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From: apak@oddjob.UUCP (Adrian Kent)
Newsgroups: net.physics
Subject: Re: quantum mechanics and all that
Message-ID: <1161@oddjob.UUCP>
Date: Mon, 10-Feb-86 11:48:25 EST
Article-I.D.: oddjob.1161
Posted: Mon Feb 10 11:48:25 1986
Date-Received: Wed, 12-Feb-86 06:37:20 EST
References: <10137@tardis.UUCP> <6746@boring.UUCP>
Reply-To: apak@oddjob.UUCP (Adrian Kent)
Distribution: net
Organization: U. Chicago: Physics
Lines: 50
Summary: 

In article <6746@boring.UUCP> lambert@boring.UUCP (Lambert Meertens) writes:
[in context of Schrodinger cat thought-experiment]
>Now do the following: make two slits A and B close to each other in the box
>that can be opened or closed from the inside.  Install equipment that
>monitors if the cat is alive (as used in intensive care) that causes slit
>A (initially open) to close, and B (initially closed) to open when the cat
>dies.  After an elapsed time such that the probabilities of life vs. death
>are equal, let an electron be fired to the slits.  It will pass through A
>or B, with equal probabilities.
>
>To an outside observer, to whom this devilish contraption is a black box,
>this is the same as a box with two open slits to which an electron is fired;

No, it's not - and this is a crucial point. A state describing two open slits
is not the same as a superposition of two states each describing one open slit.

>if measurements concerning the passing of the electron are made, it
>is equally likely to pass through A as through B.  If no such measurement
>is made but a photographic plate is placed behind the slits, we get an
>interference pattern because of self-interference of the "probability wave"
>passing through the slits, i.e., the superposition of the two waves
>corresponding to the two outlets.
>
>Now it seems to me that if this Russian roulette played on a cat results in
>the superposition of a live and a dead cat, then the output of the box is
>likewise the superposition of an electron passing through A, and the *same*
>electron passing through B.  So we should see an interference pattern here
>as well.  Is this indeed what QM predicts?  (I honestly don't know; it's
>not my field.  I have a gut feeling it does not, in which case I would
>really like to know why not.)
  You're right. QM predicts no interference. The wavefunctions |dead, A open>
and |alive, B open> are orthogonal. The electron wavefunction becomes 
correlated with them, giving a state "|dead, A open, electron went through A>
+ |alive, B open, electron went through B>", but these don't interfere.
(This is phrased loosely but can be made precise.) 
>
>If the outcome shows indeed interference, then I don't see how other positions
>than "Schroedinger's cat is in a sense both dead and
>alive" can be maintained.  On the other hand, if it does not show
>interference, that ambivalence would seem quite untenable to me.
  I think that's too strong. The standard conclusion about this type of 
experiment is that the many-worlds interpretation (cat 'both dead and alive')
and the collapsing-wave-packet picture (cat either dead or alive) make
(for all practical purposes) the same experimental predictions.            
The problem (as I see it) is that a collapsing wave-packet picture requires
a theory underlying QM (which hasn't been found), while the many-worlds
picture isn't remotely plausible as a fundamental description of nature.
(Lots of physicists disagree.)
					    Adrian Kent
>     Lambert Meertens