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From: tjr@ihnet.UUCP (Tom Roberts)
Newsgroups: net.physics
Subject: Re: Quantum mechanics, Aspect experiment, EPR paradox
Message-ID: <166@ihnet.UUCP>
Date: Wed, 10-Oct-84 10:37:52 EDT
Article-I.D.: ihnet.166
Posted: Wed Oct 10 10:37:52 1984
Date-Received: Fri, 12-Oct-84 07:55:47 EDT
References: <361@petsd.UUCP>
Organization: AT&T Bell Labs, Naperville, IL
Lines: 30

The EPR "paradox" in QM is not really a paradox, but merely a way of
pointing out that you must be VERY CAREFUL when interpreting experiments
involving quantum-mechanical systems.

Consider instead the QM system consisting of a single 2-sided coin (heads
and tails) after a random coin toss. In order to learn which side is up, 
you must perform a measurement upon the system (i.e. look at the
coin). Assuming a typical coin (i.e. VERY large on the atomic scale),
it is no surprise that a single measurement of the system is sufficient
to determine its state. You merely look at the top face of the coin; in
particular, YOU DO NOT NEED TO LOOK AT THE BOTTOM OF THE COIN, TOO.

The EPR gedanken-experiment is merely a different kind of coin-toss. If you
wish to determine whether a single particle confined in a box is in one
half or the other, it IS sufficient to measure only one half to determine
the answer (assuming negligible probability for the particle to change sides
after or because of the measurement).

The "paradox" arises when you don't keep in mind that you must take account
of the ENTIRE system: the left-half of the box, the right-half of the box,
and the particle. When you ask "why does a measurement in this half of the
box affect the situation in that half of the box (far away) ?" you are NOT
thinking of the entire system, but are (erroneously) considering two separated,
non-interacting subsystems (the two sides of the box). It isn't clear in
which subsystem the particle is in, AND THAT IS THE POINT. In this mode
of thinking, you have NOT made a proper separation of the system into
non-interacting subsystems.

	Tom Roberts
	ihnp4!ihnet!tjr