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From: tmb@tardis.UUCP
Newsgroups: net.physics
Subject: Re: quantum mechanics and all that
Message-ID: <10137@tardis.UUCP>
Date: Mon, 27-Jan-86 04:49:32 EST
Article-I.D.: tardis.10137
Posted: Mon Jan 27 04:49:32 1986
Date-Received: Thu, 30-Jan-86 05:12:24 EST
Distribution: net
Organization: Harvard University
Lines: 60

|||when one performs a measurement on a "mathematical object that represents
|||the state of the universe", the outcome is only *probable*.  That is,

||Nevertheless, abstractly, the theory only tells us how to calculate
||one future, one that incorporates both outcomes of the experiment.

|Let me put it this way:  how can a deterministic theory have it that the
|very same initial conditions (particle, measuring device setup, etc.) lead
|in different trials to the different outcomes experimenter-thinks-"E1!"
|and experimenter-thinks-"E2!"?

Let's talk about a variation on Schroedinger's cat, perhaps that will
clarify the standpoints and end this silly discussion.

Assume I put a cat into a box, together with a device that performs an
experiment whose two outcomes are equally likely, according to quantum
mechanics, and one outcome of which causes a cyanide capsule to break
within the box and the cat to die.

ALL THAT QUANTUM MECHANICS PREDICTS is that if you make an infinite number
of such boxes and perform the experiment with all of them then, after
the experiment, in one half of the boxes you will find dead cats,
in the other half you will find live cats.

Now, people are unhappy with this (I, for example, like cats). There
are some who say that you didn't make the boxes perfectly identically
in some parameter that isn't incorporated into our quantum mechanical theory
of nature. They say that if you discovered this 'hidden parameter' and
made the boxes identical in it as well, then the outcome of the experiment
would be certain: either all cats die or all cats live. This is a valid
position, although there is absolutely no evidence to support it.

Other people suggest that the cat isn't really dead until someone observed
that it is dead, i.e. that the decision about its death isn't made until
someone observes it. In different terms, we cannot say what happens inside
an isolated system until it isn't isolated anymore, because the outside
observer has to 'open' it. This point has no relevance. A physical theory
does not gain anything by assuming that the decision is made at the point
of observation; in fact, it looses simplicity. It is like argueing about
the existence of an ether with very special properties (namely such properties
that relativity works) vs. the theory of relativity. One theory isn't
any more powerful than the other, and we simply pick the one which is less
verbose.

Finally, some people suggest that instead of the cat dying or not
dying, both happen, in 'branching universes'. This, again, is like
postulating the existence of an ether: it doesn't add anything to our
theory and just introduces verbiage.  There is no experiment that I can
perform after opening the box and finding a dead cat that shows that
the cat is still alive in some 'other universe'.

ALTOGETHER, quantum mechanics is a theory which makes predictions about
the statistical outcome of a large number of identical experiments. It does
not predict the outcome of individual experiments, and you may feel free,
therefore, to refuse to call it a 'deterministic' theory. At the moment,
quantum mechanics is simply the best theory we have. We might be able
to find a theory which predicts the outcomes of individual experiments,
or we might not. There is no way to decide that question by philosophising.

							Thomas.