From: helbig@asclothestro.multivax.de   
      
   In article ,   
   Gerry Quinn  writes:   
      
   > In extreme cases it does get a bit problematic, i.e. where general   
   > relativity predicts a non-simple topology, at and inside the   
   > Schwarzschild radius of a black hole.  (That's why I've always been   
   > certain that general relativity breaks down at the Schwarzschild radius;   
   > a belief that is no longer considered particularly eccentric these days   
   > I think.)   
      
   The general view is that nothing strange happens at the Schwarzschild   
   radius.  It is, after all, only a coordinate singularity, not a physical   
   one.  In that respect, it is like a pole of the Earth: the distance   
   between meridians of longitude is zero, but it doesn't matter.  It's   
   just that in some circumstances some coordinates might be better than   
   others.   
      
   This has changed somewhat within the "firewall" controversy, where   
   indeed some people have claimed that very strange things happen at the   
   Schwarzschild radius.  I think it's fair to say that the jury is still   
   out, though my impression is that most who have investigated this don't   
   think that something strange happens there.  I doubt that that is the   
   reason why Gerry claims that GR breaks down here, though; I think he was   
   claiming that before firewalls were a hot topic (pun intended).   
      
   [[Mod. note --   
   In classical (non-quantum) general relativity, then it is unambiguous   
   that nothing strange happens at the Schwarzschild radius, or more   
   generally at an event horizon.  As Phillip Helbig says, it's just   
   a singularity in (some) coordinate systems.  Fortunately, there are   
   plenty of other non-singular coordinate systems in which it's obvious   
   that nothing strange is happening.   
      
   In quantum gravity (= quantum general relativity) things are a bit   
   trickier... because we don't yet have a working theory of quantum   
   gravity.  There are various "semiclassical approximations" (roughly   
   speaking, flat-space quantum field theory but on a curved background,   
   but not treating the backreaction of the quantum fields on the   
   spacetime curvature properly).  Working with these approximations   
   one can derive things like the firewall controversy.   
   -- jt]]   
      
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