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Article 6083 of comp.ai.philosophy:
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Newsgroups: comp.ai.philosophy
Subject: Re: Quantum mechanics (no AI here, sorry)
Message-ID: <1992Jun04.181928.5451@spss.com>
>From: markrose@spss.com (Mark Rosenfelder)
Date: Thu, 04 Jun 1992 18:19:28 GMT
References: <1992Jun2.161131.11780@guinness.idbsu.edu> <1992Jun03.203556.4561@spss.com> <1992Jun3.225545.27925@guinness.idbsu.edu>
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In article <1992Jun3.225545.27925@guinness.idbsu.edu> holmes@opal.idbsu.edu 
(Randall Holmes) writes:
>It's local properties of the _waves_ that are involved.  Such an
>experiment is carried out by setting up a state in two widely
>separated locations which depends on an unobserved factor in a single
>earlier event.  One then makes observations at the widely separated
>points, and, lo, they agree with one another.  These results make
>perfect sense (require no explanation at all, in fact) on a hidden
>variables interpretation, i.e., on the interpretation that there was a
>real underlying value to the unobserved factor in the earlier event
>which we did not in fact observe (we couldn't observe it and do the
>experiment, in fact).  The "non-locality" has to do (on my
>interpretation) with the fact that getting extra information about
>event A may immediately give me extra information about event B even
>if A and B have space-like separation (suppose that event A and event
>B are individuals listening to television broadcast years earlier from
>Earth, at the same distance from the Solar System in opposite
>directions in the conventional frame of reference of the Solar System,
>and the information that I have acquired is a copy of the TV schedule;
>I then know that if A watched "I Love Lucy" at a certain time, so did
>B).  [...]

Perhaps we are not talking about the same thing.  Let me attempt to
describe the problem in more detail.  I hope physicists reading this
will gently correct any errors I make.

An apparatus is set up which emits a stream of spin-correlated photon pairs 
in opposite directions.  Observers at points A and B are using calcite 
crystals to test the polarization of each pair of photons.  The following 
results are noted:

   Angle between orientation   Errors in their
   of A's and B's calcite      spin observations
           0                     0 in 4
          30                     1 in 4
          60                     3 in 4
          90                     4 in 4

Now, this seems to be what you are talking about: we observe a correlation
between A's and B's observations, but we have no reason to suppose any
superluminal (or even subluminal) connection between the photons once they 
leave the apparatus.

A local hidden-variable theory could explain the observations by supposing
that each photon, as it leaves the apparatus, "knows" how it is going to
react to each possible setting of the observer's calcite (perhaps by 
having an explicit angle of polarization, rather than the binary spin
value assumed by QM).  

A photon leaves the apparatus, and A twists his calcite 30 degrees left.
No problem: the photon knows how to respond to any angle.  Before either
photon is measured, but after they've left the apparatus, however, B turns
*her* calcite 30 degrees right.  If the locality assumption is true, that
can make no difference in A's observations.  But it does: the observations
are now correlated at the 3-errors-in-4 rate.  

To know how to respond to A's observation, the photon needs to react not
only to the orientation of A's calcite, but to that of B's too, and its
knowledge is not limited by the speed of light.

Hidden variables are not ruled out, but solely local ones are.


