From: eric@flesch.org   
      
   On 17 Jun 2019, PengKuan Em  wrote:   
   >Relativistic length contraction is theoretically predicted but not directly   
   >tested, which lead to incorrect interpretation of the theory illustrated by   
   >Bell's spaceship paradox and Ehrenfest paradox. But these paradoxes can help   
   >us designing experiments to test length contraction.   
      
   I expect that there are uncertainties involved which will prevent such   
   a measurement.  A quick review: special relativity shows that   
   arbitrarily large speeds are achievable within the constraint of the   
   universal boundary condition "c" in that space travellers can cross   
   the galaxy in a day as seen by themselves, whereas we observers see   
   the crossing as taking 10^5 years.  Because of the rule that what is   
   seen to happen in one inertial frame is seen likewise in all other   
   inertial frames, the spaceship is mapped (by we observers) into a   
   foreshortened object which indeed travels faster-than-c in its own   
   foreshortened frame except that the accompanying time dilation   
   technically lowers that to within-c as seen by us.   
      
   However, can the foreshortening (i.e., length contraction) actually be   
   observed, is the OP's question.  Well, the length contraction is a   
   calculational necessity which may however be enveloped by necessary   
   uncertainties.  Consider the Bohr-Einstein debates on quantum theory   
   -- Bohr beat Einstein's argued paradoxes by showing that they were   
   enveloped and thus nullified by the involved uncertainties.  Now think   
   about how hard it is to measure the relativistic length contraction --   
   I expect that physically required uncertainties will come into play in   
   any such attempt.  As a matter of fact, it's a sound speculation that   
   the positional uncertainty will be found to be exactly half of the   
   rest length of the object measured -- so that length contraction will   
   forever be unmeasurable.  That means that the required length   
   contraction doesn't need to be reified into actual physical   
   contraction.  This is just typical of quantum uncertainties.   
      
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