On Mon, Apr 10, 2006 at 10:26:17PM -0400, Jesse Mazer wrote:
> As I understand it, you don't need exactly need an observer, you just need 
> to identify various macro-variables (like pressure and temperature) which 
> can be used to "coarse-grain" the phase space of the system, with entropy 
> being proportional to the logarithm of the number of possible detailed 
> "microstates" (detailed descriptions of the positions and momenta of all the 
> particles, within the limits of the uncertainty principle) compatible with a 
> given "macrostate" (descriptions of the system which only tell you the value 
> of the macro-variables). Once you have chosen your set of macro-variables, 
> they should have well-defined values for any system, regardless of whether 
> it's being observed by anyone or not. Of course, the choice of variables is 
> based on what properties we human observers are actually capable of 
> measuring in practice, so I don't necessarily disagree with your statement, 
> but I think it needs a little clarification.

That is precisely my point. However, observers are needed to specify
the thermodynamic variables (as otherwise these things are
meaningless). I try to make this somewhat provocatively, sure, but
denying the role of the observer is bit like sweeping it under the carpet.

> Likewise, I think "the second law is interpreted as the destruction of 
> information" needs a bit of clarification--as entropy increases, there are 
> more and more microstates compatible with a given macrostate so the observer 
> is losing information about the microstate, but information is not really 
> being lost at a fundamental level, since *in principle* it would always be 
> possible to measure a system's exact microstate.
> Jesse

Information also needs an observer. Information is lost from the
observer. I would argue it is not hidden, unless you believe in the
possibility of Laplace's daemon actually existing. (Which I suspect
you are saying with your *in principle* clause).

Also note that exact measurements of microstates is *in principle*
incompatible with the Heisenberg Uncertainty Principle.

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