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

Does string/M-theory make falsifiable predictions at planck scale?

  1. Sep 12, 2010 #1
    Stephen Hawking's recent book The Grand Design uses M-theory to predict a multiverse, which then does away God via the anthropic principle.

    A common criticism that string theory makes no experimentally falsifiable predictions which is countered that experiments do not have enough energy to probe the Planck energy. The highly beautiful symmetry of strings is broken, and this symmetry breaking is why it cannot make definitive falsifiable predictions at, say, LHC energies.

    While particle accelerators do not exist, we do have the Big Bang. Big Bang had planck scale energies in the planck era. Does string/M-theory offer falsifiable predictions?

    In Grand Design Hawking outlines M-theory

    A model is a good model if it:

    1. Is elegant
    2. Contains few arbitrary or adjustable elements
    3. Agrees with and explains all existing observations
    4. Makes detailed predictions about future observations that can disprove or falsify the model if they are not borne out.

    What exactly does M-theory say about the Big Bang and Planck scale physics? I understand that M-theory’s beautiful symmetry visible on the Planck scale must be broken at lower scale. Do cosmologists use M-theory to model Planck-scale Big Bang? What does M-theory say about the Planck era where GR becomes a singularity?

    Does M-theory do what Hawking wants it to do, makes a falsifiable prediction of a multiverse, where other universes have different values of fundamental constants?

    What does M-theory tell us was the physics of the Big Bang in this energy scale?

    Can we take an M-theory prediction of planck scale physics in the Big Bang era, follow it forward in time into the future until it re-reproduces current Big Bang CDM model?

    How do the predictions of M-theory differ or similar to Loop quantum cosmology?
  2. jcsd
  3. Sep 12, 2010 #2


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    At one time the LQG folks (or mainly Smolin, I think) were pushing as a prediction of LQG the dispersion of the vacuum. This turned out not to be observed, and later theoretical work showed that it wasn't really a prediction of LQG.

    Smolin and Susskind had an entertaining debate about whether the anthropic principle was actually falsifiable: http://www.edge.org/3rd_culture/smolin_susskind04/smolin_susskind.html

    String theory predicts supersymmetric particles, and I believe that the completely supersymmetric version of string theory predicts them to have the same masses as their partners, which means that that version has been falsified. I think string theorists handle this by saying that there is some mechanism that breaks supersymmetry, which makes the predicted particles conveniently useless as a way of falsifying string theory, since they can't say how big the breaking is. But I'm sure that if the LHC discovers such particles in the energy range it has access to, string boosters will be happy to claim that as a confirmation of string theory.
  4. Sep 12, 2010 #3
    Not all that different from GUT's and proton decay and magnetic monopoles or extra dimensions.

    But at very high energies, GUT's and the higher symmetries should be experimentally observable.

    If we had an accelerator that could probe planck scale energies, what predictions does string/M-theory make that could be falsifiable (i.e if they were not observed, they would falsify strings) and would these be present in Big Bang physics?

    Stephen Hawking states in Grand Design that M-theory proves God did not create the Universe.
  5. Sep 13, 2010 #4


    User Avatar
    Science Advisor

    I think this has been explained a lot recently, but yes once the vacua of string theory is specified (eg found experimentally) the theory should in principle both postdict all lower energy experiments as well as make arbitrarily many new predictions that you could measure with a large enough collider.

    As for generic predictions, well definitely it says something about the nature of gravity perse. If you find the appropriate section in GSW, perturbative string theory gives you the quantum corrections to Einstein's equations, and is calculated order by order. This is a similar prediction to the Lamb Shift for electrodynamics and will be approximately valid up to some scale (where nonpertubative stringy effects become important).
  6. Sep 13, 2010 #5


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    It was never actually shown that dispersion in vacuum can be mathematically derived as a prediction from 4D LQG. It can from 3D LQG (Freidel and Livine 2005) so there was some hope of deriving a prediction in the interesting 4D case before GLAST--now renamed Fermi-LAT--was launched. (And there were handwave arguments based on generic QG ideas not specific to LQG, that surfaced at the time.)

    The main LQG people like Rovelli Ashtekar and associates avoided the issue (Rovelli had published a paper in 2002 saying Lorentz invariance compatible with LQG.) But a few people tried hard to derive a prediction during 2005-2007. As I said, the hope was to get a prediction before the launch.

    They did not succeed. Kowalski-Glikman was one of the main figures in this attempt. By 2008 the attempt to prove dispersion (or DSR, deformed special relativity) from LQG had largely been given up. In his May 2008 LivingReviews article, Rovelli says simply that LQG has no prediction of dispersion or any departure from Lorentz invariance.

    I do not think what you say about later theoretical work is quite right. AFAIK there has been no theoretical work earlier or later that showed a prediction, and also no theoretical work earlier or later that showed "there really wasn't".

    Instead of "later theoretical work" what you have is a motivated effort by people like Kowalski-Glikman who simply gave up trying around 2007. Not because of astronomical data (which was not in) but because they gave it their best shot and failed.
    It is still possible that a prediction of dispersion could yet be derived from some (future?) version of LQG! Nobody has shown AFAIK that it can NOT be derived. And LQG continues to develop.

    Maybe you or someone else can give me a link to some "later theoretical work" like from after May 2008 that is relevant to this. I'd be interested. To me the situation looks unsatisfactory---kind of "in Limbo".
    Last edited: Sep 13, 2010
  7. Sep 13, 2010 #6


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Nope, your summary is both more detailed and more accurate than what I offered.
  8. Sep 13, 2010 #7


    User Avatar
    Science Advisor

    Post 2008: http://arxiv.org/abs/0903.3475
    "In fact, it is now clear that these effective models of quantum gravity can in principle be falsified. Unfortunately, we are still lacking any fundamental formulation of quantum gravity that, on top of being clearly defined at the Planck scale, can produce unambiguously any of the effective models that have been proposed, thus producing falsifiable predictions."
  9. Sep 13, 2010 #8
    So according to Hawking's Grand Design, what's the basic physical picture M-theory provides in the planck era of the Big Bang?

    In the planck era, was spacetime flat 10+1, SUSY unbroken, and then 6 or 7 dimensions curled upon itself leaving 3 large spatial dimensions as it passed from planck scale to classical?
  10. Sep 14, 2010 #9


    User Avatar
    Science Advisor

    I'm sorry, I don't know what Hawking's design is (I haven't read the book).

    I also don't know how to answer the second question, and I don't think anyone really does without also specifying the vacua that corresponds to the real world.

    In so far as the very basic textbook solutions that I know a little about, well they do seem to require having supersymmetry restored at some point as well as extra dimensions, beyond that I don't know.

    I think its worth keeping in mind that string theory was originally developed to study questions that particle physicists were interested in. Like what is the cross section of this scattering experiment, or what is the decay rate of this particle or what is the Regge slope of xyz. Consequently, most of the tools are pretty good at answering questions that are decidedly local and microscopic. So for instance, perturbative string theory can spit out S Matrix elements in a quantum gravity scattering experiment.

    What is considerably harder is trying to figure out global questions, like cosmological solutions that were important during the early universe.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook