From: gerry@bindweed.com   
      
   In article <20171012225044.GA86096@iron.astro.indiana.edu>,   
   jthorn@astro.indiana-zebra.edu says...   
   >   
   > Gerry Quinn  wrote:   
   > > If general relativity is false, there exists a local property which   
   > > we may call gravitational potential (at least after lumping /   
   > > scalarisation), and this property becomes extreme at the Schwarzschild   
   > > radius[*].   
   >   
   > I don't see that we know that.  There are a lot of different   
   > non-general-relativity relativistic gravity theories, and we don't know   
   > which -- if any -- of them are "the correct theory".  So I don't see that   
   > we can say whether or not "the correct theory" has the specific property   
   > (different from general relativity) which the author suggests.   
      
   I'm referring only to theories which display close analogues to GR black   
   holes.  In such a case we must have either extreme geometry (GR) or   
   extreme physical time dilation (non-geometric but close alternatives).   
   The physical time dilation in the latter case must be associated with an   
   extreme value of a local property, and gravitational potential is as   
   good a name as any.   
      
   > Ultimately, this is a question which needs to be answered experimentally:   
   > can {general relativity, alternative theory A, alternative theory B, ...}   
   > consistently model the data from {experiment #1, experiment #2, ...}.   
   > 	[Note that figuring out whether a theory can consistently   
   > 	model the data from an experiment may be a major   
   > 	research problem on its own.  E.g., to try to model   
   > 	the recent LIGO/Virgo observations requires large   
   > 	and very complicated numerical simulations to figure   
   > 	out the predicted gravitational waves emitted by   
   > 	various candidate binary black hole systems.   
   > 	There has been some work done on extending these   
   > 	numerical simulations to also use non-general-relativity   
   > 	relativistic gravity theories, but a lot more work   
   > 	is needed.]   
   >   
   > But until we have some clear experimental evidence to let us select a   
   > specific non-general-relativity relativistic gravity theory or theories,   
   > I think the author's assertion is premature.   
      
      
   Do you have any theory in mind which would not predict extreme time   
   dilation at the Schwarzschild radius or equivalent, and yet would be   
   compatible with observations of gravitational time dilation in weaker   
   fields?  I guess any version of gravity that weakened dramatically or   
   created a repulsive force at very high densities might work at first   
   sight. But if that were the case, I'd expect the condensed objects at   
   the centre of galaxies (currently thought to be black holes) to glow   
   quite a bit...   
      
   - Gerry Quinn   
      
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