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Warping of space and special relativity?

  1. Dec 8, 2011 #1
    I saw couple of documentaries and read some books in general relativity. I find the theory quiet impressive. But I am confused with the notion that heavy and big objects warp space more than light objects. Why should heavy objects warp space more. Space is not like we feel like in earth. Why should heavy object warp space and time more than light objects? any ideas

    Please don't say silly question(I also feel like that before but now I am quiet doubtful...it might have serious consequences in physics if we find reason), please post your views.
     
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  3. Dec 8, 2011 #2

    George Jones

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    Why does an apply fall?
     
  4. Dec 8, 2011 #3
    What does apply fall mean?
     
  5. Dec 8, 2011 #4

    Fredrik

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    What does "quiet impressive" mean? :wink: (He almost certainly meant "apple").

    General relativity doesn't explain why the relationship between the matter in spacetime and the metric of spacetime is the way it is. It simply states that the relationship is given by Einstein's equation.

    The only thing that can tell us why, would be a better theory of gravity, but we don't have one. And if we did, you'd be asking about its assumptions instead.
     
  6. Dec 8, 2011 #5
    What determines if an object is heavy or light depends on how much it warps spacetime.
    Thus if all objects would warp spacetime the same level we obviously would not be able to distinguish them as light or heavy. Hence since we can distinguish light and heavy objects it must imply that they warp spacetime differently.

    Note than an object warps spacetime only locally (Ricci curvature) as there is no action at a distance in GR. However since warped spacetime also warps neighboring spacetime (but with a certain 'damping factor') the effect gets propagated by the speed of light (Weyl curvature).

    Hence not only matter tells spacetime how to curve, also spacetime tells surrounding spacetime how to curve.
     
  7. Dec 15, 2011 #6
    Suppose you have an apple and it warps space. Then you break it into two halves and separate them by a fraction. The warping should not change much, one would expect. How much of the warping is due to each half of the apple?

    [This is not meant to be rigorous, just a rough intuitive picture of an additive effect]
     
  8. Dec 15, 2011 #7
    You probably mean...warp spacetime...that is space and time....

    Why should pressure, energy,stress and energy warp spacetime??

    That's just the way THIS universe works. If you subscribe to a many worlds interpretation, then some universes may not have such relationships...we just can't be there to observe that insofar as is currently known.

    The next step beyond quantum mechanics and relativity might be quantum gravity, so far an incomplete theory....stay tuned!
     
  9. Dec 15, 2011 #8
    Wow you guys are making this complicated. Heavy objects have more mass, more mass = more gravity, gravity warps spacetime only in that an object (be it photon or basketball) traveling in an inert "straight" path will inevitably follow the easiest route. If the obect's (photon, basketball, beer can) path goes near an apple it won't really be diverted, half an apple, half the diversion, but as it travels by black hole, it will certainly appear to alter it's direction / speed. I'm not much for the quanta, but do enjoy 'dem bosons. Also interesting, check out Erik Verlinde's "Entropic Gravity", Self-Creation Cosmos, or Conformal Gravity.
     
  10. Jan 19, 2012 #9

    zonde

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    By what criterions do you judge GR as the best?
    I think that SR is a way better if we speak about space, time and motion.
    And Newtonian gravity is better theory for gravity.

    There is something to talk about. Idea that time and space are unphysical is common understanding. Relativity on the other hand redefined time and space as clocks and rulers. And certainly clocks and rulers have physical properties and so have relativistic space and time.
    And so if someone is using space and time as commonly understood it's up to you to explain that within context of relativity space and time has slightly different meaning.
     
  11. Jan 19, 2012 #10

    PAllen

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    So all the tests described in the following, showing agreement with GR and disagreement with other theories of gravity don't count? Correspondence with observation is not relevant?

    http://relativity.livingreviews.org/Articles/lrr-2006-3/ [Broken]
     
    Last edited by a moderator: May 5, 2017
  12. Jan 20, 2012 #11

    Fredrik

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    I judge all theories by their ability to make predictions about results of experiments. A better theory makes more accurate predictions about results of experiments.

    I would agree that those theories are easier to work with, and also make pretty accurate predictions about most experiments. You could argue that they're more practical, but I wouldn't call them "better" when GR makes more accurate predictions about everything.
     
  13. Jan 20, 2012 #12

    Fredrik

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    In the first half of the part I quoted, yes. In the second half, no.

    These are good points, but I have explained such things too many times to still feel like answering every person who makes that mistake. When people make claims like "matter warping spacetime is nonsense" (effectively saying that GR is nonsense), I don't particularly feel like helping them at all. People who really want to learn come here to ask questions, not to claim that the theories we are willing to explain to them are nonsense.
     
  14. Jan 20, 2012 #13
    I wonder how an instructor of physics 101 at a community college must feel, year after year, the same questions, the same answers and the same blank looks.
     
  15. Jan 21, 2012 #14

    zonde

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    It should be possible to check:
    1) consistency of theory
    2) how predictions are made (they are consistent, they uniquely follow from proposed theory)
    3) interpretation of observations intended to test the theory should be independent from proposed theory

    And if I can check it myself then I would consider theory reliable.
     
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  16. Jan 21, 2012 #15

    Dale

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    For this, what is needed is a test theory. I think that the best one we currently have is the parameterized post Newtonian theory: http://en.wikipedia.org/wiki/Parameterized_post-Newtonian_formalism

    It is not completely general because it doesn't test for deviations from the equivalence principle, and it only tests the weak field limit, but it does admit a wide class of parameters that would falsify GR.
     
    Last edited: Jan 21, 2012
  17. Jan 21, 2012 #16

    zonde

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    If domains where two theories can adequately make predictions overlap this might work. What if they don't?

    That "everything" part is not quite clear for me. Is it really so?
    I believe there are rather few exact solutions in GR. And if you can't get exact solution then predictions are not exactly uniquely determined.
    Another thing is that GR is not extension to Newtonian gravity but rather alternative approach to gravity. This makes it harder to compare two theories. And it rises a question what you would get if you would continue with approach taken by Newtonian gravity?

    Sure, such claims are not the best starting point but slipping in new concept under common name that means something different is not very good starting point either. Another example being "'surface' of last scattering", grr.
    Say why isn't spacetime of relativity called something like "Einstein's aether"?
    Do you think you would still see claims like "matter warping Einstein's aether is nonsense"?
     
  18. Jan 21, 2012 #17

    Dale

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    This is incorrect. A solution may be numerical but still unique.

    That is why you need a test theory, like PPN, which encompasses both. PPN reduces to Newtonian gravity and to GR with suitable settings of the PPN parameters. So experiments which measure the value of one of the PPN parameters where GR and Newtonian gravity differ can legitimately compare the two theories. So far, all such tests have agreed with the GR values.
     
  19. Jan 21, 2012 #18

    zonde

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    No, you need rival theory for point 2) i.e. what predictions are worth testing.

    But you express prediction in terms that you don't question.
    So for point 3) you need previously established theory that you don't question. Like you don't question how your telescope is working (optics).

    In real life straw-mans are used for point 2) and you would assume that theory is true when interpreting observations in point 3). But that of course makes theory less reliable.

    (Weak) equivalence principle is prerequisite for Newtonian gravity and GR. We can test it but it is not prediction. If equivalence principle holds then predictions are valid according to theory. If equivalence principle does not hold the theory is silent (not wrong).
    But certainly we take principles as reasonable and usually we would have to test them only when we can't make sense assuming they are true.
     
  20. Jan 21, 2012 #19

    Dale

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    No, you need a test theory. A theory which has some parameter which leads to one theory in one limit and a large class of rival theories in other limits.

    Your point 2 can be determined purely mathematically and doesn't require any experimental testing at all, nor does it require reference to any other theories.
     
  21. Jan 22, 2012 #20

    zonde

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    It seems I misunderstood what you meant by test theory. But still why proposed theory should follow from other theory as parametrization of that theory? Maybe with parameters you mean the same thing as measurements allowed by that theory?
     
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