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Theories for Mechanisms Underlying SR/GR?

  1. Jan 22, 2012 #1
    If I understand correctly, SR & GR describe with accuracy (ultra-relativistic) motion, acceleration and gravity but do not explain the underlying mechanisms that may be causing it.

    What is the present state of theorising regarding the mechanisms which may underly SR/GR?

    To take the speed of light for one, has anyone proposed a mechanism which would explain "why" its speed is invariant in all frames of reference?

  2. jcsd
  3. Jan 22, 2012 #2


    Staff: Mentor

    No. At this time GR is a fundamental theory meaning that there are no theories to explain GR.
  4. Jan 22, 2012 #3
    I disagree. Quantum gravity is a theory which could explain GR. But I think that it should not give much more new elements than GR.

    would explain "why" its speed is invariant in all frames of reference?

    Because all inertial systems are equivalent.
    Last edited: Jan 22, 2012
  5. Jan 22, 2012 #4


    Staff: Mentor

    Possibly, sometime in the future. Right now that is merely a hope.
  6. Jan 23, 2012 #5


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    No theory explains its own "underlying mechanisms". If we had a better theory of gravity than GR, then it might explain GR, but then you'd be asking about its underlying mechanisms instead.
  7. Jan 23, 2012 #6


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    There are several attempts to formulate such a theory but as of today there is not one complete candidate (leave aside experimental testability). Some candidates try to explain gravity in terms of a deeper structure (LQG), some do nothing else but quantize GR (asymptotic safety), string theory tries to harmonize gravity with other forces, but I think we cannot say that it explains gravity.
  8. Jan 23, 2012 #7
    Grammar mistake: I think that it will explain GR, when it will be discovered (if it will be discovered.) I hope that you agree with this.

    I hope on simpler theories than string theory and even LQG, but this is not important here.
  9. Jan 23, 2012 #8


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    No, no grammar mistake, I don't think so; not in it's current formulation where it fails to (fully) incorporate background independence.
  10. Jan 23, 2012 #9
    But doesn't string theory say that background independence is only approximately true at the scales that GR operates in, but that it fails in other length/energy scales?
  11. Jan 25, 2012 #10
    I also believe that quantum gravity should be background independent. But, quantum mechanics is not background independent - do you so think that there are problems with quantum gravity background independence.

    What if we said that space inside of black hole does not exist similarly as tahions (the most probably) does not exists. This eases same problems at fusion of GR and QM.
  12. Jan 25, 2012 #11


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    Yes, I think so.

    String theory (from which gravity does emerge) is usually formulated in terms of a classical background (10 dim. spacetime with a certain geometry, e.g. flat 3+1 Minkowski space + compactified 6-dim. Calaby-Yau as a simple example). There are different possibilities and backgrounds which are all 'present' in string theory but except for rare cases I do not see that the theory can be formulated w/o first fixing a specific background.

    In loop quantum gravity one starts with a 3+1 globally hyperbolic spacetime which allowes for foliations, i.e. one fixes at least the topology to R*M³.
    [It is interesting that the final theory of spin networks seems to allow for a larger class of "geometries", i.e. the spin networks need no longer be dual to triangulations and therefore need not be equivalent to PL manifolds; in addition a graph (a spin network) does not have a fixed, integer dimension; last but not least in the deep quantum regime there need not be an emergent manifold at all.]

    So these two approaches seem to depend on certain background structures which look somehow artificial and should be removed frommthe final theory.
  13. Jan 26, 2012 #12
    Yes, such a mechanism was proposed right from the start; however it cannot be directly verified, as we cannot "see" at a deeper level than the phenomena described by those theories and QM. What kind of explanation are you looking for?
  14. Jan 26, 2012 #13
    This has been one of my gripes for a long time.

    Both SR and GR describe the effects, but not the causes.
    That is one reason I say, some may agree with me, SR and GR are incomplete theories. They are tips of some massive strange icebergs.

    Suppose a few inertial systems are moving around with different speeds. A high speed spaceship and a lightbeam zoom past them. Each inertial system will record different speeds for the spaceship but the same speed for the light beam. Right?
  15. Jan 26, 2012 #14
    space itself is the underlying mechanism.

    'space is the substance, matter is the unsubstantial dream' -einstein

    all the equations describe how different forms of energy react to being in space.
  16. Jan 26, 2012 #15
  17. Jan 26, 2012 #16
    Please give a reference so we can check the context of that statement.
  18. Jan 26, 2012 #17
    What about theory of Fotini Markopoulou? Where it belongs (close to LQG?) and how it is with its background independence? I think that there is not a problem.
  19. Jan 27, 2012 #18
    Here is a question- I have a decoration shaped like an M on my wall. Yet, when I stand on my head I see a W. What is the underlying mechanism?

    Obviously, there isn't one- its just a question of reference. I can describe my room equally well from the ceiling down or the floor up.

    How does this relate to the original question? SR is the same way. There isn't a causal mechanism- the situation is the same, it doesn't make sense to ask for one. Just like standing on your head doesn't change the room your in, change frames doesn't change anything about the situation.
  20. Jan 27, 2012 #19
    We don't have a fundamental theory of reality. We just have physics. For example your question takes on a totally different meaning depending on whether the universe is four-dimensional or whether the universe is a dynamically changing 3-dimensional universe. There does not seem to be any consensus among physicists about which model is appropriate for discussing your question.

    ParticleGrl's comment is particularly relevant to the 4-dimensional universe model (there isn't a causal mechanism--it's just all there--then, you might then ask, "How did it all get here?"). But, from here we can continue with many speculations, and that's why this kind of question is not dealt with in depth on this forum.
    Last edited: Jan 27, 2012
  21. Jan 27, 2012 #20
    And you do not see a problem with this scenario?
    OP's question was exactly about this, why in one case we have different speeds and light speed is the same for all inertial frames.

    That is why I tend to think light travels by embedding itself into 'space materials', considering space is a medium. The medium we still know nothing about. This medium may also be responsible for relativistic mass increases in SR.

    Theoretical and experimental investigation into empty space may seriously surprise us in the future.
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