Hi and welcome to the third episode in my series on quantum field theory! In this episode, we will take a closer look at the propagator in 4 dimensional spacetime, and after that we will study three examples of source functions one can come up with to see how our quantum field responds to them. Because some results in this video require a larger amount of calculus to derive, I decided to add a document in which I derived the results for you. If you are interested you can have a look here: The math is not necessary to understand the conclusions drawn in this video, so if you are not that interested in the mathematical details, the video is still worth a watch! Book: Music: Made with Manim: