From: bweiner@ruhets.rutgers.edu (Benjamin Weiner)
Newsgroups: sci.physics
Subject: Re: Bell dethroned
Date: 29 Jul 92 23:10:16 GMT
Organization: Rutgers Univ., New Brunswick, N.J.

I imagine none of the sci.physics "gurus" have taken a crack at this
because it's an old idea, the post is confusing and cries out for
graphics, and they all have real work to do.  I, on the other hand,
only have a qualifying exam to study for !)

The assumptions leading to the Bell inequalities do include perfect
detector efficiency.  However, since real detectors are, well, real
detectors, useful derivations of the inequalities include a detector
efficiency factor.  (This is, as I recall, especially important
because you need to do coincidence counting to make sure you only
count photons that are genuinely paired, and inefficient detectors
will decrease your coincidences.)  This is all quite well understood.

It has also been understood for a LONG time that inefficient detection
opens the door for claims that, essentially, you're missing the
photons that make the difference.  The first Bell-inequality
experiments (Clauser and Horne?  Clauser, Holt, Shimony, and Horne) 
had considerably lower detection efficiency than Aspect.
The assumption that somehow the missing photons would tip the scales
was generally disregarded as paranoid.  (Aspect's advance was to
switch detector settings during photon time-of-flight; some people on
this group don't think he did it right, but it is immaterial for this
discussion.  You need the time-of-flight switch to rule out speed-of-
light wavefn collapse, but not to rule out generic hidden variables.)

Regardless of the niceties, one has to assume some kind of no-
mechanism, angle-dependent variation of detector efficiency, in order
to reconcile hidden-variables with QM in this experiment.  Danforth's
particular choice is this "crown model."  My opinion is, sure, if you
can assume variable efficiency you can make the results do whatever
you want.  Why should the detectors be variably efficient?  Shouldn't
this happen with any old photons, not just correlated photons?
Shouldn't, then, one be able to test this by putting a detector in
front of a known source, and just rotating the damn detector?

How can anyone possibly presume to DERIVE the efficiency of a
detector?  Do you suppose Aspect's detectors magically had the same 
efficiency as, much earlier, Clauser's?   Some of the experiments used
spin-1/2 atoms, I believe.  One wonders if the atoms and their
detectors could be following the very same rules.  An argument that
relies on conspiracy between the detector and the detectee is no
argument at all.

Finally, aficionados of EPR-tricks should read Greenberger, Horne,
Shimony, and Zeilinger's paper (Am J Phys 58, 1131) along with
Mermin's treatments (Am J Phys 58, 731; PRL 65, 3373) which show that
it is possible to construct a simple system of 3 particles of spin 1/2
and make certain measurements such that QM predicts a -1 correlation
and hidden-variables predict a +1 correlation.  That is, one could
never trick up hidden variables to reproduce QM in this case.  I
rather doubt anyone will be able to devise an experiment that 
reproduces this system, but I could be wrong.

Ben Weiner
Newsgroups: sci.physics