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From: gv@hou2e.UUCP (A.VANNUCCI)
Newsgroups: net.physics
Subject: Re: Left, Right, and Mirrors
Message-ID: <603@hou2e.UUCP>
Date: Fri, 7-Jun-85 15:53:26 EDT
Article-I.D.: hou2e.603
Posted: Fri Jun  7 15:53:26 1985
Date-Received: Sat, 8-Jun-85 03:33:17 EDT
References: <235@sask.UUCP>
Organization: AT&T Bell Labs, Holmdel NJ
Lines: 37

>An interesting, related bit of trivia is the fact that "left" and
>"right" are concepts which can only be communicated among two
>parties if both parties can "see" each other.  In other words, if
>we ever contact an alien civilization which is so far away that we
>can communicate with them only by radio, and there is no astronomical
>object which both we and they can view, then we would have no way to
>tell them what we mean by "left" as opposed to "right"--or, for that
>matter, how to tell the north pole of a magnet from the south pole.
>
>					Ken McDonald

  Not exactly true! If you can send radio waves to the distant aliens,
use right-handed circular polarization to let them know left from right.

  If you can only send them "information" as opposed to a
physical signal (such as radio waves), then you could ask them to
perform one of the parity-violating experiments involving weak
interactions. These are experiments where an axial vector (such as a
magnetic field) is aligned with a polar vector (such as the direction
of emission of particles).

   Parity violation means that if you look at the experimental
apparatus in a mirror you see a physical phenomenon that is
impossible. Only experiments involving weak interactions have this
characteristic. However, if in addition to reflecting the image the
mirror exchanges matter with anti-matter the reflected experiment is
all right.

  The moral of the story is that, when you finally meet your alien
friend after many years of discussing parity-violating experiments
and you are about to shake hands, if he stretches out his left hand
you'd better run away as fast as you can! He's made of anti-matter!

		Giovanni Vannucci
		AT&T Bell Laboratories      HOH R-207
		Holmdel, NJ 07733
		hou2e!gv