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Coupling of energy/matter to spacetime

  1. Oct 4, 2005 #1
    I have read papers regarding mechanisms responsible for maintaining the compactness of the extra dimensions and also read papers regarding how the 3 large dimensions became large.

    Mechanisms include:

    1) Casimir Energy
    2) Strings winding around the compact toroidal directions.

    Could someone PLEASE explain what the mechanism is that the matter/energy actually COUPLES to spacetime.

    Is it merely a general relativistic effect (like a contracting spacetime) or is it deeper.

    I have wandered this for a LONG time and never really received a good answer that I can say I truly understand.

    Thanks in advance.

    See for example:

    hep-th/0509225 [abs, ps, pdf, other] :
    Title: Dark energy and decompactification in string gas cosmology
    Last edited: Oct 4, 2005
  2. jcsd
  3. Dec 28, 2010 #2
    As far as I know, this question has no well understood answer. In standard QFT, matter is introduced fairly arbitrarily, and must transform under the Lorentz group in order to transform properly in space (and couple properly to the metric in QFT on curved space, i.e. maintain general covariance which comes from the equivalence principle). There is the relationship between gauged symmetries and gauge fields (force carriers), but this still requires other matter (quarks/leptons) for the symmetry to exist.
    In supergravity, matter is slightly more related to gravity in the sense that in order to be supersymmetric there must be particles in the supergravity multiplet, these are not the particles we see in the standard model however.
    In string theory, the target field is considered to be the spacetime manifold (coming from the "string worldsheet), and the particles are vibrations of the string. So the question is then reversed (why does (10-11-26d) space couple to (2d) gravity?) which is an equally valid (and poorly understood) question, although in 2d there are fewer choices for matter content (SO(2)=U(1) has only trivial irreps.). Note that the usual explanation is that string theory is "quantum mechanics of a string", so the question is then reformulated as the usual,
    "why should matter be described as a string (or brane, as it were)".
    Super String Theory also couples fermions, which underscores the non-uniqueness of the theory (five super-symmetric consistent types to be exact), but then they are all equivalent, so there appears to be some underlying reason in the end. But the choice of bulk metric kind of makes the couplings arbitrary (although they claim background independence I think it is a bit dubious).
    Loop Quantum Gravity also couples matter in a somewhat arbitrary way, although some attempts seem to incorporate it either as topology or vibrations (see the thread https://www.physicsforums.com/showthread.php?p=2766226)
    Anyways, the bottom line is that every theory has its own explanation, and none of them is really that good (especially since they can't reproduce the stuff we already know). It seems, however, that incorporating matter into the spacetime via eigen-functions (spectrum) or topology is the way to go since this provides a natural coupling to space. The main problem encountered with that approach is answering the question - what is space?
    Last edited: Dec 28, 2010
  4. Dec 28, 2010 #3
    There are many speculations according to Holographic Principle. One of them is that the space is an information background. We observe it as the fields of the virtual particles-antiparticles. The information space is created due to non-local interactions between the Compton waves of the real particles.
    The real particle and antiparticle is observed if it is defined by a sufficient amount of the related information. It doesn't mean the size of the real particle is defined but it means it is relatively more defined than the virtual particle. The virtual particle is everywhere and creates the space. The real particle has its more defined (more probably) location.
    There is nothing more than the relation between the information.
    This new approach is crazy but some physicists like Beckenstein, Hooft, Smoot, Suskind, Maldacena, Horowitz, Busso are working in this idea.
    The phenomenons like accelerated expanssion, recession of the space, dark matter effect and other are simply explained due to Holographic Principle.
    May be it is a way ?
    Who knows ?
  5. Dec 28, 2010 #4
    Sounds interesting, can you cite some of the papers explaining this idea? As far as I know there is only one dimension of space arising in holography, which still begs the question of what is the space on which the N=4 Yang Mills Theory (or any other one) is defined.
  6. Dec 29, 2010 #5
    The holographic principle states that the entropy of ordinary mass (not just black holes) is also proportional to surface area and not volume; that volume itself is illusory and the universe is really a hologram which is isomorphic to the information "inscribed" on the surface of its boundary

    I collected in my amateur website some interesting calculations and links about this idea:

    Craig Hogan from Fermilab is building the most precise clock of all time to directly measure whether our reality is an illusion.
  7. Dec 29, 2010 #6


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    you are almost right. follow this thread

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