21. The God Valued Senior Member

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    Pure Bullshit.....the concepts behind GR and Newtonian are entirely different. You do not design a theory to work almost exactly like some other theory.

    And what is that point ?

    This makes sense...not bad.
     
  22. The God Valued Senior Member

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    You did not answer, what is the altered version of Newtonian Physics ? Just the bluff ?

    Newtonian is simply Newtonian, and it matches satisfactorily with GR approximations in flat spacetime...There is no altered version of Newtonian in GR except when we talk of MOND...
     
  23. paddoboy Valued Senior Member

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    Something which as yet you have not done: Shame.
     

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