Re: superposition

[David Bowman <dbowman@tiger.gtc.georgetown.ky.us>]

James McLean writes:

> Do tell!  What are the other facets of this observation.  I.e. how do we
> know that there should be high energy gamma rays coming from great
> distances, and how is it known that their absence is due to E&M
> non-linearity?

Pardon my interposing into the superposition thread.  I'm not sure what cosmic
gamma ray sources there may be (other than the occasional supernova), but if
there are any I would expect a relatively uniform concentration of them
throughout the universe at large length scales due to the homogeneity of the
universe on such scales (unless one is looking at such great distances that
the time of emission corresponded to a time earlier in the history in the
universe than was available for such gamma sources to develop since the big
bang).

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.

David Bowman
dbowman@gtc.georgetown.ky.us