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Re: the energy



At 11:09 PM 10/29/2004, Mike, you wrote:
Well... Leigh Palmer and Jack Uretsky are certainly reading different
references than I. They both called my description of pair-production
unphysical, can't happen, etc.

* * * I [M D Edmiston] said... * * *

A photon is traveling in some region, and all of a sudden... poof...
the photon is gone and in its place we see a positron and an electron.
We discover this is energy related because not any photon can do this.
The photon must have an energy of at least 1.022 MeV which is the
E=mc^2 equivalent of the rest mass of the positron and the electron.

* * * Leigh said... * * *

I certainly dispute that this process can occur. The process you
propose can't have an energy threshold because if it had such a
threshold it could happen in any inertial frame of reference, including
all those frames in which the photon has an energy less than 1.022 MeV.
<snip>
Now, it is more conventional to say that the pair production process
you propose can't happen because it would violate the law of
conservation of translational momentum,
<snip>

* * * Jack said... * * *

1. Photons cannot be localized to "regions".
2. The photon was not observable. It is correct to say that at some
time (depending upon the detector) a vertex occurred that could be
interpreted as e+ e- production. We can measure the momenta of the
members of the pair and, using energy-momentum conservation, deduce the
energy of the photon that produced them. Here I have already
substantially simplified the detection process, but Michael's paragraph
does not correspond to physics.

* * * I [Mike] say... * * *

The "region" I am talking about is the field surrounding a nucleus or
other charged particle. That defines the frame in which the photon must
have an energy of at least 1.022 MeV. Also, the particle responsible
for the field that triggers the photon to undergo pair production will
participate to some extent in the energy-momentum conservation, and
there is no violation of translational momentum.

Jack makes the point that the original photon was not observable, then
goes on to explain that we "deduce the energy of the photon that
produced [the pair]." Indeed. Isn't deduction pretty much always the
case? With my detection system I am observing phenomena we have come to
describe as photoelectric events, and compton scattering events. I
deduce my detector is experiencing a flux of photons. If I have
performed an energy calibration and I observe that some of the photons
had energy greater than 1.022 MeV then I presume pair-production events
are also taking place. Supporting evidence for pair production can also
come from detection of 511-keV lines from annihilation of the positrons
from pair production in the shielding or other parts of the detection
system. In some detector systems (but not the ones I built) we may see
other evidence of the pair, such as tracks.

My original paragraph did not state these things, but I don't see that
my original paragraph is "unphysical" because I left out these details.

For about 6 years I designed, built, and tested gamma-ray detectors as
my primary job. The type of view I've described and the words I have
used were common at the time. I was certainly "describing physics" the
same way others were describing it. A survey of more recent literature
and a web search doesn't seem to indicate that my view and wording is
old fashioned. I don't know where Leigh and Jack are coming from.

Michael D. Edmiston, Ph.D.
Professor of Physics and Chemistry
Bluffton University
Bluffton, OH 45817
(419)-358-3270
edmiston@bluffton.edu


I enjoyed Mike's measured response with palpable pleasure.
The issue is the collision of an experimental description of a particle
collision, with a tenet of religious conviction - concerning the
relativity of kinetic energy.



Brian Whatcott Altus OK Eureka!