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

Free Parameters In String Theory

  1. May 16, 2009 #1
    Hello to all:

    I heard Michio Kaku talk on string theory recently and I thought I heard him say that the theory had no free parameters. But in thinking about the matter, I could not decide what that meant -- if it is true. Could someone please tell me what ingredients go into string theory? Are c, h and G taken to have the known experimental values, for instance?

    Thank you for your help.

  2. jcsd
  3. May 16, 2009 #2
    In string theory there is at first one free parameter: the length of the fundamental string.

    Once we start trying different ways of compactifying the extra dimensions, all these different compactifications are sort of like parameters.
  4. May 16, 2009 #3
    Okay. Thanks for the information. Could I ask you: When you say compactification, are you speaking of ways of rolling up the various dimensions? I have a fair understanding of quantum physics, but that's about it. I earned my Ph.D. in physics in 1980 -- back in the Dark Ages.

    Thanks again.

  5. May 16, 2009 #4
    Yes, compactification could loosely be thought of 'rolling up the extra dimensions.' In mathematical terms there is a compact (closed and bounded) manifold attached to each point in spacetime.
  6. May 17, 2009 #5
    Many thanks.

  7. May 17, 2009 #6


    User Avatar
    Science Advisor
    Homework Helper

    one should say I think "no dimensionless free parameters", c, G etc have dimension
  8. May 17, 2009 #7

  9. May 17, 2009 #8


    User Avatar
    Science Advisor
    Homework Helper

    so this is not the case in the standard model, where eg. mass ratios and coupling constants are fixed.
  10. May 17, 2009 #9
    You have been of much help. Thanks.

  11. May 20, 2009 #10
    It might be fun to know that the "freedom of string theory" is one of the largest problems in string theory at the moment. People are trying to come up with realistic configurations (i.e. compactifications etc) which would give a description of the Standard Model and a positive cosmological constant (General Relativity itself already comes along quite natural, I think). The point is that different configurations lead to different excitation spectra of the strings and thus different universes.

    But apart from the fact that a 'realistic' configuration has not been found, there is not even a mechanism known which explains why it's this particular configuration that corresponds to our universe. In some sense, string theoy provides way too many different ways of coming up with toy universes.

    This is known as the landscape problem.
  12. May 20, 2009 #11
    Thanks very much.

    Could I ask what kind of energies are involved to probe strings? Maybe your answer could be in Gev or Fermis -- as I am an experimentalist. And could I ask you to say a few words about what kind of particle states would be created by strings? And what do you think string theory might explain? Would all of the known particle masses come out of the theory? Would QED come out of the theory -- along with simple quantum mechanics? Would Newton's laws come out, along with special and general relativity?

    What evidence now exists to support the existence of strings?

    I am a former physics professor and am going to be on a national radio program this Friday. May use part of your answers on the air -- if I am asked any questions about string theory. My main focus on the program will relate to exotic physics phenomena to include scalar electromagnetic weapons.

    Thanks again.

  13. May 21, 2009 #12


    User Avatar
    Science Advisor
    Homework Helper

    The energies are around the Planck Scale, like 10e19 MeV

    However, String Theory 'predicts' extra dimensions to exists, and if they are "big enough" they might be discovered at LHC @ CERN.

    String Theory requires special relativity, but as a result, general relativity will come out - as the weak limit of a higher order theory of relativistic gravity.

    One hopes to solve/find QCD from string theory http://en.wikipedia.org/wiki/String_theory#Gauge-gravity_duality

    No experimental observation supports strings so far.
  14. May 21, 2009 #13
    Thanks much.

    Best wishes,

Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Similar Discussions: Free Parameters In String Theory
  1. String theory (Replies: 34)

  2. String theory (Replies: 11)

  3. String Theory (Replies: 53)

  4. String theory (Replies: 3)

  5. String theory (Replies: 8)