On Thu, Sep 15, 2011 at 6:30 PM, Stephen P. King <[email protected]>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 -- You received this message because you are subscribed to the Google Groups "Everything List" group. To post to this group, send email to [email protected]. To unsubscribe from this group, send email to [email protected]. For more options, visit this group at http://groups.google.com/group/everything-list?hl=en.
