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From: stern@bnl.UUCP (eric)
Newsgroups: net.physics
Subject: Re: Left, Right, Positive and Negative
Message-ID: <129@bnl.UUCP>
Date: Sun, 9-Jun-85 02:18:27 EDT
Article-I.D.: bnl.129
Posted: Sun Jun  9 02:18:27 1985
Date-Received: Fri, 14-Jun-85 05:01:28 EDT
References: <235@sask.UUCP>
Distribution: net
Organization: Brookhaven National Lab. Upton, N.Y.
Lines: 45

> 
> 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.

This is not strictly true, because parity and charge conjugation or
not conserved quantities.  T.D. Lee in his book "Particle Physics and
Introduction to Field Theory", discusses the case of two advanced
civilizations who can communicate using only unpolarized electromagnetic
radiation.  If further contact were to be possible, the two civilizations
would have to decide on the sign of the proton, and the definition of
left and right.  The answer lies in the breaking of symmetry between
matter and antimatter, and between left and right, in weak interactions.
A physicist in a alien civilization could decide whether his/her/its
world was made of matter or antimatter by looking at the decay of
the long-lived neutron kaon.  The neutral K has semileptonic three
body decay modes K -> e+ pi- nu, and K -> e- pi+ nu, with the ratio
between the decay rates of 1.00648 +- 0.00035.  Thus, if the alien physicist
observed a larger decay rate for the channel with the electron that
has the same sign as the proton, he/she/it knows that the world is
made of matter.  Determining left from right is a simple matter, after
the sign of the proton charge is pinned down.  The spin of the neutrino
is always pointing opposite to its direction of motion.  The convention
of the direction of spin determines clockwise and counterclockwise,
from which it is easy to figure out what left and right should be.
 
The nonconservation of Charge-conjugation and parity (CP) was discovered
in 1964 by Cronin and Fitch, see Phys. Rev. Lett. 13, 138(1964).

Nonconservation of parity was proposed theoretically be Lee and Yang in
1956 and confirmed experimentally by two groups under the leadership
of C.S. Wu and L. Lederman.


						Eric G. Stern
						Dept of Physics, SUNY
						StonyBrook, NY 11794
						stern@bnl.arpa
						stern@bnl.bitnet
						...!philabs!sbcs!bnl!stern