Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Path: utzoo!linus!decvax!harpo!seismo!hao!hplabs!sri-unix!MJackson.Wbst@PARC-MAXC.ARPA
From: MJackson.Wbst@PARC-MAXC.ARPA
Newsgroups: net.physics
Subject: Re: A question on photon decay
Message-ID: <13422@sri-arpa.UUCP>
Date: Mon, 7-Nov-83 16:55:00 EST
Article-I.D.: sri-arpa.13422
Posted: Mon Nov  7 16:55:00 1983
Date-Received: Wed, 9-Nov-83 07:52:30 EST
Lines: 24

(1) and (2):
A photon travelling in free space can't decay; the (relativistic)
relationship between energy and momentum is determined by a particle's
rest mass, and for a zero-rest-mass particle splitting apart doesn't
work, four-vector-wise.

For particle-antiparticle collisions, energy and momentum can be
conserved by producing two photons (p = 0 in center-of-mass frame, so
has to be at least two).  Other things (for which quantum numbers such
as charge and strangeness sum to zero) could be produced instead, and in
analyzing such reactions one finds it convenient to represent the
intermediate state (between input and output) as a single photon; this
is a "virtual" photon for which the E-p relationship need not be the
usual.

(3)
The zerocity of the rest mass of the photon is sensitively tested by the
degree to which n = 2 in the electrostatic field expression

	E = q/(r**n)

and that is very well established indeed.

Mark