From: helbig@asclothestro.multivax.de   
      
   In article , Tom Roberts   
    writes:   
      
   > > What causes a physical effect is the internal operation of a clock   
   >   
   > No. The internal operation of a clock is unaffected by its motion   
   > (relative to anything) -- that's the first postulate of SR.   
   >   
   >      The laws of physics govern the internal operation of   
   >      every clock, and the first postulate says they are the   
   >      same in every locally inertial frame.   
   >   
   > > The moving clock runs physical slower compared to a clock at rest.   
   >   
   > Only if the first postulate is wrong. Zillions of experiments show that   
   > it is correct.   
   >   
   > The observer will OBSERVE the moving clock to run slower than her own   
   > clock, but the moving clock ITSELF is unaffected (i.e. it does NOT run   
   > "physically slower").   
      
   I think that everyone understands purely illusory effects: A sees B's   
   clock running slower and vice versa.  It is clear that these are only   
   apparent effects and not "real".  The question of what is observed is   
   trickier, because the finite speed of light has to be taken into   
   account.  There WAS some real confusion about this, but it was cleared   
   up by Terrell and Penrose a long time ago.   
      
   [[Mod. note -- Note that in the previous paragraph, the author is using   
   the word "observed" NOT in the technical-special-relativity sense (of   
   measurements made by instantaneously co-located sub-observers so as to   
   eliminate light-travel-time effects), but rather in the sense of "what   
   a camera image would show".  That is, the author's usage of "observed"   
   INCLUDES light-travel-time effects.   
   -- jt]]   
      
   What is difficult to understand is the twin paradox: After A goes away   
   and comes back while B stays at home and they then compare clocks at   
   rest, EVERYONE agrees that A's clock has ticked less.  Recent discussion   
   here shows that acceleration is not the "cause", since the effect   
   depends on the length of the journey, and not on the acceleration.   
   Since all clocks (mechanical, electronic, atomic, biological, nuclear)   
   are equally affected, it is a) hard to imagine that some mechanism   
   affects them all equally and b) no-one has any idea what such a   
   mechanism could be.  (A similar effect exists if A spends more time in a   
   deeper gravitational potential.  This is really equally difficult to   
   understand, though since there IS an obvious cause, the gravitational   
   potential, fewer people have a problem with it.  And perhaps the   
   equivalence people suggests to them that in the SR twin paradox the   
   acceleration must have something to do with it.)  Yes, one can adopt the   
   "shut up and calculate" approach and calculate the strength of the   
   effect, and it agrees with observations, but this is not the same as   
   understanding.  The question is whether such an understanding is   
   possible.  GR is much more complicated mathematically, though   
   conceptually I find it easier to digest, agreeing with Peter Ustinov   
   here (bonus points if you spot the reference).   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)