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Is there someone who knows the difference between String Theory and LQG?

  1. Sep 19, 2010 #1
    Hi, guys. I want to do some research on quantum gravity. I know that there are two theories achieving some success in this direction. Do you know something about that? I want to learn something about that. Thank you!
     
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  3. Sep 19, 2010 #2
    There are more than two theories, you've just only heard of two. And please don't be offended but If you can't distinguish between String Theory and Loop Quantum Gravity I don't understand how you'll do research in either subjects. Especially String Theory it is known for it's fundamental complexity hence the inability for the greatest Physicists to understand it. Unless of course by research you mean aspiration. Just read through these: http://en.wikipedia.org/wiki/Loop_quantum_gravity, http://en.wikipedia.org/wiki/String_theory.
     
    Last edited by a moderator: Apr 25, 2017
  4. Sep 19, 2010 #3
    Yes, I know there are a bunch of theories, but the rest appear not to be so successful.
     
  5. Sep 19, 2010 #4

    marcus

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    Qing, there is a third approach called CDT which is simpler than the other two.

    To begin to understand quantum gravity, I suggest you first try to understand CDT.

    There is a good magazine article that makes it simple. This was in the Scientific American magazine. I always keep the link to it in my signature at the end of the message.

    http://www.signallake.com/innovation/SelfOrganizingQuantumJul08.pdf [Broken]

    Read that first. Then I (or anyone else here) can tell you about LQG. And other people can tell you about String.

    But please first this CDT article will explain that in quantum theory things can have uncertainty, and quantum gravity is about the uncertainty of geometry. We are all used to geometry being sharp, clear, fixed, unchanging according to definite laws (like the angles of a triangle add up to 180 degrees). But we think in some situations geometry can behave in an uncertain indefinite "quantum" way. For example at very small scale. Or in extreme conditions at the heart of a black hole.

    The CDT researchers use a computer simulation to model small artificial quantum universes which they can study. What they do is a good place to start understanding the problem of how to get a quantum theory of geometry.

    (gravity=the curvature of spacetime, is just one aspect of geometry, namely how geometry is affected by massive objects like the earth and sun. To really do quantum gravity one must do quantum geometry.)

    So start out by reading this:
    http://www.signallake.com/innovation/SelfOrganizingQuantumJul08.pdf [Broken]
    Then after you have a taste of CDT we will know you better (from any questions you ask or comments you make) and we can begin explaining LQG to you.
     
    Last edited by a moderator: May 4, 2017
  6. Sep 19, 2010 #5
    Great! You know I have no idea about quantum gravity. I need this introductory material. Although I just graduated from an undergraduate school, I try my best to study it. Thank you very much!
     
    Last edited by a moderator: May 4, 2017
  7. Sep 20, 2010 #6

    tom.stoer

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    Of course this will be mentioned in the review articles marcus mentioned, but I want to stress one main difference between LQG and ST:

    LQG is about quantization of gravity "only".

    ST is about the construction of a theory from which all fundamental interactions, particle content, gauge symmetries including gravity can be derived. So yes, quantum gravity is a part of ST, but ST is more ambitious than LQG and tries to unify all forces; it's not restricted to just one force.

    You should have this always in mind when comparing the two research programs.
     
  8. Sep 20, 2010 #7

    marcus

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    Qing, I didn't know you were already a graduate from college or university. We should quickly find some more challenging things for you to read---the Scientific American article on CDT that I recommended is too elementary.

    Also Tom's point is an important one to emphasize.

    Since I was talking about CDT, and you may have looked at the article already, I will make a similar comparison to Tom's, but between String and CDT.

    String tries to be more like a 5-course restaurant meal, while CDT is like a simple bowl of rice.

    Everything = soup, appetizer, salad, main dish, dessert
    Blank geometry only = bowl of steamed white rice

    1. You may have to sit and wait for the restaurant meal to be prepared. Some of the dishes have not been invented yet.
    2. Even though its goal is simple, CDT is still a valid research program. It is clear what the theory is. It has gotten interesting results and some people make their careers in it.
    3. The LQG research program comes somewhere in between. It started out being mainly about pure geometry, but now seems ready to include some limited forms of matter. It is a little more complicated than CDT (but still on that end of the spectrum, closer to CDT than to ST.)
    4. I actually do not know a good up-to-date introductory review of LQG. I guess this is because the program is progressing fast, the theory is changing, and the researchers are too busy. The introductory articles that I know (I can get you URL links when you are ready) are already "old".
     
  9. Sep 20, 2010 #8

    atyy

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    http://arxiv.org/abs/1001.4188
    "While, as we will explain, a certain amount of unification of the description of matter and gravity is achieved, in fact, the question of whether matter fields must have special properties to be consistently coupled to gravity in the framework of loop quantum gravity is an important open question."
     
  10. Sep 20, 2010 #9

    marcus

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    Atyy cites a LQG review talk that is already over one year old. August 2009. Necessarily a bit out of date but still could be very useful as an introduction if one understands the context!
    http://arxiv.org/abs/1001.4188
    Loop quantum gravity - a short review
    23 pages, 7 figures, talk delivered at the workshop "Foundations of Space and Time - Reflections on Quantum Gravity" in honor of George Ellis, STIAS, Stellenbosch, South Africa, 10-14 August 2009
    Hanno Sahlmann
    (Submitted on 23 Jan 2010)
    "In this article we review the foundations and the present status of loop quantum gravity. It is short and relatively non-technical, the emphasis is on the ideas, and the flavor of the techniques. In particular, we describe the kinematical quantization and the implementation of the Hamilton constraint, as well as the quantum theory of black hole horizons, semiclassical states, and matter propagation. Spin foam models and loop quantum cosmology are mentioned only in passing, as these will be covered in separate reviews to be published alongside this one."

    In fact the reviewer, Sahlmann, takes a restricted view of LQG, he only talks about one approach out of several that are being pursued by LQG community. This could be misleading if one takes his words out of context. Much of the action in LQG during the past 3 years has been in the area of spinfoam models and the application of LQG to cosmology. This is what Sahlmann does NOT discuss because those topics were assigned to other speakers at the conference.
     
    Last edited: Sep 20, 2010
  11. Sep 20, 2010 #10

    MTd2

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    In ST, quantum gravity relies on the existence of other fields, whereas LQG doesn't.
     
  12. Sep 20, 2010 #11
    I also know that string theory is spacetime background dependent, while LQG is spacetime background independent. I think this is one of the great properties of LQG. What you pointed out is also great! Thank you!
     
  13. Sep 20, 2010 #12
    It sounds that LQG community is divided into different group. um, feel bad...
     
  14. Sep 20, 2010 #13
    Actually String Field Theory is Background Independent (http://en.wikipedia.org/wiki/String_field_theory), but it most likely isn't the appropriate approach to constructing a Theory of Everything. I feel that Superstring Theory/M-Theory is to some extent correct but will reveal a much more simplistic yet complex approach with the inclusion of Loop Quantum Gravity's geometrical approach, although, this is very wishful thinking.
     
  15. Sep 20, 2010 #14

    marcus

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    It sounds to me as if Hanno Sahlmann does not disagree with the others. He is cooperating with the other LQG people in a division of labor. They are at a conference. He talks about canonical LQG (the original 1990s approach) and someone else talks about spin foam models (SFM), and someone else talks about the cosmology application (LQC).

    You can think of it as 3 different work teams (canonical-LQG, SFM, LQC) but there is no hostility or disagreement. In fact one individual can spend some time on c-LQG, and then work on SFM, and then go over and work on LQC. An example of someone who does this is Carlo Rovelli.

    His March 2010 paper was on a combination of SFM and LQC. His April paper was a combination of SFM and c-LQG.

    He calls all these things LQG, since in the past 3 years they have tended to come together into a single approach (all contributing to each other's development.)

    Some people, when they talk about LQG they mean only canonical-LQG, and this can be confusing unless they explain. (But notice that Hanno Sahlmann explains that he is leaving the SFM and LQC topics for others.) It is a single community engaged in several different lines of investigation.

    So I see no reason for bad feelings, in this case. :biggrin:
     
  16. Sep 20, 2010 #15

    atyy

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    No, it's good that many directions be explored! There are some who are looking for unification in group field theory, which is related to spin foams, which is related to canonical loop quantum gravity.

    You can see that even possible links to string theory have been considered in the past http://arxiv.org/abs/gr-qc/9508064 . Although many of the technical details, especially the hope that gravity would be a topological theory, are probably wrong, this is very fine speculation.

    More modern speculation on possible links between LQG and string theory are found in http://www.emergentgravity.org/drupal/sites/default/files/EGIV_presentations/Vidal.pdf
     
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  17. Sep 20, 2010 #16
    atyy, which approach to Quantum Gravity do you prefer? Superstring Theory or LQG? I know Superstring Theory is an idea for a theory of everything but which is more convenient and elegant in your perspective?
     
  18. Sep 20, 2010 #17

    marcus

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    Rather than the Sahlmann review---which is written up from one short talk given over a year ago---I would prefer to recommend the Doná Speziale review.

    http://arXiv.org/abs/1007.0402
    Introductory lectures to loop quantum gravity

    This is also explicitly limited to the canonical-LQG. It only goes up thru spin networks. It does not get to spinfoams and to cosmology. But they are clear about that in their summary so it hopefully causes no confusion. Also it is based on a series of several one hour lectures. There is more to it, and it is more pedagogical.

    It has some of the hard stuff, if the reader wants it, in appendixes. It has some of the newer stuff too, like reference to the new paper by Bianchi Doná Speziale about quantum polyhedra.

    This Doná Speziale paper: the Introductory Lectures to LQG, is, I think, the most up-to-date introduction to regular canonical (spin-network-type) LQG. It even has a section mentioning a paper I am trying to read now, that came out yesterday! In case anyone is curious, here is the thread discussing that paper:
    https://www.physicsforums.com/showthread.php?t=430679
    The thread is called "Building space with quantum chunks" and it discusses the new paper called Polyhedra in Loop Quantum Gravity.
     
    Last edited: Sep 20, 2010
  19. Sep 20, 2010 #18

    atyy

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    I'm partial to Xiao-Gang Wen's approach :smile: But AdS/CFT is definitely in the spirit of "emergence", and one that works technically too, so I find that fascinating. The parts of LQG I like are those that are more emergent in spirit, like Livine and Oriti's approach to group field theory, and Markopoulou's attempt at emergent locality.
     
  20. Sep 20, 2010 #19
    Xiao-Gang Wen, a Chinese physicist! Very good! I am proud of him! Wish I would be a student of him.
     
  21. Sep 20, 2010 #20

    MTd2

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    But you sound like a Green Dragon!
     
  22. Sep 20, 2010 #21
    You are a Chinese guy? Are there many Chinese in this forum?
     
  23. Sep 20, 2010 #22

    MTd2

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    I am a Brazilian, without imediate chinese ancestors! Well, maybe 14,000 years ago! :)
     
  24. Sep 20, 2010 #23
    I am sorry. But you know the meaning of my name. You learned Chinese?
     
  25. Sep 20, 2010 #24

    MTd2

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    Yes, a little bit! :) I need practice though. If you want to help me, send me a PM!
     
  26. Sep 20, 2010 #25
    I am very glad to help you. But what does PM mean?
     
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