Could Someone Please Explain Exactly What String Theory and LQG Are Trying To Do?

  1. Could Someone Please Explain Exactly What String Theory and LQG Are Trying To Do?

    I have recently had the opportunity to ask this question to a couple prominent physicists, and they could not answer it.

    What is the purpsoe of these theories? What do they explain that is not explained by SR, GR and QM?

    What are the postulates of String Theory? Are there any? Is there any premise as simple or beautiful as Einstein saying "The speed of light is constant in all frames," or "energy is quantized," or Bohr stating, "The electron orbits the nucleus at only discrete energy levels."

    Does String Theory or LQG have any simple forumlas associated with them, such as E=mc^2, E=hv, or something like Shroedinger's Equation?

    Do they account for non-locality as demonstarted in EPR, Bell's Theorem, and Aspect's experiments?

    Does either theory unify Rel. and QM?

    Finally, why should we have to unify relativity and QM?

    Must we also unify Hiphop and Classical Music in some new musical genere? Or can we realize that each works in a given venue, and that music and the human heart underly both?
    Last edited: Jul 13, 2005
  2. jcsd
  3. selfAdjoint

    selfAdjoint 7,521
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    GR and QM are apparently not able to combine into a unified theory. One basic purpose of both strings and quantum gravity programs is to remedy this by achieving a theory that spans both and reduces to each as special cases. In addition string theory is motivated the belief that it does describe basic nature better that the previous theory, the Standard Model of Particle Physics. The Standard Model has about 19 undefined parameters in it, and another goal of physicists is to find explanations for these. It is one of the boasts of string theory that it does not have ANY undefined parameters. These claims of string theory are not belied by its present multivacuum difficulties.

    The premise of string theory is contained in the Nambu-Goto Lagrangian; The local action of a string worldsheet is proportional to its change in area.

    See above.

    QM and all the research you site does not assert nonlocality as local realist theories would have it. It shows that local realist theories are false.

    String theory claims its graviton reproduces Einstein's GR physics in flat spacetime. The quantum gravity workers are all trying to couple quantum matter to their various quantum gravities.

    From the time of Galileo science has been monist, looking for unified explanations. It bothers physicists that the two best theories around are so contradictory to each other.

    This confuses an artistic field where there's no one right answer with a rational one where you do seek for THE answer. BTW all through my lifetime classical composers have been trying, with variable success, to unify their music with the everchanging manifestations of popular music, starting with rag time and jazz.
    Last edited: Jul 13, 2005
  4. Thanks for the answers!

    There is only one science of the heart, and that is art. :)

    I'm not sure that any physical theory will ever unify poetry and physics, without oversimplifying poetry and leading physics astray.

    Has string theory had any successes in anything its attempted?

    Has it unified quantum mechanics and relativity?
  5. Pengwuino

    Pengwuino 6,942
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    Incorrect. It is called cardiology
  6. Hurkyl

    Hurkyl 15,987
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    The basic programme of String Theory, as I understand it, is to take the wildly successful Standard Model, and tweak in a way that is likely to preserve the successful features of the Standard Model, yet also includes a graviton from which General Relativity can emerge. Also, it would like to provide a way to derive the fundamental constants of the Standard Model.

    The basic programme of Quantum Gravity, as I understand it, is the modest goal of simply trying to figure out how to quantize General Relativity. The hope is that once this task is accomplished, it will be more clear how to merge Quantum Gravity with the Standard Model.

    Loop Quantum Gravity is the branch of Quantum Gravity that postulates that basic geometric excitations take the form of loops.
  7. When you say "LQG wants to quantize gravity," what exactly do you mean?

    Why must gravity be quantized?

    How will it be quantized?

    Did quantum mechanics arise because somoene wanted to quantize photons/particles, or did quantum mechanics arise because photons/particles are quantized?
  8. Pengwuino

    Pengwuino 6,942
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    Its not because they WANT to quantize photons and particles, its because it is quantized. We do things to find out how things truely are. Gravity doesnt have to be quantized but people do these things and study things and do various experiments to find out IF it is. Also, if we knew if it were quantized, we will know how its quantized and we cant just tell it how to be quantized.
  9. Instead of qunatizing gravity, couldn't we just quantize time, space, or space-time?

    This would explain the quantum behavior of small particles in and short distances, while allowing for the classical behavior of large objects and long distances.
  10. Pengwuino

    Pengwuino 6,942
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    Well im far as hell from an expert but that woudlnt explain quantized energy levels of say, electrons. Someone else probably has a better explanation.
  11. Hurkyl

    Hurkyl 15,987
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  12. Hurkyl

    Hurkyl 15,987
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    Don't forget that, according to GR, gravity is space-time.

    LQG has achieved a quantized space-time: the basis states are "spin networks" -- graphs whose nodes represent a "chunk" of space-time, and whose edges represent the boundary between neighboring chunks. The area and volume operators of LQG have quantized spectra.
  13. Hurkyl

    Hurkyl 15,987
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    By the way, the quantum behavior of bound particles in the Standard Model is not suggestive of quantized space-time. Furthermore, this neglects free particles for which things like energy do not have discrete spectra.

    (Incidentally, I do not know of a bound system for which the position operator has a discrete spectrum, but then again I don't know a vast array of examples)

    AFAIK, people have tried to directly put QFT onto a discrete lattice, with little success.
    Last edited: Jul 13, 2005
  14. Chronos

    Chronos 10,348
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    LQG is [at least here] usually taken to mean Loop Quantum Gravity. I'm not aware of any association between Linear Quadratic Gaussian theory and string theory.
  15. vanesch

    vanesch 6,189
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    It is not so much that we have a choice, and the "unification of gravity and quantum theory" is not a matter of "what if" ? If gravity and quantum theory could exist peacefully together, that would disturb nobody, but they don't: both theories (general relativity and the standard model) give wildly incompatible predictions (even inconsistencies) when you push them into domains where both are relevant.

    It is a bit analogous to the following story. On a faraway planet there's a civilisation of beings, the size of an ant and two geniuses have established 2 theories: Euclidianus has a theory of the surface on which they live (2-dim flat surface), and this works well for road and field work and coastal navigation. Newtonibus has a theory of planets being points and this works well to predict the motions in the sky. But both theories (flat earth vs. points in space) are INCOMPATIBLE, however, if you speculate in domains which are far beyond the experimental reach of these creatures, like "what happens when I make a very very long road, how come that I cannot reach another planet" and so on.
    So comes a new theory: planets are spheres !
    This unifies both previous theories: in small enough areas, the surface of the sphere approaches the flat surface of Euclidianus, and at far away distances, the spheres look like points as in the theory of Newtonibus.
    That doesn't change the fact that for all practical purposes, the theories of Euclidianus and Newtonibus are sufficient to explain about all phenomena in the sky and on the land, and that it will be damn difficult for these creatures to TEST the new theory.

  16. Hans de Vries

    Hans de Vries 1,135
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    Every major String Theory revolution seems to be aimed at making it more
    difficult to falsify it... Job security?

    In the seventies so many nice theories ended up in the waste bin because
    of non-renormizability. The first String Theory revolution in the eighties then
    made it easy to renormalize about anything. One could develop theories
    without fear again.

    But it gave an explosive number of theories. Surely from N different theories
    there must be N-1 wrong? So who's right and who is wrong? Infighting
    and conflict was the result. Then, the second String Theory revolution
    in the nineties unified them all in 11 dimensions and peace returned.

    Nevertheless impatience grew because it predicted no SM parameters.
    Then, luckily, the latest String Theory revolution adequately solved this
    nuisance with the Landscape model which tells the world there's no need
    to predict anything since it's just all "random". And so, it relieved the
    community from the outside pressures.

    Two days ago there was a panel discussion about what the next String
    Theory revolution could be:

    Why should you want yet another one when you've already achieved that
    String Theory can't be falsified in a thousand years? Making it THE theory for
    the next Millennium... :smile:

    Regards, Hans
  17. Hello Hans,

    I pretty much figured this as String Theory has not achieved anything in the realm of physics.

    It's sad that it's given so much prominence.
  18. Who are the top young String Theorists? What are their definitive papers?

    Einstein wrote a few definitive papers. Debroglie wrote a definitive paper. Maxwell and Planck wrote definitive papers.

    Have any string theorists written any definitive papers? I'd love to read the definitive papers, although I think I already have.

    Who are the big names in string theory these days? Who are the young guns?

    What's the latest of the latest? What's the buzz, hype, and new new thing?
  19. Hurkyl

    Hurkyl 15,987
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    You could try searching for them. Marcus, for example, manages to bring up an amazing wealth of papers from people working on Quantum Gravity, and shares his findings in this very forum. I can't imagine it would be that hard to find papers on String Theory, if that's what you would prefer to see. would probably be a good place to look.
  20. But with Relativity and Quantum Mechanics, we know the definitive papers without having to google them.

    And even if I found the greatest papers on String Theory, how would I know?

    What are the leading journals for String Theory?

    I think I know, but I might not. Thanks!
  21. Hurkyl

    Hurkyl 15,987
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