From: tjroberts137@sbcglobal.net   
      
   On 7/6/18 9:34 AM, Gregor Scholten wrote:   
   > Nicolaas Vroom wrote:   
   >> [... twin paradox] the reason why A's clock is ticking faster is physical.   
   >   
   > Physical, but frame-dependent.   
      
   I disagree with both claims, and I think SR disagrees with both of these   
   statements, too. This is partly about English word usage, but there is an   
   important aspect of physics here:   
      
   A's clock is NOT ticking faster (than B's), it is ticking at its usual rate,   
   the   
   same rate as clock B. This is required by the first postulate of SR.   
      
   	One can say: "A ticks more times than B". One can also say   
   	"As measured in frame A, A ticks faster than B". But when   
   	one says "A ticks faster than B", one is talking about JUST   
   	THE CLOCKS; it is wrong: clocks tick at the same rate because   
   	the laws of physics that govern their ticking are the same.   
      
   The reason why clock A ticks more times is geometrical, not physical (for any   
   reasonable meanings of these words) -- clock B took a shorter path through   
   spacetime than clock A.   
      
   Note also that frames and coordinates are artificial human constructs used to   
   DESCRIBE experiments and measurements; Nature uses no frames or coordinates. So   
   anything that is frame- or coordinate-dependent cannot possibly be physical (in   
   the sense that it appears in some law of physics).   
      
   All too many popular descriptions of relativity get this wrong -- "Moving   
   clocks   
   run slow" is WRONG. These authors do students no favors by ignoring the first   
   postulate -- to advance to physics beyond SR, it is essential to understand   
   the coordinate independence of the laws of physics.   
      
   Tom Roberts   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)