Online reading assignment: Feynman diagrams, quantum electrodynamics (QED)

Physics 205B, spring semester 2013
Cuesta College, San Luis Obispo, CA

Students have a weekly online reading assignment (hosted by SurveyMonkey.com), where they answer questions based on reading their textbook, material covered in previous lectures, opinion questions, and/or asking (anonymous) questions or making (anonymous) comments. Full credit is given for completing the online reading assignment before next week's lecture, regardless if whether their answers are correct/incorrect. Selected results/questions/comments are addressed by the instructor at the start of the following lecture.

The following questions were asked on (re-)reading presentations on Feynman diagrams (Phillip "Flip" Tanedo, Cornell University/USLHC Collaboration) and quantum electrodynamics (QED) (Christopher "Bot" Skilbeck, cronodon.com).

Selected/edited responses are given below.

Describe something you found interesting from the assigned textbook reading or presentation preview, and explain why this was personally interesting for you.

"At first I didn't realize how simple it was to draw these diagrams, but now I am realizing that I can do this, so it is becoming more interesting to me!"

I find it interesting that someone has found a way to explain this concept as a game."

"I don't like this section."

"I think the idea of antimatter and virtual particles is interesting."

"It is interesting how such simple diagrams can represent such a seemingly complex subject."

"It was interesting to read about the virtual particles on the QED site because it provided a better description of how and why they come in and out of existence so quickly."

"It seems interesting but I'm quite confused on what its all about."

"I find it interesting that particle physics can be explained in such a simple way."

"I really like Feynman diagrams, they make sense to me."

"I think the general motion and interaction of subatomic particles is interesting because they end up behaving in pretty funky ways and it is cool to know how those behaviors happen."

"I thought it was interesting how we were told our entire science life that unlike charges attract but after reading the article from the hw question it was a lil confusing to understand the rest of the article."

Describe something you found confusing from the assigned textbook reading or presentation preview, and explain why this was personally confusing for you.

"I'm not sure I understand all of the different ways of drawing the electrons and photons. Is there a reason the have the angles they have? What about the drawing their directions?"

"Following the diagrams but I am getting it."

"The Feynman diagrams are confusing. the positrons moving backward in time is weird."

"I don't understand exactly what a pion is or the forces pions create."

"The virtual particles are confusing to me."

"The QED site was confusing because it broke down the simple components on neutrons and protons into deeper classifications that I just didn't understand because it's like too much information and just confused me."

"The idea of a system borrowing a lot of energy for a short period of time or a little for a long period of time...what?"

"Everything, especially the QED article."

"How can you tell if a particle is virtual or not?"

"I am still very confused on how these Feynman diagrams work. I have been trying to figure out what everything means but I just cant seem to fully grasp it."

Describe how the path of a virtual particle is drawn on a Feynman diagram.

"With a vertical photon line."

"The path of a virtual particle is drawn in between the 'incoming' and 'outgoing' side of the diagram. It is simply a particle that is so briefly in existence that it is not observed but allows the incoming particle to become the outgoing."

Describe the difference between first-order process and second-order process Feynman diagrams.

"The first order is an exchange of one photon. The second order is an exchange of two photons."

"The first-order events denote one interaction happening, while the second-order events denote two interactions happening simultaneously. Luckily, the higher order events do not take place very often in nature, so we can disregard them."

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.

"Why are we using the Feynman diagrams?" (We have to end the semester with something. That something is Feynman diagrams.)

"I'm still trying to wrap my head around how a diagram can depict a spontaneous formation of an electron and a positron from a photon." (Wrap your head around these bubble chamber tracks in liquid helium, where two high-energy photons (which leave no tracks) spontaneously form electron-positron pairs, which curve in opposite directions, due to the right-hand and left-hand rule forces exerted on them by an external magnetic field.)

"Virtual particles are very confusing...they exist, but not really?" (Virtual photons emanating from a charge don't exist, unless they encounter another charge (and can borrow energy/momentum, and thus exert a force on that other charge), or if the charge is suddenly moved, leaving the virtual photons behind (creating bremsstrahlung, "braking radiation," as in your dentist's x-ray machine.)

"Can you describe annihilation, is that just when two particles become one or is it specific to what is happening at their vertex interaction?" (Yes, and yes?)

"I know the cutoff for an A is 600 points, but what happens if someone is at 595? do you ever have any leeway for grading when it comes to final grades?" (That's what extra-credit points are for.)