Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10 beta 3/9/83; site frog.UUCP Path: utzoo!linus!decvax!genrad!mit-eddie!cybvax0!frog!john 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