From: terry@aslws01.asl.dl.nec.com (Terry Bollinger)
Newsgroups: sci.physics
Subject: Re: QM Spin-Observer Problem
Date: 28 Aug 92 20:36:19 GMT
Organization: NEC America, Inc. Irving, Texas

Hi folks,

  Observer   Pol filter  Photon     Pair     Photon   Pol filter   Observer
     A       at 45 deg     A      Generator     B     at 135 deg       B

    <@         (/)         *  <------ ! ------> *         (\)          @>
   |=====================================================================|
       Platform linking pair generator, filters, and observers A and B

> By the rules of QM, each of the two filter/observer pairs will "see" a
> photon that is 100% polarized at (45 or 225) degrees for observer A, or
> (135 or 315) degrees for observer B.

NOT!  My original version of this used fermions, not photons, and Stern-
Gerlaugh magnets + 2 base state detectors for the observers.  Should have
stuck with that, cause I royally messed up the detectors for the photon
case.  Why?  Because the detectors suggested are for all pratical purposes
identical -- they differ only in whether a photon of a particular spin
(polarization) will pass through or be stopped.

The intent of the thought experiment is not terribly profound -- it's just
an attempt to ask what happens when the normal vector arithmetic used to
balance out angular momentum gets a bit wonkered by quantization effects.

You can apparently still ask my basic question with the above arrangement
if you, say, change the position of the left filter to 0 degrees instead
of 45.  It gets a bit messier, but the same idea of "what happens when
dual observation results in misalignment of the original spin vectors?"


My original 1/2 spin fermion version, which is uses a setup similar both to
some Bell Inequality experiments and Dr. Feynman's nice Lectures Vol. III
discussion of base states, looked like this:

  Observer  Stern-Ger.  Neutron     Pair     Neutron  Stern-Ger.   Observer
     A      at 45 deg      A      Generator     B     at 135 deg       B

    <@         (/)         *  <------ ! ------> *         (\)          @>
   |=====================================================================|
       Platform linking pair generator, filters, and observers A and B

For each pair of neutrons "generated" the observers should see one of
only two possible base states for each neutron hitting a detection screen
placed after the Stern-Gerlaugh magnet:

    Observer A:   * 45 deg          Observer B:  * 135 deg
                 /  -or-                          \  -or-
                * 225 deg                          * 315 deg

That much I'm pretty sure is safe.  The peculiar "either-or" quantized
behavior of fermions is well documented and not the issue.  My question,
quite possibly a simple one, is just whether dual detection of correlated
spins in a QM system *might* leave you with oddball residual vectors.
In the case of 1/2 spin fermions the nominal remainder state for the
entire platform for each pair production would be be taken with equal
(.25) probability from the set:

    sqrt(2)/2 spin units oriented { up, down, left, or right }

And again, you could in principle accumulate an indefinite number of any
one of these nominal residual vectors simply by running a very large
number of experiments.  I emphasize that simply to point out that if such
a residual vector is in fact implied by such problems, simply saying that
it is "negligable" is not really the question -- rather, the question is
either why it cannot arise, or why it has real physical meaning, or why it
cannot accumulate to macroscopic scales.

One physicist to whom I described the Stern-Gerlaugh version simply said
that the magnetic field is the culprit -- that is, that the field twists
the particle around and thus takes up the needed reciprical spin.  While
that may or may not explain the Stern-Gerlaugh case, such an answer still
seems to avoid the issue of whether quantized dual detection of very small,
correlated spin vectors could in general lead to oddball residual vectors
showing up at the multiple-detector level.  For example, I don't see off
hand how it could be used to explain the modified photon-pair detection
problem (with 45 degrees between polarizers instead of 90) that I mentioned
briefly above

Comments?  Clarifications?  (Flames and/or major abuse?  :-)  )

				Cheers,
				Terry Bollinger