On Mon, Apr 10, 2006 at 09:45:50PM -0700, Brent Meeker wrote:
> Russell Standish wrote:
> > On Mon, Apr 10, 2006 at 12:03:47AM -0700, Brent Meeker wrote:
> > 
> >>Russell Standish wrote:
> >>
> >>>Unitary evolution preserves information. It is only through
> >>>measurement by an observer that information can be created or
> >>>destroyed. Usually, the second law is interpreted as the destruction
> >>>of information (anyone observing a closed system will over time know
> >>>less information about the system), so it puzzles me that you have the
> >>>sign the other way.
> >>
> >>What?  You're saying that if I observe a system, then I know less about it. 
> >> You 
> >>must be using some non-standard meaning of "know".
> >>
> >>Brent Meeker
> >>
> > 
> > 
> > Yes - in the case of milk being stirred into coffee. Strange as it may
> > seem, you know more information when the system is initially
> > structured than after that initial structure  has dispersed.
> What's that have to do with observing it?  Stirring milk into coffee isn't 
> observing it - and as you point out below, entropy depends on "observation", 
> i.e. on some coarse grained constraint.
> Your answer seems to consist of non-sequiturs.  ISTM that my knowledge is 
> increased when I observe something.  Physically this corresponds to some 
> small 

Your total knowledge increases, assuming perfect memory (which is
itself debatable, but beside the point). But your knowledge of the
current state of the system decreases. The information content of the
system decreases (exactly offset by the rise in entropy). 

My point is that this is precisely because it is observed.  If it
weren't observed, one simply has a quantum superposition undergoing
unitary evolution.

A/Prof Russell Standish                  Phone 8308 3119 (mobile)
Mathematics                                    0425 253119 (")
UNSW SYDNEY 2052                         [EMAIL PROTECTED]             
Australia                                http://parallel.hpc.unsw.edu.au/rks
            International prefix  +612, Interstate prefix 02

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