On Thu, Sep 15, 2011 at 9:05 PM, Stephen P. King <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>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=falseand
> 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.


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