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From: gwyn@brl-tgr.ARPA (Doug Gwyn )
Newsgroups: net.physics
Subject: Re: Heisenberg Uncertainty Principle
Message-ID: <556@brl-tgr.ARPA>
Date: Thu, 8-Aug-85 20:05:18 EDT
Article-I.D.: brl-tgr.556
Posted: Thu Aug  8 20:05:18 1985
Date-Received: Sun, 11-Aug-85 07:01:51 EDT
References: <3506@decwrl.UUCP>
Organization: Ballistic Research Lab
Lines: 30

> In over-simplified terms Heisenberg's Uncertainty Principle says that we
> cannot know the simultaneous position and momentum of an individual
> elementary particle with unlimited accuracy.  Yet, we are able to
> determine the simultaneous position and momentum of conglomerations of
> these elementary particles.

No, the same constraint holds.  Why do you think otherwise?

> Is it strictly a case of the measurement process itself disturbing the
> individual particle, or is something else going on here?  For example, it
> seems to me that if it is simply a matter of the measurement process
> disturbing the particles we are trying to measure, then we just have to
> find a measurement process that uses small enough particles so that they
> won't disturb the particles we are trying to measure.

But you can't!

> But in my opinion saying that "in principle,
> it is impossible to measure the simultaneous position and momentum of a
> particle" is quite different than saying that "the means to measure the
> simultaneous position and momentum of a particle does not exist".

Yes, these are different.  QM says that you cannot simultaneously
determine (the same component) of the position and the momentum of
any object with absolute precision; indeed, because the two are
Fourier transform pairs, the simultaneous uncertainties have to obey
	 

>= (constant on the order of 1) * h, where h is Planck's constant. means the RMS uncertainty of a quantity. This is a matter of fundamental principle, not of insufficient cleverness on the part of the measurer.