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Unification of GR and classical E&M

  1. Feb 7, 2005 #1
    It is my view that the road to a unified quantum theory of gravity and electromagnetism begins with unifying GR and E&M classically (i.e. a non quantum mechanical formulation). They show so may similarities that they may be more closely related than we think.
     
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  3. Feb 7, 2005 #2

    dextercioby

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    Interesting.Guess what,Theodor Kaluza thought of it 86 years ago and 79 years ago F.Klein approved giving mathematical consistency to his theory... :smile:

    Yes,the two CLASSICAL fields have many things in common,but so many different.

    Daniel.
     
  4. Feb 7, 2005 #3
    The two differences that come to mind right away are that gravity is only attractive and that there is no analog to the magnetic field in gravity. But just because these have not been detected does not rule out their existence. We need to keep the options open until they are explicitly rulled out.
     
  5. Feb 7, 2005 #4

    pervect

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    Theoretically, there is an analog to the magnetic field in gravity. This is sometimes called gravitomagnetism, and other times called frame dragging.

    Some detailed study of the orbits of the Laegos satellites seems to confirm the theoretical predictions of gravitomagnetism/frame dragging, and Gravity Probe B should provide a more direct and precise measurement of this effect.

    See for instance

    http://www.phys.lsu.edu/mog/mog10/node9.html

    for more details.
     
  6. Feb 7, 2005 #5

    selfAdjoint

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    This is what Einstein and Schroedinger were working on. J. A. Shifflett has carried the work farther and claims to have achieved a full EM field in the combined theory by careful choice of parameters.
     
  7. Feb 8, 2005 #6

    Chronos

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    GR and EM cannot be unified classically.
     
  8. Feb 8, 2005 #7
    What if a new theory came along, that explains relativit, gravity, and EM, QM, and everything of that matter :surprised Maybe in 1000 years, til then, there must be a unification. Its like saying gravity and relativity have no unification, until it came along (Einstine). Perhaps the current theories make unification close to imposible, because they dont describe everything 100% accuratly. Im my opinion, there will be a unification, when the right theory comes along, but classicly, probally not.
     
  9. Feb 8, 2005 #8
    Not a very scientific statement.
     
  10. Feb 8, 2005 #9
    Certainly yes, but a correct statement.
    For a more complete explanation, it should be (for example) good to consider the mathematical difficulties encountered by the Theory of Spinors (TS) to connect with the GR. As one knows that the TS is an excellent tool to describe rotations and reversals, that the Faraday's tensor (the EM strength tensor; 4-D representation) accepts a (4-4) matrix representation that can be considered as the representation of a bivector and that this one is equivalent to an infinitesimal rotation... one can may be guess the end of this unsuccess story and understand the statement. Regards
     
  11. Feb 18, 2005 #10

    Yes they can.

    Kaluza-Klein, by extending GR to five dimensions you get Maxwell's equations as well.

    Unless i am misunderstanding Kaluza-Klein?
     
  12. Feb 18, 2005 #11

    dextercioby

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    The point is:do you understand this theory...??Altogether,not only the results...

    Daniel.
     
  13. Feb 18, 2005 #12
    We certainly could have a long discussion as long as we would not have a common definition of the word "classical". If this word is a reference to a certain period of the history, this must be to the period +- 1890- 1920. EM theory was formulated in a 3-D space with consideration for the evolution of the things that is with consideration for the time. The GR is a kind of generalization of the Special Relativity which is introducing the time as an "adult" independant dimension in the discussion and we get with this a 4-D Theory. If we stop the history here, then the Klein-Kaluza Theory which is obviously an interesting generalization of the GR is no more classical. As you see just a question of definition concerning "classical".... Regards
     
  14. Feb 18, 2005 #13

    In a word: no. I'll admit that.

    But if my understanding of what it means is incorrect, please reeducate me.
     
  15. Feb 18, 2005 #14

    dextercioby

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    No,the result is okay.Indeed,Kaluza was able to write a Lagrangian action which,when reduced to 4D,would yield both H and Maxwell actions...


    Daniel.
     
  16. Feb 18, 2005 #15

    Classical in physics generally means non-quantized. Continous quantities and the like.
     
  17. Feb 18, 2005 #16

    dextercioby

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    The K-K theory is very classical,simply because at 1919,the QM was inexistent...So yes,history decides,even not knowing what the theory itself was about...

    Daniel.
     
  18. Feb 18, 2005 #17

    Chronos

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    Indeed, but Kaluza-Klein does not unify GR and EM any more than Einstein-Maxwell and produces results that do not agree with observation. Einstein spent half his life trying to classically unify GR with EM without success. No modern theorists, AFAIK, seriously consider it possible. Of course you need to define what is 'classical' to lend any meaning to such a statement. I generally think of it as the pre-quantum physics era. Perhaps it would be more rigorous to define it as solutions possible using only the mathematical tools available before the later half of the 20th century.

    The problem inherent to both GR and EM is they are incomplete. It is understandably difficult to unify theories that are themselves incomplete. Neither GR or EM are quantum theories, hence they are not considered theories of fundamental interactions, as are quantum theories. Maxwell's EM is an effective field theory or "classical" limit of a renormalizable quantum field theory - quantum electrodynamics [QED]. A renomalizable quantum field theory of gravity has not been found and there is no apparent way to unify EM and GR without a quantum theory of gravity. Bear in mind the only sucessful unification theory developed to date - GWS electroweak - came from QED. I seriously doubt any other successful unification theory will be achieved using a 'classical' approach. Perhaps string theory will ultimately triumph. But, you can hardly call string a classical approach with a straight face.
     
  19. Feb 18, 2005 #18
    Ok, this i did not know. I had assumed that KK was simply not pursued because with the discovery of quantum mechanics, maxwell's equations would have been outdated(the 'classical' limit), and so unifying GR with Maxwell would have been moot.

    I have read about good things coming in the field of loop quantum gravity, which surpisingly, i rarely ever hear mentioned here, while string theory is brought up all the time.
     
  20. Feb 18, 2005 #19

    Chronos

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    There are a number of us here that are very interested in LQG, but, most discussions are inspired by recent advances. String advocates are less frequently so inspired, but more passionate.
     
  21. Feb 20, 2005 #20
    This is a ridiculous statement. How can you say string advocates are more passionate? This is completely ill-logical.
     
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