XPost: sci.astro.research   
   From: newspam@nezumi.demon.co.uk   
      
   On 02/06/2017 11:07, Phillip Helbig (undress to reply) wrote:   
   > In article ,   
   > Gerry Quinn  writes:   
   >   
   >>>> To put it another way, the 'clumpy' states in the non-gravitational   
   >>>> universe have lower entropy than the smooth state, but the clumpy states   
   >>>> in the gravitational universe have higher entropy than the smooth state.   
   >>>   
   >>> Imagine a clumpy universe with no gravity.  It has low entropy (lower   
   >>> than the smooth universe).  Now G starts increasing from zero to, say,   
   >>> its current value (at which point the clumpy universe has a higher   
   >>> entropy than the smooth universe).  At some value of G, the clumpy   
   >>> universe must have the same entropy as the smooth universe (which you   
   >>> say has the same entropy with or without gravity).  So for this value of   
   >>> G, the entropy is independent of the clumpiness.   
   >>>   
   >>> Someone has made an error somewhere.   
      
   It is a failure of intuition rather than of physics. The apparent   
   paradox is because a self gravitating clump of material gets hotter as   
   shrinks under the influence of its own gravity. Adding gravity makes the   
   smooth uniform matter distribution metastable wrt perturbations.   
      
   >> Why should it not be independent of the clumpiness?   
   >   
   > Because it's not.  A room full of air with the same density everywhere   
   > has higher entropy than a room with all of the air squeezed into one   
   > corner.  (In the case where gravity can be neglected.  When gravity   
   > plays a role, then the clumpier distribution has higher entropy.)   
      
   The difference is that once gravity gets involved there is potential   
   energy available to be released when a clump of matter collapses under   
   the influence of mutual gravitational attraction (gravity is always and   
   attractive force). The shrinking material heats up as it is compressed.   
      
   The original uniform maximum entropy state is not the lowest energy   
   state for the system and so it is vulnerable to collapse if density   
   fluctuations arise sufficient to allow self gravitating clumps.   
      
   It would behave like a short lived star collapsing in on itself and then   
   getting smaller and hotter as a result without any nuclear fusion to   
   hold it up for longer. Martin Rees describes this far better than I can   
   on p116 of Just 6 Numbers in the section about Gravity and Entropy.   
      
   You now have a significant temperature difference between your new   
   gravitational star and the background which can be used to do work.   
      
   Originally it was Lord kelvin that did the lifetime computation of a   
   star powered only by gravitational collapse as a means of discrediting   
   the very long geological timescales needed for Darwinian evolution.   
      
   --   
   Regards,   
   Martin Brown   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)