From: "JACK L. URETSKY (C) 1996; HEP DIV., ARGONNE NATIONAL LAB, ARGONNE, IL 60439" <JLU@hep.anl.gov>
Date: Tue, 21 Jan 1997 21:32:35 -0600 (CST)
Hi all-
In response to David Bowman, who writes:
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As far as the E&M non-linearity goes it isn't that the gamma photons interact
*directly* with the cosmic microwave background. Since the U(1) EM gauge
theory is abelian there is no direct interaction of the photon field with
itself. (The pure electromagnetic theory is purely linear and
noninteracting.) The interactions that Leigh is referring to are, I believe,
due to indirect interactions due to quantum effects where the photon field
interacts with virtual electron-positron pairs in the vacuum and these virtual
e-/e+ pairs interact back on the photon field. In essence, the interaction
between the gamma photons and the cosmic background photons occurs via the
mediation of the virtual electron/positron Dirac field. This process is
effectively the opposite of the usual interaction between electric charges
(such as electrons and positrons) interacting with each other via the
mediation of the photon field.
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Yeah! Let's not confuse old dame nature (what actually happens)
with our mathematical description of her (the way we calculate).
The fact is that there are photon-photon interactions, and they
are observable. It happens to be true (today) that we explain these
interactions by iterating a tree Lagrangian that obeys a U(1) symmetry.
So the fine structure constant is small and we can get away with that.
But, hey man, the paradigm is being reworked. Maybe someday folks will
work with a tree Lagrangian that has a photon-photon interaction built
in. We're only beginning to understand the mathematicas of all this stuff.
The wonder of physics is all the stuff that isn't clear yet.
The rituals that we know how to perform at present are for the engineers
(and lecturers).
Regards,
Jack
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It's 4 A.M. Nazruddin leaves the tavern and walks the town
aimlessly. A policeman stops him. "Why are you out wandering the streets
in the middle of the night?" "Sir", replies Nazruddin, "if I knew the
answer to that question, I would have been home hours ago!"
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-The Essential Rumi-