From: spicedham@dualboot.net   
      
   "Earl Colby Pottinger"  wrote:   
   > That is silly, if I throw a ball at you and it bounces off your head, does   
   > the fact that the ball is still moving at the same speed prevent the pressure   
   > on your head?  The direction of the photon's momentum and to balance it the   
   > momentum of the sail must also change.   
      
   Quite.  If a perfect mirror were able to reflect photons   
   and not change their energy, it would still be forced to   
   generate thrust via conservation of momentum.  Similarly,   
   if you hit someone with a ball and it bounced off and no   
   energy was lost, then you'd expect the target to recoil   
   due to conservation of momentum.  You simply cannot   
   change momentum of an object without keeping the books   
   balanced.  Changing the direction of photons changes   
   their momentum drastically, and that has to be balanced   
   somewhere.  If Gold's theory were correct then Newton's   
   3rd law would be violated, and that would be quite   
   serious indeed, quite a lot more serious than any of the   
   evidence he has put forth to back up the theory that such   
   a violation would be possible.   
      
   However, there is also thermodynamics at play.  To   
   simplify greatly, thermodynamics says that you can't do   
   work without generating an appropriate amount of   
   entropy.  And here Gold would be correct in stating that   
   a perfect mirror which changes only the direction of   
   reflected photons but which is moved would be a violation   
   of the 2nd law of thermodynamics (it would be a closed   
   system with decreasing entropy).  However, as has been   
   pointed out many times, the reflected photons are red   
   shifted, so entropy does increase in the system.   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)