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From: desj@brahms.BERKELEY.EDU (David desJardins)
Newsgroups: net.physics
Subject: Re: Does the moon exist?
Message-ID: <12773@ucbvax.BERKELEY.EDU>
Date: Sat, 29-Mar-86 07:31:32 EST
Article-I.D.: ucbvax.12773
Posted: Sat Mar 29 07:31:32 1986
Date-Received: Sun, 30-Mar-86 10:43:35 EST
References: <12628@ucbvax.BERKELEY.EDU> <1483@mhuxt.UUCP>
Sender: usenet@ucbvax.BERKELEY.EDU
Reply-To: desj@brahms.UUCP (David desJardins)
Organization: University of California, Berkeley
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In article <1483@mhuxt.UUCP> js2j@mhuxt.UUCP (sonntag) writes:
>     Could someone who thinks they understand this stuff translate for us
>laymen?  In particular: this wave-function which collapses is merely a 
>measure of our ignorance of the true state of the system, right? 
>Schroedinger's cat will continue to live or continue to rot in the box, even
>if we delay our observation for a day or two, right?

   Well, quantum mechanics is *far* more than "quantified ignorance."
That said, I believe (but am not sure) that it *is* possible (although not
usual) to do QM from this point of view (and get the right answers).  It is
not the usual approach because it is much easier to do actual calculations
from the point of view that the outcome is determined at the moment of
observation.
 
>> Wrong.  The moon is as subject to quantum law as photons.  And the reach
>> of quantum law is the entire universe: one can observe a quasar billions
>> of light-years away split in two by gravitational lensing and then run a
>> delayed choice experiment on the double images.  Quantum mechanics has a
>> very long reach indeed.  [WIENER]
>
>     Sure you can run the experiment.  Do you think it will have any effect
>on the quasar?  If so, will the effect propogate at the speed of light, or
>what?  

   No, the effect propagates instantaneously (whatever that means).
   I understand EPR has been previously discussed on the net, so I am
going to give only a brief summary.  There are certain processes which
produce pairs of photons, and from certain symmetry considerations we
know that the two photons in the pair must have opposite polarizations.
But we do not know how the individual polarizations of the photons will
come out until we pass them through polarizers to measure them (this is
what QM is all about).
   It turns out, therefore, that if we take two photons produced in this
way, store them and then separate them by a great distance, and "simul-
taneously" measure their polarizations, that the polarization of either
of the two photons individually is randomly determined, but they will
always come out opposite to one another even though there is not time
for information about one outcome to propagate to the site of the other
experiment.
   All of the above is essentially a confirmed fact (the experimental
evidence is still a little shaky, but the theoretical predictions are
firm).  But I do have to admit that I don't really know what Matt is
talking about above, about light from the distant quasar.  Maybe he
will clarify his example for us.

>> The problem now is what happens when you don't look at it.  If you refuse
>> to look at the moon, does quantum mechanics force you to conclude it does
>> not exist?
>
>     If the answer to this question is yes, could someone describe the 
>experiments and the chain of reasoning which forces this conclusion?

   My answer is no, so I am not the one to answer, but I will say that
it depends exactly what you mean when you refer to "the moon."  The point
is that, according to the standard interpretation of quantum mechanics,
at times when the moon is not being observed its characteristics (position,
momentum, etc.) are not exactly determined.  Thus it is not clear if it
makes sense to say that it "exists."

   -- David desJardins