The pleasures of reading Feynman on Physics – II

If you’re tired of hearing and thinking about COVID-19 24/7 even when you don’t want to, do what I did when I was a neurology resident 50+ years ago making clever diagnoses and then standing helplessly by while patients died.  Back then I read topology and the intense concentration required to absorb and digest the terms and relationships, took me miles and miles away.  The husband of one of my interns was a mathematician, and she said he would dream about mathematics.

Presumably some of the readership are chemists with graduate degrees, meaning that part of their acculturation as such was a course in quantum mechanics.  Back in the day it was pretty much required of chemistry grad students — homage a Prof. Marty Gouterman who taught the course to us 3 years out from his PhD in 1961.  Definitely a great teacher.  Here he is now, a continent away — http://faculty.washington.edu/goutermn/.

So for those happy souls I strongly recommend volume III of The Feynman Lectures on Physics.  Equally strongly do I recommend getting the Millennium Edition which has been purged of the 1,100 or so errors found in the 3 volumes over the years.

“Traditionally, all courses in quantum mechanics have begun in the same way, retracing the path followed in the historical development of the subject.  One first learns a great deal about classical mechanics so that he will be able to understand how to solve the Schrodinger equation.  Then he spends a long time working out various solutions.  Only after a detailed study of this equation does he get to the advanced subject of the electron’s spin.”

The first half of volume III is about spin

Feynman doesn’t even get to the Hamiltonian until p. 88.  I’m almost half through volume III and there has been no sighting of the Schrodinger equation so far.  But what you will find are clear explanations of Bosons and Fermions and why they are different, how masers and lasers operate (they are two state spin systems), how one electron holds two protons together, and a great explanation of covalent bonding.  Then there is great stuff beyond the ken of most chemists (at least this one) such as the Yukawa explanation of the strong nuclear force, and why neutrons and protons are really the same.  If you’ve read about Bell’s theorem proving that ‘spooky action at a distance must exist’, you’ll see where the numbers come from quantum mechanically that are simply impossible on a classical basis.  Zeilinger’s book “The Dance of the Photons” goes into this using .75 (which Feynman shows is just cos(30)^2.

Although Feynman doesn’t make much of a point about it, the essentiality of ‘imaginary’ numbers (complex numbers) to the entire project of quantum mechanics impressed me.  Without them,  wave interference is impossible.

I’m far from sure a neophyte could actually learn QM from Feynman, but having mucked about using and being exposed to QM and its extensions for 60 years, Feynman’s development of the subject is simply a joy to read.

So get the 3 volumes and plunge in.  You’ll forget all about the pandemic (for a while anyway)

 

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Comments

  • Mark Thorson  On May 4, 2020 at 10:50 am

    . . . and why neutrons and proteins are really the same.

    I’m sure you meant “neutrons and protons”.

  • luysii  On May 4, 2020 at 12:11 pm

    You bet thanks

  • MadRocketSci  On May 4, 2020 at 10:06 pm

    I *wish* I had a clear explanation of fermions. They’re really very strange, when you think about them. All quasi-particle excitations of any medium which you can come up with are naturally bosons. (You can sort of make “fermions” by imposing a short-range repulsive force, but they’re really bosons.) Everything that lives in angular-space-as-we-know-it is a boson. We can paste a counterintuitive mathematical object over a fermion’s internal state, but no one can describe to you what the hell is going on “on the surface of an electron”, other than juggling operators to “prove” things via commutation relations. (Or various non-sequitur hand-tricks).

    With bosons, angular momentum has the exact same meaning as it has in classical physics – it’s the moment of the way something is moving around.

    With fermions, angular momentum (the interconversion nothwithstanding) is supposed to be something completely different. Spin-1/2 is strange. IMO, either geometry is not what we think it is, or fermions aren’t what we think they are.

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