Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Strings or LQG and why?

  1. Sep 13, 2004 #1
    Strings or LQG and why???

    I am currently studying LQG and i would like to have an idea on how opinions are devided between String Theory and LQG.

    I would like to use the outcome of this poll as a reference to the introductory text i am writing on LQG. View the Loop Quantum Gravity-thread if you are interested.

    So here's the question. What theory do you think is the best approach in order to unify QM and general relativity ( to describe the quantum gravity).

    Thanks a lot in advance

    marlon :smile:
    Last edited: Sep 13, 2004
  2. jcsd
  3. Sep 13, 2004 #2


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    String is more of an approach towards unifying the treatment of all fields and forces. By contrast, LQG has the limited aim of applying QM to general relativity and seems more apt to achieve that narrowly defined goal. But that is what your poll is explicitly asking about!

    So although I answered Loop, Marlon, I don't think it is really a fair contest as the poll is stated. That's because quantizing GR is not String's main purpose, interest, or value. If, in the future, some version of String theory should emerge that makes experimentally testable predictions, it would be valuable based on other considerations than what you mention here.
  4. Sep 13, 2004 #3
    Yes, marcus takes the words right out of my mouth. String theory is a TOE, where lqg is a theory of quantum gravity. Now, a good question might be.... I think I'll start another poll.

    Paden Roder
  5. Sep 13, 2004 #4
    I feel LQG might demonstrate that there is no need for strings. Yet, string theories are so beautiful and have produce so many mathematical results, that this attempt might be considered better. I totally agree with Marcus too, and answered strings for other reasons :smile:
  6. Sep 14, 2004 #5
    So LQG does not have this feature to describe point particles, where a one dimensional string includes gravity.

    According to Wikipedia:

    1.loop quantum gravity makes too many assumptions
    2. according to the logic of the renormalization group, the Einstein-Hilbert action is just an effective description at long distances
    3. loop quantum gravity is not a predictive theory
    4. loop quantum gravity has not offered any non-trivial self-consistency checks
    5. loop quantum gravity is isolated from particle physics
    6. loop quantum gravity does not guarantee that smooth space as we know it will emerge as the correct approximation of the theory at long distances
    7. loop quantum gravity violates the rules of special relativity
    8. the discrete area spectrum is not a consequence, but an assumption of loop quantum gravity
    9. the discrete area spectrum is not testable
    10. loop quantum gravity provides us with no tools to calculate the S-matrix
    11. loop quantum gravity does not really solve any UV problems
    12. loop quantum gravity is not able to calculate the black hole entropy, unlike string theory
    13. loop quantum gravity has no tools to answer other important questions of quantum gravity
    14. the criticisms of loop quantum gravity regarding other fields of physics are completely misguided
    15. loop quantum gravity calls for "background independence" are misguided
    16. loop quantum gravity is not science

    The numbered points are connected to deeper explanations.

    Criticisms of string theory can follow in someone else's post. With the group in favor of LQG, they should be able to gather their heads and come up with lots of things :smile:
  7. Sep 14, 2004 #6
    You are really unfair sol2. It is absolutely necessary that some of us check deeply that we really need new hypothesis such as : supersymmetry, extra-dimensions, extended objects.

    Especially your first point is wrong. Your all post is not a scientific argument. The simple argument I quote above justify that great scientists as Rovelli and Smolin among other spend time on LQG. Please read Rovelli's paper.
    Last edited: Sep 14, 2004
  8. Sep 14, 2004 #7
    sol2, I think you are gonna have to learn a little bit about LQG. This is not hwo you do science.

    Did you know that when Einstein published his General Relativity many scientists of whom we will never remenber the name were against it. 100 scientist published a paper in order to convince everybody else that Einstein was wrong.

    Einstein replyed : Why a hundred scientists??? If I was wrong, 1 would be enough !!!

  9. Sep 14, 2004 #8
    Of course I have to rebuttel you too :smile:

    I present the source, and you call that unfair? I didn't write it :smile: I only repeated the claims. Now it is up to the people that signed their name to the LQG perspective to present there reasons why they like it. I expect these people to be deep thinkers

    I have been more then fair in my research, and do not fault Smolin and the road he has choosen. It is a honourable imention, and my posts reflect that honour, when I ever refer to Smolin.

    For heavens sake I even named my site in honour :rofl:

    People have to step up and say why.

    My plan this morning was to look for information on the energy values of the particles we assign in those colliders( this should give some clues as to why LQG cannot work here in regards to quantum gravity as a explanation).

    I am making a brief appearance here from being up all night. You must know I hold high esteem all those theoreticist, for what ever approach they come from.

    Humanino you must explain why you like strings, from being more then simple and beautiful. :smile: I a explaining myself constant onthis board when it comes to strings. A geometrical discription cannot arise from empty space, so you need a back to begin.
  10. Sep 14, 2004 #9
    First of all the claims you are referring to are not your own, so did you really think about LQG???

    It is in fact a model that is in development stage so nobody will say that it is cristal clear...just as string aren't

    As an answer to these claims, just check out the many references made by marcus in the Loop Quantum Gravity thread. I especially recommend the book of Carlo Rovelli on this matter.

    The reason I prefer LQG over strings is the fact that we adopt the notions of General relativity and rewrite the QFT in such a way that it fits the background independence and the gauge-invariance...
    String Theory starts the other way around. There, the general relativity is not considered the basis of the theory. as a matter of fact in string theory one gives up background independence, then try to quantize gravity in some way and finally try to restore the backgorund-independence. ofcourse I do realize that the main attempt of string theory is not to quatize gravity but to unify the fundamental interactions...

  11. Sep 14, 2004 #10
    You are probably more competent that I am to talk about strings. I have some books, I did not read them entirely yet. You ask, I have to try : the main reasons why I claim strings are elegant :

    1) It is very natural from the assumption of extended objects, and when one lists the axioms leading to QFT, only point particles looks non-fundamental
    2) Once the free theory is settled, there is no need to introduce interactions
    3) There is only one diagram at each order given by a simple quantity : the genus
    4) It has one, or even no free-parameter
    5) I work in QCD. The Maldacena conjecture takes us very near to describe strongly coupled objects with strings, alas one supersymmetry is required

    OK, would it only be those, this already justify why I love strings. However, the previous argument I quoted still holds : given the fact that there is not (yet) any experimental test, we must explore theories without new physical hypothesis.

    Besides, what has strings done : who said "predicted gravity" ? See, I am not competent.

    Also, LQG has reproduced Hawking result without the assumption of extreme black-holes, contrary to strings (AFAIK)

    Again, I am not competent to judge. I think it is important to keep up for all physicists. After all, the only reason why strings and/or LQG might have some relevance, can be due to the fact that there are models for Connes' geometry. But here, I am totally incompetent.
  12. Sep 14, 2004 #11

    Your points reveal competency to me, as much as any who are endeavoring to understand this issue.

    Thank you for responding
  13. Sep 14, 2004 #12
    Oh yes, Marlon I have, and others perspective as well. Smolin was very kind to us in showing us those three roads and his distilliation process he went through. Because I give sound reasons for LQG rejection should not matter what my opinion is? I recognize the source of who wrote it, so I knew that from that perspective it would have been a legitmate one, and any disgust with me was a sore point about facing reality then?

    I have been monitoring and have spent a few year piece mealing, as strings has been piece mealing :smile: You had to look for the obvious differences, and who do you thnk brought Glast to this board? :smile:

    Again, if you look you will see my name along Marcus's at almost every turn. I understood his position very well and did not need this thread to tell me what he thought :smile:

    In fact if you find the post of the soccer ball by John Baez and the Monte Carlo demonstration, maybe come back to be about your perspective. I mean that in :smile: a nice way

    Then I have to ask how did the model of LQG ever think it could have some geometrical association that begins from nothing? Did Loops always exist? That's a fundamental question about it's logic. :smile:

    As I said above, the search for geometical consistancy has been my bend one could say, and there is no perspective here that has shown itself. But I have some ideas here that I am working. Also you must understand, that the one dimensional strng already includes gravity, and you must also undertsand that this is a discription of a point particle. How shall you, with LQG show me this feature in quantum gravity?

    In joing QM with GR, strings recognize the background dependancy as necesaary feature in how the geometry must emerge? There is a version of string theory that B.I., although I woud have to dig deeper for this. Maybe someone could bring it forward here, above in sci physics strings?

    to unify all of them of course and where shall we do this. We have set targets for ourselves and how shall the standard model arise. J Pierre has a graph for your consideration, which I shall bring later. It includes gravity.


    Last edited by a moderator: May 1, 2017
  14. Sep 14, 2004 #13
    meteor, PRodQuanta, setAI, yanniru, YZer

    So we have a couple here, who have not really address the issue, but have voted?
  15. Sep 14, 2004 #14

    Yes, but LQG can incorporate the other interactions. In the paper above, is proposed an unification of LQG with Yang-Mills theory. QCD and electroweak theory, the 2 pillars of the Standard model, are both Yang-Mills theories. If this path becomes fruitful then LQG will also become a TOE
  16. Sep 15, 2004 #15
    A really good argument for string theory is that the five original theories have all been connected to form M-Theory by means of duality: isn't that a lot of coincidence?
  17. Sep 15, 2004 #16


    User Avatar
    Staff Emeritus
    Gold Member
    Dearly Missed

    If string physics stood by itself it would be. But it is an instantiation of a lot of interconnected mathematics, so the fact there is a non-perturbative zone that unifies the five original perturbative theories is not quite so surprising.
  18. Sep 15, 2004 #17
    Self Adjoint,

    You seem a bit reserved to voting here? :smile:

    I have always known you to be open to the possibilties, but always well grounded in that respective view. Would this thread present something of a
    dilemma, becuase you might have not voted on either, as there could be other possibilities? :smile:
  19. Sep 15, 2004 #18

    You are right in saying that ST describes particles, thus is coherent with our standard model. Well, LQG doesn't exactly define the particles, but more of the strings themselves.

    String theory is formulated with the need for supersymmetry.

    String theory is not a background independant theory. I know, I know, neither is LQG. But it is moreso than ST.

    I don't know, maybe both theories are a part of one greater theory.

    My $.02

    Paden Roder
  20. Sep 15, 2004 #19
    Oh you cannot revote. You made your bed, so you have to sleep in it. The only hope is that you wake from your dreams in peace :rofl:

    There is no other way in which to look at the cosmo in my book. The idea is to look where we might have found this trail that leads us to where this supesymmetry could exist. You had to go to the early universe and this is not inconsistent with the idea of how to tap that energy(oops I let it out)?:)

    Every feature around us leads us to some consideration about how it all began. If not at this singuarity, then how? It had to ask us to consider, if existance always was, and if so how we might have interpreted it.

    Splitting the atom takes us to the the very realization of supersymmetry, and how such events, take us to the doorway, but have never allowed us to tap it. So in the mean time, there is this vast world out there developing mathematcially. If you cannot geometrically construct the foundations of this geometry from that event, then you do not hold much hope of getting anywhere but understanding the basis of photon intersection at the glast level?

    Quantum mechanics recognizes the string, and the oscillatory nature this energy defines for us. That's my point of view anyway. Pure speculation :smile:
  21. Sep 15, 2004 #20
    Unless of course, there is no fermions in your world :uhh:
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook