From: g.scholten@nospam.gmx.de   
      
   Luigi Fortunati  wrote:   
      
   > I'm standing in front of the mirror at a distance  with my clock   
   > that marks the time .   
   >   
   > The clock image in the mirror marks a delayed time of 2d/c (compared   
   > to t) due to the time it takes to light to get to the mirror and   
   > return (as long as i stand still and the mirror as well).   
   >   
   > But what if I am running at constant speed parallel to the mirror? The   
   > distance is always equal to  but the light does not go and goes   
   > back to the same point, because in the meantime I have moved away.   
   >   
   > And then the light has to travel a greater distance due to the   
   > inclination of the path and therefore the delay time of the clock   
   > image is greater than 2d/c.   
   >   
   > This longer delay measures my speed compared to the mirror.   
   >   
   > What if the mirror was moving while I was still standing?   
   >   
   > In that case, the light should no longer extend its path because it   
   > would leave me firm and return from me always.   
      
   In both cases (you are moving and the mirror is moving), you can   
   consider things either in your own rest frame or in the mirror's rest   
   frame.   
      
   In your own rest frame S, the path of the light has the length s = 2d,   
   and therefore, the light takes the time interval   
      
   Delta_t = s/c = 2d/c   
      
   for the path.   
      
   In the mirror's rest frame S', the path of the light has the longer   
   length s' > 2d/c, and therefore takes the longer time interval   
      
   Delta_t' = s'/c > 2d/c   
      
   This difference in the time interval is what is known as relativistic   
   time dilation.   
      
   This is true for both cases, so both cases are fully equivalent. It's   
   just a matter of the frame of reference you chose (your own or the   
   mirror's).   
      
      
   > If all this was true (and it is not possible) it would be enough to   
   > compare the time of the watch with that of the image to find out if   
   > one moves or moves the other: if the delay is equal to 2d/c it moves   
   > the mirror, if it is the bigger the clock moves.   
      
   In your own rest frame, the time delay is always Delta_t = 2d/c.   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)