# Reading through Feynman lectures on physics, trying to make sense of it, help me

Richard Feynman is a great explainer, and I got through the first 2 volumes without problems. The 3rd one, treating the subject of quantum mechanics, gets very complex and he doesn't always provide enough explanations to my liking.

My questions may be very stupid, but keep in mind that I'm trying to teach myself, and I don't have a teacher to answer questions for me. So I'd like to know if what I think I understand is good or not. So, this is like a "true or false" multiple questions post. Please tell me if I am wrong about anything, and what is the proper answer. I searched quite a bit on this site, and have lurked on it for quite a while before joining today, and I finally decided to ask a question, because I couldn't find an answer to a lot of things.

[I should mention that I have posted on vBulletin-based forums before, but stackexchange is quite different. If my question needs re-wording or if it doesn't conform to the required format, I'm sorry: I checked all the FAQs and it didn't seem to forbid asking multiple questions in the same post.]

So am I correct to say:
- the H11 and H22 Hamiltonian matrices represent the time evolution of a state from state 1 to state 1, and from state 2 to state 2. The H12 and H21 represent the time evolution to go from state 1 to 2, and 2 to 1, respectively.
- The ammonia molecule represents a 2 state system. An electron spin-half as a free particle, without an EM field, would correspond to the 2-state system of an ammonia molecule without an EM field.
- The probability amplitude of the ammonia molecule varies sinusoidally with time, between its 2 states, without an EM field. In a varying EM field, it can experience resonance at a given frequency that corresponds to the energy difference between the two states, divided by Planck's constant.

Does that mean the probability amplitude electron's z-spin (which he treats a few chapters later, but in an EM field, with Pauli spin matrices) also varies sinusoidally with time, even without any EM field?

Since the probability amplitude goes to 0 and back to 1 for both state 1 and 2, I guess that means the state itself must constantly be changing back and forth? If this is correct, what is the frequency at which the electron would oscillate between spin up and down?

• You point out perhaps one of the greatest values of the PSE, the ability to serve as a community teacher for the self learner. I also self studied the Feynmans lectures, agree he had a remarkable ability to explain things, but the 3rd volume is difficult. Quantum physics in general difficult, so you need to compliment your study with other books, and questions here. Bravo! Oct 23, 2016 at 15:24
• Thank you for the warm welcome. I also have Dirac's "Principle of Quantum Mechanics" and Griffiths' "Introduction to Quantum Mechanics". I've also found some course notes on the Standard Model, chromodynamics, etc., but I definitely felt like I needed to clarify this before reading more. I'll need to re-read a few chapters of Feynman's book now that I understand. Oct 23, 2016 at 15:36
• You are welcome. Another good book, quantum physics by Stephen Gasiorowicz Oct 23, 2016 at 17:17