From: g.scholten@gmx.de   
      
   Tom Roberts wrote:   
      
   >> Twin A is at the base of the tower where the watch  of twin B goes   
   >> slower than that at the top.   
   >   
   > 	[This is garbled -- is B at the top or bottom? which   
   > 	 watch goes slower than which? -- Let me stipulate   
   > 	 that A is at the bottom and B is at the top, and your   
   > 	 claim is that A's watch goes slower than B's.]   
   >   
   > Not in GR. In GR, the local laws of physics are the same everywhere,   
   > including the laws that govern the ticking of watches. So the two   
   > watches tick at the same rate (assuming they are identical).   
      
   Measured in the proper time of an observer, the clock of this observer   
   always ticks with the same rate, that is correct. However, one can   
   consider two clocks on different positions in a gravitational field -   
   e.g. one clock at the bottom of a tower and the second clock at the top   
   of that tower - and construct a coordinate system, e.g. Schwarzschild   
   coordinates, to describe the spacetime region around the two clocks, and   
   see one clock ticking slower compared to the coordinate time than the   
   other clock.   
      
      
   > All too many elementary books and discussions talk about "clocks ticking   
   > slower" than other clocks. In GR this is just plain wrong -- all clocks   
   > tick at their usual rate, no matter where they are located or how they   
   > might move   
      
   Once again: this holds for the tick rate of a clock measured in the   
   clock's own proper time. But it does not hold e.g. when comparing the   
   clock's proper time to the coordinate time of some coordinate system.   
      
   --- SoupGate-Win32 v1.05   
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