
Let’s draw Feynman diagrams! - geekpressrepost
http://blogs.uslhc.us/?p=3712
======
ewjordan
I'm not really sure why anyone would want to bother drawing Feynman diagrams
if they didn't need to calculate the amplitudes associated with them. There
are very few physical effects that can be explained through Feynman diagrams
without understanding a lot more than this article offers about the math that
these diagrams involve.

Typically Feynman diagrams are offered as a way to simplify the accounting
involved in doing quantum field theory calculations. To treat them as a view
of what is "really happening" is a bit misleading, especially to an audience
that is not intimately familiar with the oddities of quantum mechanics. I'm
not saying everyone needs to understand path integrals to "get" the basics of
QFT, but it would be a real mistake to think that you could understand
anything nontrivial about the theory by looking at mere Feynman diagrams -
QFT's magic is in how you calculate the amplitudes from diagrams, not in the
construction of the diagrams themselves.

And at the _very_ least, some justification for ignoring the higher level
Feynman diagrams needs to be offered, especially since in certain quantum
field theories we can't pull the same kind of crap that we can get away with
in QED, and it doesn't make any sense to ignore the more complicated diagrams.

~~~
randallsquared
I can't tell if you're joking, but in case you aren't:

 _To treat them as a view of what is "really happening" is a bit misleading,
especially to an audience that is not intimately familiar with the oddities of
quantum mechanics._

There is no simplified layman's view of any complex subject which is not at
least a bit misleading.

 _And at the very least, some justification for ignoring the higher level
Feynman diagrams needs to be offered_

Audience. The post starts out by mentioning that he's talking only about stuff
that could be done by school children, or the whole family. It's a bit tongue-
in-cheek, but I didn't get the impression that he was completely unserious
about that.

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micheles
It worries me when people attach too much Physics to Feynman diagrams. Feynman
diagrams arise when you try to compute perturbatively the correlation
functions in a field theory. They appears both in quantum field theory and in
statistical field theory and in general in any theory which can be expressed
via a functional integral. I see them more as a property of the perturbative
expansion than a description of the "true" Physics going on.

~~~
pfedor
What's "true" physics and what's just a computational technique is a
distinction which seems rather philosophical, and I don't mean it as a
positive thing.

In any case, there's more to Feynman diagrams than your comment implies. For
example, think of the optical theorem. You can take a diagram with a loop in
the middle and cut it in half, and get two diagrams of a lower order, say
originally you had A + B -> C + D and you get A + B -> X and X -> C + D (where
X can be any number of particles), and the amplitude for the former is the
integral over all momenta of the amplitudes for the sequence of the latter.
What it means is you can think of a "true" physical sequence of events, A + B
-> X followed by X -> C + D, which contributes to the cross-section for A + B
-> C + D and the math behind it is the same as used for the A + B -> C + D
scattering with a not-"true" X (i.e., X with non-physical momenta) in the
middle of the Feynman diagram.

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mhartl
If you're interested in how these really work, a good place to start is
_Introduction to Elementary Particles_ by David Griffiths. For the full-
strength stuff, go with _An Introduction to Quantum Field Theory_ by Peskin
and Schroeder.

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Maro
The real deal:

[http://www.amazon.com/Quantum-Electrodynamics-Advanced-
Book-...](http://www.amazon.com/Quantum-Electrodynamics-Advanced-Book-
Classics/dp/0201360756/ref=sr_1_26?ie=UTF8&s=books&qid=1267097328&sr=8-26)

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rms
<http://lesswrong.com/lw/pk/feynman_paths/>

------
Jun8
"Make things as simple as possible, but not any simpler!"

