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From: stekas@houxy.UUCP
Newsgroups: net.physics
Subject: Re: A question on photon decay
Message-ID: <158@houxy.UUCP>
Date: Mon, 7-Nov-83 10:02:47 EST
Article-I.D.: houxy.158
Posted: Mon Nov  7 10:02:47 1983
Date-Received: Tue, 8-Nov-83 22:31:30 EST
References: <195@denelcor.UUCP>
Organization: Bell Labs, Holmdel NJ
Lines: 17


        Real photons do not decay.  The process of creating an
electron-positron pair from a photon is called "conversion", and
it occurs when a photon collides with a charged particle.
During the collision, the photon is "absorbed" by the charged particle
which later emits a massive "virtual" photon. The decay of the
massive "virtual" photon gives the electron-positron pair.

        Photon decay would violate the conservation of energy-momentum.
The simplest way to see this is to realize that the electron-positron
system has a non-zero mass (by mass I mean the "rest mass") while the
photons mass is zero.  Only when some energy and momentum can be shed
to some other particle can an electron-positron pair be created from
a photon without violating energy and momentum conservation.


				Jim