From: tjroberts137@sbcglobal.net   
      
   On 8/25/17 1:43 AM, Nicolaas Vroom wrote:   
   > On Tuesday, 15 August 2017 20:08:47 UTC+2, Tom Roberts  wrote:   
   >> On 8/9/17 12:55 AM, John Heath wrote:   
   >>> The SR effects - GR say the traveling west bound twin's clock was   
   >>> running faster not slower.   
   >   
   >> Actually neither clock runs faster, and neither clock runs slower -- ALL   
   >> clocks run at their usual rate, regardless of how they might move or where   
   >> they might be located.   
   >   
   > If you try to measure time with two identical clocks and the two don't   
   > measure the same than IMO you should disgard the slowest.   
      
   Why? Identical clocks should have equal weight in the measurement. But note   
   that   
   for this to apply they must be measuring THE SAME time -- in the "twin paradox"   
   they don't.   
      
   > That is why the GPS clocks are continuously synchronised?   
      
   They aren't. Small updates (~ few nanoseconds) are uploaded once a day; these   
   are dwarfed by the GR correction to their internal dividers (~ 38   
   microseconds/day).   
      
   > If you want to measure the revolution time of two objects and you place on   
   > each a clock to measure this, don't you want to be sure that the definition   
   > of 1 year for both is the same?   
      
   If they are different objects rotating differently, then the best you can do is   
   use identical clocks. Then the "definition of 1 year" will be the same for both   
   CLOCKS. Whether that corresponds to either object's rotation depends on the   
   objects.   
      
   	"1 year" is a measure of time, not rotation.   
      
   >> IOW: "time dilation" does NOT affect the clock itself, it is a geometrical   
   >> projection of the interval between a moving clock's ticks onto the inertial   
   >> frame used for the measurement.   
   >   
   > I doubt this.   
      
   Then you should learn about Special Relativity. It is SOLIDLY established   
   experimentally.   
      
   > Length contraction (if it exists?) is as far as I see, a geometrical   
   > projection and not something physical.   
      
   "Length contraction", like "time dilation" is purely a geometrical projection,   
   in both SR and GR. So the object being observed is not physically affected. But   
   still, such projections can have physical consequences (e.g. a ladder fits   
   through a narrow doorway only if the geometrical projection of its length onto   
   the doorway is smaller than the latter's width).   
      
   Tom Roberts   
      
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