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GUT vs Quantum gravity unification

  1. Apr 14, 2013 #1

    ftr

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    So why the hot pursuit of QG while GUT unification does not look even attainable. None(AFAIK) of the QG theories seem to address this question (GUT), or am I wrong.
     
  2. jcsd
  3. Apr 14, 2013 #2
    is GUT mean grand unified theory ?
     
    Last edited: Apr 14, 2013
  4. Apr 14, 2013 #3

    ftr

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    yes, that is correct.
     
  5. Apr 15, 2013 #4

    tom.stoer

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    String theory claims that quantum gravity must emerge from a theory addressing unification.

    However there are good reasons to believe that quantum gravity (= a consistent, well defined theory for the quantization of the gravitational field which resolves issues like perturbative non-renormalizability and singularities) can be constructed w/o addressing unification. It seems that non-perturbative techniques as applied in loop quantum gravity, asymptotic safety etc. allowes one to construct a consistent quantum theory.

    So it's reasonable to have more than one gemein town ...
     
  6. Apr 15, 2013 #5

    ftr

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    Thanks for the clarification. So it is clear now, QG thoeries (as they stand today) do not address full unification in the GUT sense, the origin (prediction) of the SM constants or the nature of QM, only the specific goal you mentioned. It seems to me then, the GUT should have more priority, since it is clear by now that QG theories are not able to address fundamental questions and they are very far away from TOE. Sorry if my question is not clear.
     
  7. Apr 15, 2013 #6

    tom.stoer

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    Your question is clear.

    But I do not see why a certain research program should have priority. It depends on the perspective, of course. If you are interested in particle physics, then 'GUT' is more important. But if you are interested in quantum gravity (+ other interactions on top), then QG should have priority.

    We do not know which research program will succeed, so most experts would agree that there should be more than one game in town ;-)
     
  8. Apr 15, 2013 #7

    ftr

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    Yes, I agree variety is good. But To my mind it sound easier and more important to UNIFY electroweak and the strong force in an unambiguous way (at least as first step). If we cannot do that, then there is a hint of either something is wrong with the formalism or the nature of these forces is not what we think it is. That might even shed some light on gravity. In another word, why not try another route, which seems to be little researched if not neglected.
     
  9. Apr 15, 2013 #8

    tom.stoer

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    a matter of taste
     
  10. Apr 16, 2013 #9

    PAllen

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    What makes you think GUT's are little researched? They have been enormously researched and continue to be researched. Just because they are not written up in popular press doesn't mean they aren't researched.
     
  11. Apr 16, 2013 #10

    ftr

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    What I meant is that GUT is not discussed by high profile scientists or Theoretical centers. Can you cite some, I appreciate it.
     
  12. Apr 16, 2013 #11

    PAllen

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    Super symmetric GUT's are the current favorite (a number of earlier, simpler, GUTs have been ruled out by experiment - they predicted too short a lifetime for proton decay). Here is an example of current theoretical research:

    http://arxiv.org/abs/hep-ph/0605139

    and an even more recent one:

    http://arxiv.org/abs/1302.2574
     
    Last edited: Apr 16, 2013
  13. Apr 23, 2013 #12
    ftr: There is a sort of standard idea of what lies beyond the standard model. It is, supersymmetry appearing around 10^3 GeV, grand unification appearing around 10^16 GeV, string theory around 10^19 GeV. That's a general paradigm that developed in the 1980s, and thousands of papers were written and are still being written which fit into that paradigm.

    Of course there are also many papers, and whole schools of thought, which disagree with one or more parts of the paradigm. But it has a lot of internal logic and some mild empirical support:

    There are several GUT ideas, known since the 1970s, which explain features of the standard model. SU(5) or SO(10) can explain the generation structure (quantum numbers and group transformation properties of the SM fields). Specific GUTs can also explain the Weinberg angle and some ratios of fermion masses.

    If the running of the coupling constants is extrapolated to high energies, the couplings come close, as if derived from a single unified force, and if supersymmetry is added to this, the convergence becomes very good. Supersymmetry can also stabilize the low mass of the Higgs scalar against quantum corrections that would make it heavier, and can provide dark matter particles.

    Finally, GUTs and supersymmetry naturally appear in string theory, along with gravity.

    This is the dominant paradigm in high-energy physics for many years now, and the GUT ideas are definitely part of the current research. But GUTs aren't new any more and so people don't get excited about them (compared to more recent topics). Also, the basic possibilities for GUTs - mostly I mean the different possible symmetry groups - have been known for a long time. If someone proposes a new model with a GUT component, the GUT part won't be the new part.

    Also, unless someone manages to observe proton decay, the evidence for GUTs is going to be somewhat indirect. We won't be seeing e.g. the X bosons of SU(5) in colliders, they are much too heavy. Instead, people mostly hope to arrive at the correct model by gradually extending the mild successes of 1970s GUTs that I already mentioned. See the abstract of the second link from PAllen. They talk about a choice of a "Yukawa flavor texture".

    A texture is a hypothesis about the matrix of "yukawa couplings" e.g. in the standard model, there are 3x3 yukawa matrices for up-type quarks, down-type quarks, and charged leptons, with the matrix elements being Higgs couplings between left-handed and right-handed components of the three generations. So in a contemporary GUT theory, you might choose the group and the "representations" (fields specified by their transformation properties under the group), and then add some theory about what the yukawa matrices look like at the GUT scale, e.g. that they contain a certain distribution of zeroes. Then you would extrapolate this to lower energies via "renormalization group flow" and see if it could be consistent with the measured properties of the particles. And ultimately, according to the paradigm I have described, you would try to explain the GUT group, representations, textures, etc., in terms of string theory (shape and size of the extra dimensions).
     
  14. Apr 23, 2013 #13

    ftr

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    Thanks Mitchell, a very nice summary.

    But I hope I am not much bother, can you list quickly the other main "school of thoughts", I would really appreciate it.
     
  15. Apr 23, 2013 #14
    No, that is too much to ask, there are too many variations. Just look at the threads in this sub-forum, there are many discussions of theories which don't fit the paradigm that I described.
     
  16. Apr 24, 2013 #15

    ftr

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    Yes, that was a laborious question. I will ask more specific ones later. Thanks, you were really helpful.
     
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