From: fortunati.luigi@gmail.com   
      
   > [[Mod. note --   
   > To put it another way, can you explain why you think it's paradoxical   
   > that observer B1 observes something different from observer A?   
   > -- jt]]   
      
   I do not consider it paradoxical that observer B1 observes something   
   different from observer A.   
      
   I think it is paradoxical that observers B1 and A see different things   
   both real: if one thing is real the other must be apparent and vice   
   versa.   
      
   This is why I speak of reality and appearance.   
      
   A simple and clear example is the following.   
      
   If observer B1 stands on the carousel, he sees the earth rotate a full   
   360 degrees in 5 seconds and sees the carousel stationary.   
      
   Instead, observer A on the ground sees the carousel rotate 360 degrees   
   in 5 seconds and sees dry land.   
      
   One sees one thing, the other sees another: can both be real? Obviously   
   not.   
      
   If what observer A sees is real, what observer B1 sees can only be   
   apparent.   
      
   In fact, if the Earth really rotated at that angular velocity, it would   
   shatter!   
      
   [[Mod. note -- What do you mean when say that something is or isn't   
   "real"?  In special relativity we don't really have such a concept.   
   Rather, we have the concept of what some specified (inertial) observer   
   observes,   
      
   ALSO, note that B1, B2, B3, ... observing the body X to be Lorentz-   
   -contracted does *not* imply any special internal stresses in X.   
   There are several two different ways of obtaining ("proving") the   
   statement in my previous sentence.  For example:   
   (a) We can introduce the concept of "stress tensor", note (show)   
       that it is in fact a tensor, note that Lorentz transformations   
       can be viewed as tensor transformations of coordinates, and make   
       use of the elementary tensor calculus result that a tensor that's   
       zero in one coordinate system (basis) is zero in any other coordinate   
       system (basis), so that the stress tensor vanishing in A's inertial   
       reference frame implies it must also vanish in any other inertial   
       reference frame.   
   (b) More generally, to reason about internal stresses in objects, we   
       need a theory of (relativistic) continuum mechanics, and we can   
       use this to work out the internal stresses in the body X, computed   
       in A's intertial reference frame, B1's inertial reference frame,   
       B2's intertial reference frame, etc etc.   
   (c) At a microscopic level, the size of an object is determined by the   
       lengths of the chemical bonds between the object's constituent atoms.   
       These lengths are in turn determined by the Schroedinger equation   
       for the atoms' valence electrons.  So, we could work out how   
       solutions of the Schroedinger equation change under Lorentz   
       transformations.   
      
   (a), (b), and (c) should all give the same answer, namely, the usual   
   Lorentz contraction does *not* imply any special internal stresses in   
   the object.   
   -- jt]]   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)