From: g.scholten@gmx.de   
      
   Nicolaas Vroom wrote:   
      
   >>> When they compare clocks at P2 A's clock has counted 40 ticks   
   >>> and B's clock has counted 30 ticks.   
   >>> What is wrong by claiming that A's clock (for some reason)   
   >>> has ticked faster during the time that both A and B travelled   
   >>> from P1 to P2?   
   >   
   >> Let A be the stationary twin and B the travelling twin. One the one   
   >> hand, you can go into the frame of the stationary twin A and say that   
   >> during the whole trip, the clock of twin B ticks slower than the clock   
   >> of twin A, or in turn, the clock of twin A ticks faster than the clock   
   >> of twin B.   
   >   
   > IMO the two figures in paragraph 9 and 10 show this situation.   
   > What you have is one observer/clock A which stays at point P1   
   > and a second observer/clock which moves in a straight line to a point   
   > P3 and back to point P1.   
   > In paragraph 9, A's clock counts 12 and B's clock count 9   
   > In paragraph 10, A's clock counts 17 and B's clock count 12   
   > That means on average B's clock counts slower.   
   > Actual experiments should validate this claim.   
   >   
   > The explanation is in the physics of the clock used.   
   >   
   > The clock used, is described in Figure 1.3 of the book SpaceTime Physics   
   > by E.F Taylor and J.A. Wheeler II edition.   
   > The clock consists of a lightsource with emits two lightflashes which   
   > oscillates between two mirrors.   
   > (See also paragraph 5.6)   
   > The problem with this clock is when the mirrors are placed perpendicular   
   > to the direction of movement that when the clock moves with the speed   
   > of light, the light flash will never reach the mirror in front   
   > and the clock will not count.   
   > On the return path you have the same.   
      
   You describe the situation seen from twin A's frame, let's denote this   
   frame S: twin A is at rest, twin B is moving, and therefore, the   
   lightflashes in B's clock take longer from one mirror to the other than   
   the lightflashes in A's clock, making B's clock ticker slower.   
      
   However, we can as well consider the situation from the frame S' that is   
   co-moving with twin B during the trip out: during the trip out, B is at   
   rest and A is moving, therefore the lightflashes in A's clock take   
   longer from one mirror to the other than the lightflashes in B's clock,   
   making A's clock ticker slower. During the return trip, both, A and B,   
   are moving, where B has the higher velocity, so the lightflashes in B's   
   clock take longer from one mirror to the other than the lightflashes in   
   A's clock, making B's clock ticker slower. So, B's clock ticks   
   temporarily faster and temporarily slower than A's clock.   
      
   And as well, we can consider the situation from the frame S'' that is   
   co-moving with twin B during the return trip.   
      
      
   >> So, the claim that A's clock is ticking faster than twin B's clock is   
   >> not correct in a frame-independent sense. Depending on the frame, A's   
   >> clock may tick faster during the complete trip, or tick even slower than   
   >> B's clock temporarily.   
   >   
   > As explained above the reason why A's clock is ticking faster is physical.   
      
   Physical, but frame-dependent.   
      
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