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From: thomasmc@netcom.com (Tom McFarlane)
Subject: Re: Collapse of wave function occurs once only
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Date: Sat, 31 Dec 1994 03:54:18 GMT
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Peter Norton (pnorton@beaux.atwc.teradyne.com) wrote:

: weare@galaxy.ucr.edu (christopher weare) writes:
: > So it is no suprise that the QM nature gets washed out
: > in our macroscopicworld.

: If I read right, your statement is not a consensus, there still is controversy
: on whether or not the Cat is really a 'paradox'. Isn't there?

OK, here's the deal:

When particles are combined in a single quantum system, their states 
become entangled or correllated with each other.  This correllation is 
what leads to the bizarre nonlocal quantum effects we all know and love.
When bizillions of particles get together, though, their correllations 
tend to almost cancel each other out, leading to a state that is almost 
classical in the sense that almost all quantum correllation is gone.  
Rather than being smeared around all-over, the state is most likely to be 
in just one state or the other, without much correllation between them. The 
catch is that the correllations (the off-diagonal terms of the density 
matrix for you physicists out there) never completely vanish, so the 
state never really becomes classical.  (It's like when a small area of 
the earth is examined, the curvature is small -- but no matter how small 
you get, a sphere never really becomes flat.)

Now the problem here is that the Schrodinger equation is the law that 
governs how the state of the system evolves, and it prohibits any change 
in state that simply cancells all correllation (i.e. sets the off-diagonal 
terms to zero).  In other words, the difference between classical and 
quantum is not a matter of degree but a matter of kind.  In simple terms, 
you can't get to a classical world from a quantum world without some 
ad-hoc rule that collapses the wave function or cancells all the 
off-diagonal terms of the density matrix.  We're then left trying to 
explain this ad-hoc rule.  This is where physicists part company.
(How do you get from a round earth to a flat earth?)

It simply isn't settled because there is no real theory of quantum 
measurement.  Schrodinger's cat will have to sit in suspended animation 
for a while longer yet, it seems.  Maybe the next revolution in physics 
will clear the matter up...only to create more mind-boggling paradoxes.

Tom McFarlane
