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It is possible from the general relativity reach a ToE?

  1. Oct 29, 2015 #1
    Hello everyone!
    As an admirer of string theory, I have strong interest in the theories that purport to unify general relativity with quantum mechanics. In the case of string theory, the goal is to find a form of describe the force of gravity according to the principles of quantum mechanics, ie, we start from quantum mechanics to later incorporate general relativity. My question lies at this point. It is possible from the general relativity reach a ToE? What are the reasons to try the first approach? What are the theories that use this approach? I once heard in a documentary that one reason for Einstein's failure to find a ToE was just to have opted for this approach. This fact proceeds? Is possible to perform the reverse path of string theory? I appreciate any response and apologize for any theoretical errors contained in my question. :smile:
     
    Last edited by a moderator: Oct 29, 2015
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  3. Oct 31, 2015 #2
    Loop quantum gravity is a logical approach, but it is not yet a complete theory of gravity as opposed to GR which is extremely accurate and for all physical knowledge the simplest and complete as it incorporates SR up to the speed of light!
    String theory is basically a theory of anything, as MWI of QM says a possible infinite number of universes all exist apart from our one single universe at any one point in time. Far from the goals of saying, specifically, "This is all that is possible in our universe." But is that goal realistic, or does the inherent probability of random quantum fluctuations prohibit the clarity of observing through the shroud of uncertainty?
     
  4. Oct 31, 2015 #3

    mfb

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    The different options for string theory (with different physics) are a completely different thing than the different worlds (with the same laws of physics) in MWI.
     
  5. Oct 31, 2015 #4

    MathematicalPhysicist

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    @mfb can MWI and the different options for string theory coexist?
     
  6. Oct 31, 2015 #5

    mfb

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    You can have multiple different universes or multiple options for the same one, and interpret quantum mechanics in every one with MWI, sure.
     
  7. Oct 31, 2015 #6
    There is a difference between the multiverse in string theory and in MWI of QM. In string theory or in M-Theory, the idea of the multiverse arises from the requirement of the presence of 10 or 11 spacetime dimensions. As we live in (3+1)-dimensional spacetime, we may consider two alternatives to understand the existence of the extra 6 or 7 dimensions:
    (1) the extra dimensions are compacted into a very small scale.
    (2) our universe is localized on a dynamical (3+1)-dimensional object (D-brane).
    The first alternative reminds us of Kaluza-Klein theory and the second alternative opens up the possibility that there are other branes which could support other universes and these universes could be subject to other laws. Another point to be considered is that we can try to build a deterministic approach to quantum mechanics (local hidden variable), eliminating the uncomfortable possibility of MWI. However, my main question is why starting from quantum mechanics to later incorporate general relativity has been a most widely used approach? The reasons are historical or structural?
    ps.: I appreciate any response and as I am not an expert on the subject, I apologize for any theoretical errors contained in my comment :smile:.
     
    Last edited by a moderator: Oct 31, 2015
  8. Oct 31, 2015 #7

    mfb

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    It is (relatively) easy to write down something gravity-like in quantum field theory. It is unclear if that is well-defined and how we can use it for calculations, but the start is easy. You have everything together.

    GR is purely classical - it is missing all the features of quantum-mechanics, and you cannot express those as spacetime curvature.
     
  9. Oct 31, 2015 #8
    this is gauge gravity correct?
     
  10. Oct 31, 2015 #9

    mfb

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    I don't understand your question.
     
  11. Oct 31, 2015 #10
    I fully agree with your comment. We cannot express the features of quantum mechanics as spacetime curvature. However, we cannot express the features of general relativity in a quantized form. In fact, it is extremely difficult to quantize gravity and for me this is the biggest problem of this journey. It should be possible to express the features of quantum mechanics as spacetime curvature and the features of general relativity in a quantized form. As I heard once in a documentary that should not exist a theory to explain the stars and a theory to explain the particles, because the stars are composed of particles. After all, that's the hope of string theory. Most people agree that there is an incompatibility between quantum mechanics and general relativity. And most people believe that to correct the incompatibility, one theory should be reformulated, and this theory should be in principle general relativity. However, I can not understand the reasons for this choice. In my mind, general relativity is more consistent than quantum mechanics. As always, I appreciate any response and I apologize for any theoretical errors contained in my comment :smile:.
     
  12. Oct 31, 2015 #11
    what is an example ?
     
  13. Oct 31, 2015 #12
    As much as I love GR, I don't think it's possible to start with GR and end up with ToE. They're too many issues with GR involving what certain quantities actually mean. If you believe that GR is more logically, and mathematically consistent than QFT (I agree), then you will want to check out LQG.

    However, that being said, no matter what theory you look at will have it's flaws. ST, LQG, EFT, they all have problems!

    If you want to read some background on LQG, here is a great book to start: http://www.cpt.univ-mrs.fr/~rovelli/IntroductionLQG.pdf
     
  14. Oct 31, 2015 #13

    mfb

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    I'm not a theoretician, but the Lagrangian should be written down at several places.

    Example presentation for an effective field theory.
     
  15. Oct 31, 2015 #14
    i thought you were alluding to gauge gravity, gravity written as a yang-mills field
     
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