## 20140512

### Online reading assignment: Feynman diagrams (2)

Physics 205B, spring semester 2014
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 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 what you understand from the assigned textbook reading or presentation preview. Your description (2-3 sentences) should specifically demonstrate your level of understanding.
"Quantum field theory: forces between particles are mediated by virtual particles."

"I am fairly confident with the easier Feynman diagrams, and can determine what is going on with moderate ease."

"A proton exchanges photons with an electron which makes a Coulomb force of electrostatic attraction. Virtual particles temporarily violate energy conservation."

"Quantum physics is awesome and crazy! Electrons can teleport, photons can spring into existence from nothing, and nothing is ever where it 'should' be on a quantum level."

"I like how they really helped me to remember that there can only be one photon particle line per junction where electrons are. And that if the arrow on a straight line is going against time, then it is a positron."

"I understand valid and invalid Feynman diagrams and the reasoning for their validity."

"I understand that the direction of time is from left to right, and that electrons have an arrow pointing to the right and positrons have an arrow pointing to the left. I also understand that first order processes are more common, and that second-order and other higher order processes occur, but are not as common."

Describe what you found confusing from the assigned textbook reading or presentation preview. Your description (2-3 sentences) should specifically identify the concept(s) that you do not understand.
"All of it."

"I'm confused about what the virtual particles do."

"The beginning paragraphs of the text is pretty thick. I guess I just don't know which parts to pay attention to and learn. A lot of vocabulary is thrown at us in this material. Also, still a bit confused about annihilation vs absorption. Finally, tell me more about pions."

"I don't understand most of the mathematical explanations. I also don't understand what the different fundamental forces are and what the "spin-1 gauge boson" is. A lot of the theoretical explanations were beyond me."

Describe how the path of a virtual particle is drawn on a Feynman diagram.
"The path of a virtual particle is drawn as a squiggly line on a Feynman diagram."

"A virtual photon is the wavy line. Forces between particles are mediated by the exchange of virtual particles (virtual quanta). (copied from text). Virtual particles in general are connected to straight lines that are either protons or neutrons or in some cases positrons or electrons."

"Virtual particles may be mesons or vector bosons"

Describe the difference between first-order process and second-order process Feynman diagrams.
"I don't know."

"More photons being exchanged."

"The difference between the numbers of ordered process is the number of photons or virtual particles exchanged between two electrons."

Compare first-order pion (π) exchange processes, and describe the first-order process [n-p interaction by exchange of π0 from proton-to-neutron].
"I don't understand this."

"Okay, obviously I didn't understand much."

"Is it just flipped over [compared to n-p interaction by exchange of π0 from neutron-to-proton]? But how does the story change?"

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"Is there a limit to the number of order processes a Feynman diagram can reach?" (No, but each additional order results in a decreasing probability of likelihood, so much like including terms in an infinite series calculation, many Feynman diagrams don't go past a certain number of orders for a sufficient result.)

"How are virtual particles are produced from nothing? How is it possible to make something out of nothing." (They temporarily violate energy conservation, as long as they eventually go back into a(nother) real particle to "pay back" what they borrowed. More energetic virtual particles can exist briefly and travel only short distances, while less energetic virtual particles can exist longer and travel farther distances. This is why forces between real particles (carried out by the exchange of virtual particles) get weaker over increasing distances. This is the quantum physics answer to the "action-at-a-distance" problem--the exchange of virtual particles!)

"I could use some help on the more complicated Feynman diagrams." (I will do my best!)

"So is this stuff quantum physics?" (Yes--at least one very elegant representation of quantum physics processes.)