From: nicolaas.vroom@pandora.be   
      
   On Monday, 15 July 2019 02:56:16 UTC+2, Nicolaas Vroom  wrote:   
   > On Sunday, 14 July 2019 02:53:29 UTC+2, Tom Roberts  wrote:   
   > >   
   > > But SR predicts that the moving rod (or train) does NOT "become   
   > > shorter".   
   > > It also predicts the moving rod WILL BE OBSERVED to "look" shorter.   
   >   
   > [[Mod. note -- The reason for the apparent conflict between the two   
   > statements is that they are answering different questions.   
   > 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.   
      
   I agree.   
      
   > 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   
   > then she measures the position of the right end of the rod   
   > and she subtracts the two positions.   
      
   The question is how exactly does she makes this measurement?   
   Does she performs this measurement using a set of standard rods?   
      
   > 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).   
      
   I assume you mean its rest length?   
   The question is how is this rest length measured?   
   Is this measurement performed using a set of standard rods?   
      
   I expect if both measurements are performed in the same way   
   (Using both standard rods) no length contraction will be measured   
   because if there is any length contraction both her moving rod   
   and the moving standard rods will change, making it disappear.   
      
   > 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.)   
      
   I agree. I think finally both require the same precision, specific is no   
   length contraction is observed (which could be the case in my proposal)   
      
   > 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.   
      
   That is what I'm proposing in my experiment.   
   I want to measure (compare) the positions when the center of the rod is   
   exactly in between the rod at rest.   
   That is moment t.   
      
   > 	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.   
      
   See above how that moment is defined in my proposed experiment.   
      
   > 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 central collation point computes L_you := x_right - x_left   
   >   
   > According to special relativity, if you do this, you will find that   
   > this length will show Lorentz contraction.  Answering *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... so it's no longer surprising or contradictory   
   > that their answers are different.   
   > -- jt]]   
      
   When I understand everything correct at every moment t there is always   
   one observer at the front end of the rod and one at the back end of each   
   rod such that we can measure the length of two rods, one at rest and one   
   moving in the same (inertial) frame simultaneous.   
   What I also understand that the measured length of the moving rod   
   is shorter than the measured length rod at rest and a function of   
   the speed v of the moving rod. That means this demonstrates length contraction.   
      
   This leaves open the question what is the physical explanation.   
      
   What is difficult to understand the experiment, which is a thought   
   experiment, should be in agreement, as SR predicts, while the experiment   
   I propose, which is much simpler, is supposed to physical fail (?) IMO   
   every experiment is very difficult to perform in reality   
      
   Nicolaas Vroom.   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)