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

What's the latest?

  1. Jun 30, 2005 #1
    Have experimental findings and theoretical developments in the very recent time (like last three years) been in favour for or against string theory? Has optimism been growing or has it decreased?
     
  2. jcsd
  3. Jun 30, 2005 #2

    selfAdjoint

    User Avatar
    Staff Emeritus
    Gold Member
    Dearly Missed

    No experimental evidence in favor of SST. Theoretical discovery of huge number, maybe infinite number, of inequivalent vacua causes dismay among some SST physicists and circle-the-wagons psychology in others. Revisions to estimated mass of top quark (experimental, from Tevatron) tighten the window for Higgs mass and may force supersymmetry theorists into unwanted fine tuning.
     
  4. Jun 30, 2005 #3

    George Jones

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    String theory seems to be mathematically consistent only when there are extra (to those normally perceived) spatial dimensions. How could these extra spatial dimensions be detected? One way is by look for modifications (for semi-technical reasons why, see below) in Newton's inverse square law for gravity at small distances. There ongoing investigations looking for such modification, but, so far, none have been detected.

    I, too, have a question. If evidence for either supersymmetry or extra spatial dimensions is found, string theorists will jump all over the results and claim vindication. But, since (as far as i know) they are not sufficient conditions, will either of these results really be enough?

    It seems that superstring theory implies supersymmetry and superstring theory implies extra spatial dimensions, but how likely is that these necessary conditions can be "true" without symmetry being true? Scientific theories are often "verified" (note the scare inverted commas) for a certain domain of validity by a finding a number of necessary conditions. In this case, how much is enough?

    For what it's worth, finding *both* supersymmetry and extra spatial dimensions would go a long way towards bringing me around.

    Now, why do extra dimensions modify Newton's inverse square law? First, a somewhat wordy motivation for the inverse square force in three spatial dimensions. Consider gravitational lines of force coming from a point mass. Consider two (massless) imaginary balls with radii R and r that both have the point mass at their centres. The lines of force cross the surfaces (boundaries) of the balls at points. The (relative) strength of gravity at these surfaces will be proportional to the number of crossing point per unit area. Since the same lines of force pierce both surfaces, the number of crossing points is the same for the two surfaces. However, the surface areas scale as radius squared, and thus the strength of gravity (number of points per unit area scales inversely to radius squared.

    Now consider gravitational lines of for from a point mass in a universe that has four spatial dimensions. A ball now is a 4-dimensional spatia volumel, and the boundary ("surface") of a ball is a 3-dimensional object. Consequently, the number of crossing points per unit "surface area", i.e., the gravitational force, now scales inversely to radius cubed.

    In practice, the extra spatial dimensions, if they exist, are probably folded up in a complicated manner, so the precise deviation from Newton's inverse square law is probably impossible to calculate.

    Regards,
    George
     
  5. Jun 30, 2005 #4

    marcus

    User Avatar
    Science Advisor
    Gold Member
    2015 Award
    Dearly Missed

    compliments on the prose style, which is as concentrated and salty as a boullion cube. if you hadna been a math/physicist you could have been a writer about science.
     
    Last edited: Jun 30, 2005
  6. Jul 2, 2005 #5

    George Jones

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Oops, I meant to say "... how likely is it that these necessary conditions can be "true" without string theory being true?"

    Opinion, anyone?

    Regards,
    George
     
  7. Jul 2, 2005 #6

    selfAdjoint

    User Avatar
    Staff Emeritus
    Gold Member
    Dearly Missed

    Supersymmetry by itself, at least in its full-bore form, seems to require 10 dimensions. SST does not imply supersymmetry, it uses it as an assumption, and some of the counterintuitive stuff we ordinarily blame on SST is actually imported with supersymmetry. It is supersymmetry that is having problems due to the revised top quark mass. Not anywhere near falsification yet, but the SS partisans are running their fingers around their collars.
     
  8. Jul 2, 2005 #7

    Chronos

    User Avatar
    Science Advisor
    Gold Member
    2015 Award

    The light at the end of the Theory of Everything tunnel has been receding instead of approaching for the past couple years. It's been like trying to find one particular quarter on the bottom of a wishing well [and a really, really big one at that]. The search itself further muddies the water. A number of theorists have shelved their TOE ambitions and focused on more managable puzzles, like a workable theory of quantum gravity. I would say that is a good thing. The bull has too many horns to be defeated head to head.
     
  9. Jul 2, 2005 #8

    Haelfix

    User Avatar
    Science Advisor

    If large extra dimensions are found experimentally, its a pretty good bet SST is on to something. If its just a vanilla SuSY, then it would take a lot more to convince me.

    Incidentally there is a lot more wiggling room in the higgs sector for the SuSy/GUT models w.r.t the recent top quark revisions than there is in just the standard model.

    I will say this, finetuning to one or two orders of magnitude doesnt really bother me, I mean there could in principle be all sorts of other physics going on at very high energies that has been neglected and influencing those numbers. Otoh, more than that starts becoming damn disturbing.
     
  10. Jul 3, 2005 #9

    ohwilleke

    User Avatar
    Gold Member

    The problem, and the declining optimism doesn't just come from the experimental developments. Equally important, string theorists are finding it much harder than anticipated to derive theoretical predictions from the models. If you theory can't make concrete predictions, no amount of experiment does you any good.

    One of the big virtues of the Loop Quantum Gravity program is that it has, a least, started to get to the point where it can say something about what the theory says about real life.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: What's the latest?
Loading...