From: tjroberts137@sbcglobal.net   
      
   On 9/24/17 9/24/17   3:56 PM, Phillip Helbig (undress to reply) wrote:   
   > [...] Length contraction here is obviously an illusion.   
      
   Not so. An "illusion" could not have physical consequences, but   
   "length contraction" does. For instance the magnetic forces from   
   current-carrying wires, the correspondence between fixed-target and   
   intersecting-beam cross-sections, and the frequency/wavelength of   
   free-electron lasers.   
      
   Of course "length contraction" is most definitely NOT an illusion   
   in SR, it is a geometrical projection. Just like "time dilation",   
   which is a similar projection but onto the time coordinate rather   
   than a length coordinate. Such projections have more substance than   
   illusions, and have physical consequences in appropriate situations,   
   but aren't a physical change in the object, either.   
      
   Tom Roberts   
      
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