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From: rimey@ernie.berkeley.edu.BERKELEY.EDU (Ken &)
Newsgroups: net.physics
Subject: Re: quantum mechanics and all that
Message-ID: <11661@ucbvax.BERKELEY.EDU>
Date: Sat, 1-Feb-86 16:36:49 EST
Article-I.D.: ucbvax.11661
Posted: Sat Feb  1 16:36:49 1986
Date-Received: Mon, 3-Feb-86 06:27:04 EST
References: <10137@tardis.UUCP> <6746@boring.UUCP>
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Reply-To: rimey@ernie.berkeley.edu.UUCP (Ken Rimey)
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In article <6746@boring.UUCP> lambert@boring.UUCP (Lambert Meertens) writes:
>In article <10137@tardis.UUCP> tmb@tardis.UUCP writes:
>> Assume I put a cat into a box, ...
>
>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.

Yes.  Nothing profound here.

>[Now consider two slits through] which an electron is
>fired; 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.

Yes, one of our favorite thought experiments.

>Now it seems to me that if this Russian roulette played on a cat
>	[ where the electron is constrained to go through slit A if
>		the cat is alive, and slit B if dead ]
>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?

No, according to quantum mechanics you will NOT get an interference pattern,
because the electron's choice of slit is correlated with the state of the
cat.  If there is so much as a neutrino that would be going this way if
the electron went through the left slit, but that way if the right, then
you will not get the double-slit interference pattern.  (This observation
suffices to kill most amateur hidden-variables theories.)

If there are two paths to exactly the same final state, you add their
complex amplitudes (interference).  But if these paths really lead to
two different final states, the probability of getting to either is
the sum of the probabilities of each (no interference).

						Ken Rimey