Re: Feynman diagrams and space-time

Thus spake Hontas Farmer <hfarme2@xxxxxxx>
Happy holidays to all.

I have just finished a formal course based on Peskin and Schroeder's book on
Quantum Field theory. So far the Feynman diagrams have had no explicit
reference to space or time in them. Earlier in my career the Feynman
diagrams I was shown were drawn a certain way with a set of space-time axes
at least implied. The particles proceeding forward through time
anti-particles backward in time, Massless bosons propagating at 45 degree
angles to the axes and such. In this way not simply specifying the
cross-section of a interaction but it's location in space-time.

What is the distinction I should see between a Feynman diagram drawn with
space-time axes implied and without them?

The internal ordering of points in a Feynman diagram is irrelevant. Only
the topology matters. There is certainly no reason to draw lines at
45deg angles, unless it is one of graphical clarity. However, when used
to describe a particular physical interaction, there is a defined in
state and a defined out state, and the external lines will be assigned
either to the in or the out state. Clearly there is a time ordering in
that.



Regards

--
Charles Francis
substitute charles for NotI to email

.