From: merritt@macro.bu.edu (Sean Merritt)
Newsgroups: sci.physics
Subject: Re: Yet Another Preposterous Post
Date: 7 Jan 93 15:48:18 GMT
Distribution: sci
Organization: Boston University Physics Department
In-reply-to: hagerman@ece.cmu.edu's message of 6 Jan 93 22:48:24 GMT

In article <HAGERMAN.93Jan6174824@rx7.ece.cmu.edu> hagerman@ece.cmu.edu (John Hagerman) writes:



   paul@mtnmath.uucp says:

   > Consider a singlet state photon pair that was emitted from a distant
   > galaxy.  The photon paired with this has been traveling in the
   > opposite direction for billions of years. QM predicts that these two
   > are still connected. If you detect one photon in this pair by
   > looking at the night sky with polarizing sun glasses you will
   > instantly impose a constraint on the polarization of the other
   > photon billions of light years away according to existing theory. To
   > me such a prediction requires extraordinary justification.

#   "Requires" is pretty strong, and invites a reactionary response.  But
#   that's why sci.physics is so much fun, right?
#
#   Here's the chain of thought that I think leads to posts about collapse
#   being real.  "QM is a good theory, but it has a flaw (it includes the
#   concept of instantaneous collapse, which is cause for concern).  But
#   rather than consider it a flaw, what happens if we make it a postulate
#   and explore the implications?  That's what Einstein did with the speed
#   of light, right?"  The first step is easy, and leads to posts.  The
#   second step is hard; perhaps beyond most readers of sci.physics, but
#   surely not beyond the experts.  So what do the experts say?  If I have
#   understood the discussion, instantaneous collapse does *not* imply the
#   ability for ftl communication of information, so there is no violation
#   of relativity.  (Is this related to discussion about the difficulty in
#   defining/measuring delays?)  What other implications are of interest?

I've been staying out of this one but there is an approach offered
by Aharonov while controversial, does offer some hope of a resolution
to the problems of wave function collapse.

let 

Psi1 = exp [ - i Int(t1 ->t)d(tau) H(t) ] |A=a> = |P1>

that is operator with non degen. eigenvalues produced a measurement
O|A> = a|A> at t=t1.

at later time t=t2

a eigenvalue was produced from a wavefuntion <B=b| having the form

Psi2* = <B=b| exp [ -i Int(t ->t2)(dtau) H(t)] = <P2|.


Now Aharonov and his collegues define a NEW concept which the call
a "weak" measurement using these two wave functions.

Given that |P1> was pre-selected(prepared) and that <P2|
was postselected an expectation value 

   _  <P2|A|P1>
Aw =  ---------
         <P2|P1>

is defined as weak measurement and does not collapse the wavefunction.

It starts to get weird after that and I think people should
read the Papers for themselves:

See Y. Aharonov,L. Vaidman Phys Rev A 41,11(1990) 


-sjm


--
Sean J. Merritt		         | "Every revolt is a cry of innocence    
Dept of Physics Boston University|  and an appeal to the essence of being."
merritt@macro.bu.edu             | Albert Camus, The Rebel
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