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

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