From: helbig@asclothestro.multivax.de   
      
   In article <47d06be9-b89d-41ce-bc90-236772ee9182@googlegroups.com>,   
   Nicolaas Vroom  writes:   
      
   > > 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.   
      
    From the point of view of another observer, yes, you could think of it   
   this way.  But from the point of view of the observer moving with the   
   rod, the better answer is that of course there is no difference, because   
   there is no way to determine whether she is moving or at rest.  That's   
   the whole point of "relativity": inertial motion is relative.   
      
   You might be implying that there is some physical change, but it affects   
   both the thing measured and the measuring device.  Nope.  There is no   
   physical change.   
      
   (In the "twin paradox", where two observers meet up later and compare   
   clocks, then there IS a physical difference: the elapsed time is   
   different.  This is easy to understand in the context of relativity, but   
   it seems like that something must have physically affected the   
   clocks---all clocks, whether mechanical, biological, atomic, whatever.)   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)