home *** CD-ROM | disk | FTP | other *** search
- Path: sparky!uunet!stanford.edu!rutgers!dziuxsolim.rutgers.edu!ruhets.rutgers.edu!bweiner
- From: bweiner@ruhets.rutgers.edu (Benjamin Weiner)
- Newsgroups: sci.physics
- Subject: Re: Bell dethroned
- Message-ID: <Jul.29.19.10.15.1992.21798@ruhets.rutgers.edu>
- Date: 29 Jul 92 23:10:16 GMT
- References: <1992Jul27.041310.7281@riacs.edu>
- Organization: Rutgers Univ., New Brunswick, N.J.
- Lines: 54
-
- 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
-