From: nicolaas.vroom@pandora.be   
      
   On Sunday, 14 July 2019 02:53:29 UTC+2, Tom Roberts  wrote:   
   > On 7/12/19 5:11 PM, Nicolaas Vroom wrote:   
   > > [...] The final step is to perform this experiment with two rods,   
   > > with the same length at rest.   
   >   
   > This experiment will fail, for the same reason as before: you cannot   
   > make the ends of the rods flat to the accuracy required to observe the   
   > "length contraction" of the moving rod.   
      
   That can be a practical reason when there actual is physical length   
   contraction involved.   
   But the experiment can also fail because there is no physical LC.   
      
   > > The physical issue if the moving rod (or train) becomes shorter   
   >   
   > But SR predicts that the moving rod (or train) does NOT "become shorter".   
   Let me propose/rephrase this sentence:   
   SR predicts that the physical length of a moving rod does not become shorter.   
   (What is meant that the length does not change. Newton would agree with this.)   
      
   > It also predicts the moving rod WILL BE OBSERVED to "look" shorter.   
   Let me propose/rephrase this sentence:   
   SR predicts that the measured physical length of a moving rod, does become   
   shorter.   
   (All human aspects are removed)   
      
   IMO if we agree both about the new proposals there is a problem, because   
   the two predictions are in conflict which each other.   
   IMO what counts are the results of an actual experiment.   
      
      
      
   [[Mod. note -- The reason for the apparent conflict between the two   
   statements is that they are answering different questions.  To elucidate   
   this, it's important to be very precise about just what question we are   
   considering.  That is, a phrase such as "the measured physical length   
   of a moving rod" is not sufficiently precise: it's necessary to specify   
   precisely who (what observer) is making that measurement, and how that   
   measurement is made.   
      
   Suppose that you are an inertial observer, i.e., that you observe that   
   Newton's 1st law holds for motion measured relative to you.  And for   
   simplicity, let's suppose that the rod is also in inertial motion, i.e.,   
   an observer riding on the rod also observes that Newton's 1st law holds   
   for motion measured relative to her.  And (again for simplicity) let's   
   say the rod is oriented horizontally in your reference frame, and is   
   moving lengthwise from your left to your right.   
      
   Then the simplest case is one where the observer-on-the-rod measures   
   the length of the rod.  In this case, measuring the rod's length is easy   
   (for her): she measures the position of the left end of the rod   
   	[VERY IMPORTANT: it doesn't matter *when* she makes   
   	this measurement, because she is riding along on the   
   	rod and hence the rod is stationary with respect to   
   	her inertial reference frame],   
   then she measures the position of the right end of the rod   
   	[again, it doesn't matter when she makes this measurement],   
   and she subtracts the two positions.   
      
   If she does this, then (according to special relativity) she will find   
   the rod's length to be unchanged from its "basic" length (its length   
   before it was in motion with respect to you).  (We're assuming that the   
   accelerations involved in setting the rod into motion were sufficiently   
   gentle to not permanently distort it.)  This means, for example, that   
   if the "rod" is actually made up of a number of eggs placed end-to-end,   
   that she will NOT observe these eggs to have been crushed.   
      
   This framework -- defining "the length of the rod" as that measured   
   by an observer riding along with the rod (this is sometimes called a   
   "proper observer"), is the basis for the statement you quoted above   
   > But SR predicts that the moving rod (or train) does NOT "become shorter".   
      
   Now let's consider what *you* might observe.  That is, you see the   
   rod moving from left to right, and as it's moving you (somehow) measure   
   it's length.  What will you measure?   
      
   As I noted above, answering this question requires being very precise   
   about just how the measurement is made.  (It turns out that this case   
   requires rather more precision than was the case for the proper-observer   
   measurement.)   
      
   One obvious way for you to measure (define!) the length of the rod is   
   for you to measure the position of the left end of the rod at some time   
   t, and for you to measure the position of the right end of the rod at   
   the same time t, and then for you to subtract the two positions.   
      
   	[In the previous sentence I'm using "measure" in the   
   	special-relativity sense, i.e., we assume a (gedanken)   
   	infinite set of sub-observers, all at rest in your   
   	inertial reference frame, one at each spatial position.   
   	Each sub-observer has her own clock, and all these   
   	clocks are synchronized to each other via the Einstein   
   	clock-synchronization convention in your inertial   
   	reference frame.   
      
   	The position of an end of the rod at a time t is   
   	then measured by that (unique) sub-observer who is   
   	coincident with that end of rod at her clock's time t.   
   	Since that sub-observer is coincident with her end of   
   	the rod, there are no light-travel-time delays from   
   	the moving rod to the sub-observer.   
      
   		[It's precisely this avoidance of light-travel-time   
   		effects that's the motivation for introducing the   
   		infinite set of sub-observers.]   
      
   	After making their measurements, the sub-observers then   
   	send their observations back to some central collation   
   	point for analysis.]   
      
   In relativity, "time" is observer-dependent, so we need to be specific   
   about whose (which reference frame's) "time t" we're using.  The simplest   
   choice is to say that since *you* are making this measurement (in *your*   
   inertial reference frame), then you should measure the left and right   
   rod ends' positions at times which are simultaneous in *your* inertial   
   reference frame, i.e., your sub-observers should each measure the   
   rod ends' positions at the same clock reading (t).   
      
   That is, to recap,   
   * the subobserver who is coincident with the left end of the rod at her   
     clock reading t sends a message back to the central collation point   
     to that effect (the message includes the subobserver's position, say   
     x = x_left, in your inertial reference frame)   
   * the subobserver who is coincident with the right end of the rod at her   
     clock reading t sends a message back to the central collation point   
     to that effect (the message includes the subobserver's position, say   
     x = x_right, in your inertial reference frame)   
   * the central collation point computes L_you := x_right - x_left   
      
   This (L_you) is a reasonable operational definition of   
   "the length of the rod as measured by you".   
      
   According to special relativity, if you do this, you will find that   
   this length will show Lorentz contraction.  Aswering *this* question   
   is the basis of the other statement you quoted above,   
   > It also predicts the moving rod WILL BE OBSERVED to "look" shorter.   
      
   It should now be clear(er) that the two apparently-contradictory statements   
   you quoted above, are in fact the answers to two *different* questions...   
      
   [continued in next message]   
      
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