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From: sra@oddjob.UUCP (Scott R. Anderson)
Newsgroups: net.physics
Subject: Re: Electron radius
Message-ID: <954@oddjob.UUCP>
Date: Mon, 2-Sep-85 12:18:04 EDT
Article-I.D.: oddjob.954
Posted: Mon Sep  2 12:18:04 1985
Date-Received: Tue, 3-Sep-85 01:41:32 EDT
References: <522@sri-arpa.ARPA>
Reply-To: sra@oddjob.UUCP (Scott R. Anderson)
Organization: University of Chicago, Department of Physics
Lines: 33
Summary: 

In article <522@sri-arpa.ARPA> TERRY%LAJ.SAINET.MFENET@LLL-MFE.ARPA writes:
>
>I see no basis for the assumption that the rest mass of an electron is
>due entirely to the mass of its electrostatic energy.

If an electron is a classical sphere of charge of radius r, it will have a
"self-energy" of approximately e^2/r.  This is the energy needed to hold the
charge together (by some unknown force).  From the relativistic equivalence
of mass and energy, this binding energy will appear as a mass, just as the
mass of a nucleus is different from the total of the masses of its component
protons and neutrons.

>The following equation is then given:
>
>        m*c^2 = e^2/r
>
>but a previous message by the same author says that electron radius
>is ~ e^2/(m*c^2).  Where did the approximation come in?

The approximation comes in because e^2/r is not exactly the self-energy;
there is some unmentioned constant factor involved (for a uniform sphere,
I believe it is 3/10).  However, the above expression gives rise to a useful
arrangement of universal constants, e^2/mc^2, which is referred to as the
"classical electron radius".

>The value for r is then given as ~ 3*10^15 meters.  Is this really
>correct?

3*10^(-15) m.  As mentioned before, the upper limit on the electron's
radius is much smaller than this.

				Scott Anderson
				ihnp4!oddjob!kaos!sra