From: ram@zedat.fu-berlin.de   
      
   Richard Livingston  writes:   
   >I was not taught this when first learning about QM in the 1970's.  As   
   >best as I can tell the idea comes from the idea that the QM wave   
   >function evolves per a unitary operator that can, in principle, be   
   >reversed to recover the past state as well as calculate the future state   
   >of the system.   
      
     I think this is correct.   
      
   >It seems to me that this argument is missing two important facts:   
      
     It's not an argument, it's an observation about the   
     properties of the time evolution.   
      
   >                                                                  -The   
   >wave function is not real, it is only a mathematical tool for predicting   
   >the probabilities of future states   
      
     If you believe that one of those future /states/ will be real,   
     then you should also acknowledge that the /wave function/ is   
     real, because a /wave function/ is just a particular   
     representation of the /state/ of a system. (I assume the   
     simplified case of a pure state.)   
      
     But honestly, I do not care if it's called "real" or not.   
     It's an element of the theory, and this theory gives correct   
     predictions. These predictions do not depend upon whether   
     someone calls the wave function "real", so I deem that   
     question to be irrelevant.   
      
     In one sense it really is not "real". It's values are not   
     real but complex numbers, and observables must have real   
     values.   
      
   >It seems to me that each time the wave function "collapses" that   
   >information is lost.  Is there a good argument why this is wrong?   
      
     Ah, the "collapse"! No one understands this yet. If an   
     observer interacts with a quantum system, he might observe   
     a collapse. But when the whole combination of the observer   
     and the quantum system is seen as a single quantum system   
     from the outset, there is no collapse.   
      
     So, there are two possible descriptions of this history.   
      
     A tentative guess for an explanation might go as follows:   
     When the observer O measures the quantum system with two   
     possible outcomes A and B, the world is split into two worlds:   
     One world where the observer (now called OA) observed A,   
     one world where he (now called OB) observed B. Information   
     is not lost. However, from the point of view of OA,   
     information is lost. This is because he now only sees a part   
     of the unitary evolution - that part with the outcome of A.   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)