# Feynmann diagrams interpretation and diagramm symmetry order

1. Dec 24, 2007

### kvl

I'm trying to understand Feynmann diagrams interpretation in terms of virtual particles. What I still can't understand is how this interpretation is compatible with the fact that one should divide each diagram by its symmetry order.

As far as I understand, diagram symmetry order means how many different physical processes are represented by this diagram. But than one should multiply the probability by this number, not divide !

Probably someone can clarify this a bit...

2. Dec 24, 2007

### blechman

Yeah, symmetry factors are a b&tch! What you have to remember is that they come from an OVERCOUNTING, so you have to divide. You can see this by explicitly looking at Wick's theorem - a single feynman diagram can represent MANY terms in the Wick expansion, and we only want one of these terms. That's why we divide.

I've been doing loop calculations for years, and just last week, my collaborator corrected me on my symmetry factors! I doubt I'll ever get them right first-time!

3. Dec 24, 2007

### kvl

Yes, I understand the Wick's theorem and why we should divide. But what I don't understand is how can it be compatible with interpretation of diagrams in terms of virtual particles creation/anihilation !

4. Dec 24, 2007

### masudr

Feynman diagrams are literally terms in an expansion.

It is misleading to think these are diagrams of actually what is happening.

5. Dec 24, 2007

### blechman

I agree. The point is that a feynman diagram overcounts the number of ways lines can attach to vertices. And this is the symmetry factor.

6. Dec 24, 2007

### QuantumDevil

Wick theorem used for "external connection" (as shown in Peskin &Schroeder books) shows that your interpretation is correct.