From: tjroberts137@sbcglobal.net   
      
   On 6/30/19 2:53 AM, Eric Flesch wrote:   
   > On 17 Jun 2019, PengKuan Em  wrote:   
   >> Relativistic length contraction is theoretically predicted but not   
   >> directly tested, [...]   
   >   
   > I expect that there are uncertainties involved which will prevent   
   > such a measurement.   
      
   There are, but not as you describe. The "uncertainties" are due to   
   classical measurement resolutions, not quantum effects as you claim.   
      
   > 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.   
      
   This last sentence is frame hopping, and thus incorrect. We (on earth)   
   only observe in our own frame, not using any sort of "foreshortened   
   frame" of the space travelers. Relative to our frame, those travelers   
   always travel with speed less than c -- neither "time dilation" nor   
   "length contraction" is involved, we simply use instruments at rest in   
   our own frame.   
   > 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.   
      
   Those are quantum uncertainties, which are irrelevant for considerations   
   of special relativity using macroscopic objects (such as space travelers).   
      
   > Now think about how hard it is to measure the relativistic length   
   > contraction --   
      
   Yes, it is so hard that nobody has succeeded in measuring it directly.   
   But it is difficult for classical reasons (not quantum effects).   
      
   > 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.   
      
   Such a speculation is not "sound" at all. For macroscopic objects there   
   is no reason to expect that quantum uncertainties will be important   
   (much less "half").   
      
   There are no quantum effects preventing us from measuring "length   
   contraction", just technological limitations.   
      
   	Indeed, as pointed out elsewhere in this thread,   
   	measuring a magnetic field is in essence measuring   
   	the "length contraction" of conduction electrons   
   	moving at a few cm/sec. No quantum effects prevent   
   	such a measurement or interpretation.   
      
   > That means that the required length contraction doesn't need to be   
   > reified into actual physical contraction.  This is just typical of   
   > quantum uncertainties.   
      
   Typical for QUANTUM uncertainties, but not here. SR is a purely   
   classical theory.   
      
   In a different post you said:   
   > I must add the obvious, that objects travelling near c are seen to   
   > rotate so that we view not their sides, but the front end when   
   > approaching and the rear end when receding.   
      
   That is true when using eyeballs (or equivalent) to look at the object   
   via light rays. But it does not apply to the usual methods of measuring   
   in SR using an array of assistants: they make measurements as an object   
   passes nearby, and then communicate the measurements to a common   
   observer who correlates them and makes conclusions. That method is used   
   in gedankens, of course, to avoid the complexities of light propagation   
   affecting the observations.   
      
   > You don't get to see or measure the length contraction -- nature   
   > does her elegant sleight-of-hand once again to confound us   
   > baryonoids.   
      
   You cannot SEE "length contraction" (i.e. with your eyeballs or   
   equivalent), but with appropriate assistants or apparatus you can   
   MEASURE it.   
      
   > "Length contraction" works great for our models and experiments, but   
   > you'll never measure it -- it stays hidden behind the uncertainty   
   > curtain.   
      
   Not so. There's no such "curtain", its just that we don't have any   
   technology capable of measuring "length contraction".   
      
   Tom Roberts   
      
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