Quantum field theory, Lecture 9

This winter semester (2016-2017) I am giving a course on quantum field theory. This course is intended for theorists with familiarity with advanced quantum mechanics and statistical physics. The main objective is introduce the building blocks of quantum electrodynamics.

Here in Lecture 9 I introduce and prove Wick's theorem and begin the discussion of Feynman diagrammes

[jacobvandijk]

I find it amazing that this can be useful in describing Nature.

[leander]

So what happens if we have three fields phi_1*phi_2*phi_3, there are no full contractions right? That means the amplitude is always zero - is there an intuitive reason why that is the case?

[andoverite]

When the lecturer speaks of a  "phi(z) to the 4th power" interaction theory do we interpret that to mean that a particle, say an electron, interacts with the phi field at one z site four separate times, or does it mean that the particle interacts with the phi field at four (possibly) different z field sites, a la pin ball machine? Or, maybe any combination of z field sites adding up to four interactions?

[markolinen]

@Tobias Osborne Did you do also the Path Integral formulation and the Renormalization Group?

[simonb.]

In my Quantum mechanics 2 lecture we spent around 4 lectures on the topic of second quantization and with its it looks very similar to QFT. But at the same time i feel like what we did there was just a basis change of the involved operators. But then again to calculate expectation values the approach was very similar: creation operators to left and a. operators to the right and you're left with some delta functions ...
I know this isn't a whole lot to go from, but do you know of any argument for me to distinguish the two ?

[samapanbhadury]

This marks the half way to the lecture series.

[九色火]

From 21:06, the form of the field operator under interaction picture was used. But the only place where I could find anything about interaction picture was mentioned is in the 8th lecture, and this conclusion was not obtained there. Was this suppose to be a easy piece of work or did I miss something? Because I found it not so easy for me to come to this conclusion myself. Anyone would like to help?

[PeeterJoot]

Kind of you not to practice psychological warfare on the students!

[SuperMaDBrothers]

This is by far the best source of this topic I have found, online or anywhere. 10/10. But some explanations/ending remarks you give are not helpful, and might instead be frustrating. For instance, 1:10:39 'that needs you to think about it for a second' serves no purpose other than to frustrate a student who didn't already understand this.