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

String threory assumptions about spacetime

  1. Jun 18, 2007 #1
    I just read that string theory "started out as a generalization of quantum field theory where instead of point particles, string-like objects propagate in a fixed spacetime background".

    Is this true? And if so, did anyone try to do the same with a curved spacetime? What did they get? Or perhaps modern string theory already accounts for that, having incorporated (or arriving at) that at a later stage?
    Last edited: Jun 18, 2007
  2. jcsd
  3. Jun 18, 2007 #2

    String theory is supposed to accommodate the graviton as one of the modes of vibration of strings in this fixed spacetime background. So it would seem that the curved spacetime of gravity via the graviton is derived from a pre-existing fixed spacetime background. Seems like a bit of a contradiction of terms, wouldn't you say? Where did the fixed spacetime background come from in the first place?
  4. Jun 18, 2007 #3
    I took the quote from http://en.wikipedia.org/wiki/Quantum_gravity. I know that there may be errors in wikipedia, but I found this to usually be a reliable source. In any case, the full quote reads:

    It seems the starting point was a QT in fixed Minkowski spacetime, which they expanded by modeling the particles as 1D strings rather than 0D points. So the same Minkowski spacetime was probably assumed as the background. I know that they got to the gravitons later. And yes, if gravitons produce curved spacetime, and they got to that from a fixed background, that would seem a bit of a contradiction - unless perhaps if you assume the curvature as a sort of superposition on top of the flat substrate (like pouring sand on a table-top)...

    But all of that is just my own interpretation, and doesn't really tell me what is really the case. :confused:

    P.S.: Am I correct in assuming that a curved spacetime cannot be fixed (this also seems to be implied later in that article), and hence fixed means flat, or am I grossly confusing things?
  5. Jun 18, 2007 #4


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    I am not an expert on string theory, so take what I will say with a grain of salt. Hopefully someone else will clarify things up more but, unfortunately, there are very few people knowledgeable in string theory visiting this board.

    But as far as I know, one could in principle do string theory using any spacetime obeying Einstein's equation as a background (renormalization group applied to string theory yields Einstein's equation). I think that starting from a flat background is just a matter of convenience.
    Of course, the detractors of string theory will say that this is still not satisfactory because they expect a fundamental theory to be background independent to start with.
  6. Jun 19, 2007 #5


    User Avatar
    Science Advisor

    A curved background in string theory is viewed as an APPROXIMATION that approximates a coherent state of gravitons in flat spacetime.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook