From newshub.ccs.yorku.ca!ists!helios.physics.utoronto.ca!news-server.csri.toronto.edu!utgpu!pindor Thu Feb 20 15:20:50 EST 1992
Article 3738 of comp.ai.philosophy:
Newsgroups: comp.ai.philosophy
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>From: pindor@gpu.utcs.utoronto.ca (Andrzej Pindor)
Subject: Re: QM nonsense
Message-ID: <1992Feb14.190712.13821@gpu.utcs.utoronto.ca>
Organization: UTCS Public Access
References: <jbaxter.697533284@adelphi> <406@tdatirv.UUCP> <65812@netnews.upenn.edu> <17872@castle.ed.ac.uk> <1992Feb13.202401.28326@organpipe.uug.arizona.edu>
Date: Fri, 14 Feb 1992 19:07:12 GMT

In article <1992Feb13.202401.28326@organpipe.uug.arizona.edu> bill@NSMA.AriZonA.EdU (Bill Skaggs) writes:
>
>  This is not exactly related to AI, but there seems to be a lot of
>puzzlement related to quantum mechanics on this newsgroup, so it might
>be useful to lay out some of the issues.
>
>  Most of the philosophically perplexing aspects of QM arise in one
>simple, easily understood experiment, the "two-slit experiment."  The
.....
    Description of two-slit experiment follows..
...
>
>  It is important to realize that the occurrence of collapse is, in
>principle at least, empirically verifiable, because it implies the
>loss of interference between the different possible states of the
>system.  Quantum mechanics (i.e. the Schrodinger equation) predicts no
>such loss of interference, so collapse, whenever it occurs, is a
>violation of QM.  
>
There is more to QM than Schroedinger equation, for instance the principle
of superposition of states is not part of Schr. eqn. And the statement that the
collapse of wave function is a violation of QM is far too strong. 
...
>
>  My feeling is that collapse probably does not occur at all.  Quantum
>mechanics has always in the past been correct when it predicted
>interference between states; why should it be wrong now?  There may be
>a sort of *effective* collapse, though.  It may be generally
>impossible for a measuring device to detect interference between its
>own internal states.  If this is so, then whatever you look at is,
>from your point of view, collapsed.  But somebody else, looking at
>you, could still in principle detect interference between various
>possible states of you.
>
I am afraid that you see things in too simple terms. As it seems at present,
the collapse of wave function results in _destruction_ of its part which is
ortogonal to space span by eigenvectors of a current observable.
The most paradoxical side of collapse of the wave function involves apparent
conflict with causality: wave funcion may be extended in space and then
observation at some point of space forces a transition in the whole space 
simultaneously! To refer to the two-slit experiment described above, note that
wave function would be filling the whole space between the barier with two
slits (both of them open) and the screen at which we detect electrons. Now
even if we had detectors covering the whole screen, only one of them
would fire at a time - interaction with any of them (which one - being totally
random) causes an immediate collapse of the _whole_ wave function into a state 
localized in this detector. The problem is how do the distant 
parts of wave function 'know' that somewhere a detection already occured. 
A thought experiment showing that QM requires such instantenous effects prompted
Einstein to doubt QM (famous Einstein-Podolsky-Rosen paradox). However, in 
recent years there have been a number of experiments which _do_ confirm this
unortodox behaviour (so much for introspection, even Einstein's).
 
>	-- Bill


-- 
Andrzej Pindor
University of Toronto
Computing Services
pindor@gpu.utcs.utoronto.ca


