From: helbig@asclothestro.multivax.de   
      
   In article , Tom Roberts   
    writes:   
      
   > 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.   
      
   This is just a difference in words; your "projection" is my "illusion".   
   What I meant is that effects which depend on relative motion only, such   
   as length contraction, are not real from the point of view of that which   
   is being contracted.  The physical effects you mention are observed by   
   others, not by that which is contracted (or an observer at rest relative   
   to that).   
      
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