XPost: sci.physics.relativity   
   From: marc182@cox.net   
      
   In article ,   
   toby_kelsey@ntlworld.com says...   
   > For the purpose of a science fiction story I was wondering what a   
   > black-hole 'solar' system would be like for humans.   
   >   
   > Suppose we start with a 10-solar-mass BH and a Jupiter-size gas-giant at   
   > 8 AU, and put a human settlement of one of its moons.   
   >   
   > How would such a system form: Would a gas-giant survive a   
   > black-hole-forming nova; Would a planetary system survive a collision   
   > beteen a normal star and a black-hole; Would a binary system formed with   
   > a black-hole have stable planetary orbits?   
   > Which mechanism is plausible and what sort of system would result?   
      
   The first extra-solar planet discovered was in orbit around a pulsar.  A   
   pulsar is just a failed blackhole, so yes, a black hole probably could   
   have a planet.  However, the supernova that formed the blackhole would   
   blast away the atmosphere of any gas-giant and you'd be left with a   
   core.  A core of what?  Rock and iron, metallic hydrogen, diamond, I've   
   heard all of these suggested. It might have some kind of atmosphere,   
   boiling out of what's left of the surface, but it wouldn't be a gas-   
   giant anymore.   
      
   Moons, I don't see any way one could survive. The change in mass of the   
   gas-giant alone would tend to throw them off. Not to mention being   
   evaporated by the heat of the supernova.   
      
   What's left after a supernova would be refractory, fused and baked. Not   
   too many fluffy atoms of hydrogen, nitrogen or carbon left around. If   
   the supernovae was recent there might be useful amounts of transuranic   
   elements there on any solid bodies ready to be swept up. Your human   
   settlement could keep warm with fission reactors.   
      
      
      
   > The event-horizon is where the escape velocity exceeds the speed of light.   
   > However the escape velocity is defined as escaping to an infinite   
   > distance.  For someone close to the black hole, light can reach them from   
   > below the 'infinite' horizon. Suppose satellites were positioned such that   
   > the delta-V between adjacent ones was c/2?   
   >   
   > We have   
   >   
   > 0.5*m*(c/2)^2 = GMm/r_1 - GMm/r_2   
   >   
   > which becomes   
   >   
   > (c*c)/(8GM) + 1/r_2 = 1/r_1   
   >   
   > so the reciprocal of the radii are separated by a constant amount, and   
   > the radii can become arbitrarily small.  Placing satellites at these radii   
   > allows signals to be relayed past the nominal event horizon.  Is this   
   > correct?  What would the inner satellites see of the outside world?>   
      
   The event-horizon is a wall.  You can't fool it with relay satellites.   
   No information comes out. Even if you did the calculus, with an infinite   
   number satellites, that very last satellite of the infinite series   
   wouldn't quite be able to push a photon past the event-horizon.   
      
   Marc   
      
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