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From: FAILOR%LLL@LLL-MFE.ARPA
Newsgroups: net.physics
Subject: Re: Do Positrons Have Negative Mass?
Message-ID: <553@sri-arpa.ARPA>
Date: Thu, 19-Sep-85 15:20:00 EDT
Article-I.D.: sri-arpa.553
Posted: Thu Sep 19 15:20:00 1985
Date-Received: Mon, 23-Sep-85 00:26:36 EDT
Lines: 25

From: Bruce Failor 
To: Physics@SRI-UNIX.ARPA
In-Reply-To: Message from "Physics@SRI-Unix" of Thu 19 Sep 85 00:50:28-PDT

By the way, does anyone know how good a vacuum intergalactic 
(and interstellar) space is, say in protons or electrons per cc?
According to what I have heard, it is mainly H.

One of the professors on a friend's oral examination committee asked
a question about the time of arrival of high energy photons.  The prof
maintained that the arrival time was a function of energy with a
threshold at the energy required to produce an electron-positron pair.
If pair production is the process that causes the lag, there needs to
be protons or other nuclei available to allow for momentum and energy
conservation.  He stated that the high energy photons produce pairs
which in turn annihilate which in turn produce pairs, etc.
Since matter travels at less than the speed of light, the photons that
spend the most time as electon-positron pairs will experience the most
time lag.

Has anyone else heard of this/know of references to actual
measurements?

                               Bruce Failor
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