From: ram@zedat.fu-berlin.de   
      
   Mike Fontenot  writes:   
   >Inertial observers can legitimately use the famous time-dilation result   
   >of special relativity to determine simultaneity at a distance.   
      
     While there are no rigid bodies in general relativity (GR),   
     special relativity (SR) is a simplification. An inertial   
     observer is a idealized theoretical construct identified   
     with a rigid and non-accelerated frame (like a coordinate   
     system or a scaffolding) not subject to gravity and filling   
     the whole space.   
      
     Using this frame, one can find the middle of two objects,   
     both of which are at rest (relative to the frame), and emit   
     a flash of light in this middle to define simultaneity for   
     those two objects, which are not in the same place.   
      
     When a - more realistic - body then somehow is moving in a   
     possibly accelerated manner along a curved worldline, at   
     every point of its worldline (as a mapping from proper   
     time to events) the /tangent vector/ defines an inertial frame.   
      
     Every confusion arrises when one tries to identify idealized   
     theoretical concepts (such as space-filling rigid frames)   
     with real people (who listen to music and eat food).   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)