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:

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


               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.


    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



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

More soon..


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