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Let’s draw Feynman diagrams! (uslhc.us)
43 points by geekpressrepost on Feb 25, 2010 | hide | past | favorite | 11 comments



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.


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.


Can't you let us mere mortals catch our glimpse of the magical forces the demigods of physics deal with in peace!


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.


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.


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.




[deleted]


In my day we used FeynDiag (http://feyndiagram.com) if we needed a printable diagram. I don't know what the cool kids use now.


Aren't Feynman diagrams already a DSL for a certain sub domain of quantum field theory?


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




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