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From: john@frog.UUCP (John Woods)
Newsgroups: net.physics
Subject: Re: Heisenberg Uncertainty Principle
Message-ID: <260@frog.UUCP>
Date: Fri, 9-Aug-85 12:13:30 EDT
Article-I.D.: frog.260
Posted: Fri Aug  9 12:13:30 1985
Date-Received: Mon, 12-Aug-85 23:32:51 EDT
References: <3506@decwrl.UUCP>
Organization: Charles River Data Systems, Framingham MA
Lines: 39

> 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, ... I can determine the
> position and momentum of my desk to a degree of accuracy limited only by the
> resolution of the measuring equipment I use, whereas the Uncertainty
> Principle says that no matter how accurate my measuring equipment, when it
> comes to the individual particles making up the desk, it is fundamentally
> impossible to even make the observation.
> 
OK, the problem is this:  Even desks are subject to Heisenberg's Uncertainty
Principle (which applies to many measurement pairs, but position and momentum
are particularly easy to grasp).  Perform the following (canonical) thought
experiment:  You have an electron under an ideal microscope, and you want to
look at it to find its position and/or momentum.  You must use a photon to
do this, which knocks the electron about, and (look up the experiment for a
clear explanation) is the derivation of the Uncertainty Principle:  you must
interact to measure, and the minimum interaction comes in units of h-bar/2.
(I am not doing this justice, sorry).

Now, put your desk under the microscope.  How do you propose to find out where
it is, or how fast it is going?  Ask it?  No, you bounce photons off of it.
It recoils...  and if you calculate the amount that your interaction screws up
the situation by, it comes out to (ideally) h-bar/2.

Why does it seem that desks are perfectly stable objects, that can have
perfectly definite positions even when they fly across your office at several
feet per second?  Because h-bar/2 is so incredibly tiny when compared to
sensible desk-measuring units (like furlong-stone-fortnights), that no-one can
possibly care about the difference.  (For reference, h-bar/2 (which is h over
4*pi), is .527E-34 joule-sec)  After all, the photons from your desk lamp
don't send your desk flying, do they?

I hope this helps.  Aggregate particles are bound by the same kinds of
behaviour required of sub-atomic particles, but they (and we) are so large
that we do not notice.

--
John Woods, Charles River Data Systems, Framingham MA, (617) 626-1101
...!decvax!frog!john, ...!mit-eddie!jfw, jfw%mit-ccc@MIT-XX.ARPA