On 9/15/2011 9:43 PM, Jesse Mazer wrote:


On Thu, Sep 15, 2011 at 9:05 PM, Stephen P. King <stephe...@charter.net <mailto:stephe...@charter.net>> wrote:

    On 9/15/2011 6:59 PM, Jesse Mazer wrote:


    On Thu, Sep 15, 2011 at 6:30 PM, Stephen P. King
    <stephe...@charter.net <mailto:stephe...@charter.net>> wrote:

        On 9/15/2011 5:17 PM, meekerdb wrote:

            On 9/15/2011 1:42 PM, Stephen P. King wrote:

                On 9/14/2011 9:49 PM, meekerdb wrote:

                    snip
                    On the contrary, the singularity is in the
                    description.  Which is why no physicist believes
                    the description (General Relativity) is valid.

                    Brent

                   Ummm, really?  Let me see if I understand this
                claim, no physicist believes that General Relativity
                (GR) is valid or no physicists believe that there are
                solutions to the field equations of GR that are
                invalid? What about Roger Penrose and Stephen
                Hawking? They wrote the paper that showed a proof
                that the field equations of GR generate singularities
                for relatively innocuous and plausible conditions and
                yet they are still great proponents of GR. So... what
                is the source of your opinion re "no physicist
                believes ..."?


            The importance of their paper was that it showed GR
            predicted a singularity under very general conditions.
             Before that,it had been widely assumed that the
            singularity prediction was just an artifact of assuming
            perfectly spherical 3-geometry with no rotation.  Of
            course I can't really vouch for what every physicist ever
            believed.  But I was in graduate school at the time
            studying GR and nobody I knew, including Penrose whom I
            met and my fellow students, drew any conclusion except
            that GR breaks down and does not apply in those
            circumstances.  And no one was surprised by this.  There
            was already an active search for a quantum theory of
            gravity, which it was assumed would avoid singularities.

            Brent


        Hi Brent,

           AH! I understand and agree with you then. But we have to
        deal with the observational evidence that space-time is
        smooth down below scales that most forms of quantum gravity
        theories, loop quantum gravity for example, predict a
granularity or foam or some other form of discontinuity.

    What observational evidence are you referring to? There was
    recently a paper by Philippe Laurent that was widely misreported
    in the media as giving evidence that ruled out "granularity" at
    the planck scale (see
    http://www.esa.int/esaCP/SEM5B34TBPG_index_0.html for example),
    but in fact if you look at the actual paper (at
    http://arxiv.org/abs/1106.1068 ) it was specifically about ruling
    out granular theories that predicted violations of the
    Lorentz-symmetry of relativity. Most forms of string theory and
    loop quantum gravity actually assume that Lorentz-symmetry is
    *not* violated (see
    
http://books.google.com/books?id=dWmZb3uQWbQC&lpg=PA320&dq=supersymmetric%20string%20theory%20lorentz%20invariant&pg=PA320#v=onepage&q=supersymmetric%20string%20theory%20lorentz%20invariant&f=false
    
<http://books.google.com/books?id=dWmZb3uQWbQC&lpg=PA320&dq=supersymmetric%20string%20theory%20lorentz%20invariant&pg=PA320#v=onepage&q=supersymmetric%20string%20theory%20lorentz%20invariant&f=false>
    and http://arxiv.org/abs/1012.1739 for instance), so the new
    findings wouldn't be a problem for them.

    For a number of physical arguments that general relativity is
    likely to break down at the Planck scale, see
    http://arxiv.org/abs/1001.1205

    Jesse

--
    See http://prl.aps.org/abstract/PRL/v97/i14/e140401

    More soon..



The abstract says that the observations "constrain certain types of relativity violations", which suggests they are again talking about violations of Lorentz symmetry (the basic symmetry of relativity). The preprint of the same paper at http://arxiv.org/abs/hep-ph/0607084 confirms this, saying on p. 2 that "A promising candidate effect is relativity violations, which are associated with the breaking of Lorentz symmetry, the invariance of the laws of physics under rotations and boosts". So, the observations wouldn't rule out versions of string theory and loop quantum gravity which preserve Lorentz symmetry, which as I said is true of the most commonly discussed versions.

Jesse
--

    Hi Jesse,

Any physically significant boost would act to alter the scale of "Plankian effects", that is what general covariance basically tosses out any physically real notion of space-time points what ever their size might be. If our laws are physics are really invariant with respect to coordinate transformations then those include boosts as well. So string theory has its own set of problems with GR. Additionally, string and brane theories *require* a fixed and flat space-time to act as a base space for the fibration of compatified torii, orbifolds or what have you. The same is true for all quantum field theories that depend of the fiber bundle formulation.

You might wish to check the date of publication of the various papers that we are throwing around. The ones you referenced are from 2006... Here is one published this year: http://arxiv.org/abs/1106.6053

    A 'money quote' from page 8 of that paper:

"The observation of the highest energy gamma rays up to 31 GeV from
a distant gamma ray burst GRB 090510 also shows that there are no ob-
servable quantum effects of spacetime down to the Planck scale [15]. The
result therefore rules out those quantum gravity theories in which the speed
of light varies linearly with photon energy. There is no evidence of violation
of Lorentz invariance down to the Planck length. Spacetime is continuous
and special relativity is right. The greatest mystery is why spacetime is man- ifestly so smooth and classical all the way to the smallest conceivable level."

Here is a 2009 article that discusses GRB 090510: http://www.nasa.gov/mission_pages/GLAST/news/first_year.html

Onward!

Stephen

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