The pleasures of reading Feynman on Physics — III

The more I read volume III of the Feynman Lectures on Physics about Quantum Mechanics the better I like it.  Even having taken two courses in it 60 and 10 years ago, Feynman takes a completely different tack, plunging directly into what makes quantum mechanics different than anything else.

He starts by saying “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.”

Not to worry, he gets to the Hamiltonian on p. 85 and  the Schrodinger equation p. 224.   But he is blunt about it “We do not intend to have you think we have derived the Schrodinger equation but only wish to show you one way of thinking abut it.  When Schrodinger first wrote it down, he gave a kind of derivation based on some heuristic arguments and some brilliant intuitive guesses.  Some of the arguments he used were even false, but that does not matter. “

When he gives the law correct of physics for a particle moving freely in space with no forces, no disturbances (basically the Hamiltonian), he says “Where did we get that from”  Nowhere. It’s not possible to derive it from anything you know.  It came out of the mind of Schrodinger, invented in his struggle to find an understanding of the experimental observations of the real world.”  How can you not love a book written like this?

Among the gems are the way the conservation laws of physics arise in a very deep sense from symmetry (although he doesn’t mention Noether’s name).   He shows that atoms radiate photons because of entropy (p. 69).

Then there is his blazing honesty “when philosophical ideas associated with science are dragged into another field, they are usually completely distorted.”  

He spends a lot of time on the Stern Gerlach experiment and its various modifications and how they put you face to face with the bizarrities of quantum mechanics.

He doesn’t shy away from dealing with ‘spooky action at a distance’ although he calls it the Einstein Podolsky Rosen paradox.  He shows why if you accept the way quantum mechanics works, it isn’t a paradox at all (this takes a lot of convincing).

He ends up with “Do you think that it is not a paradox, but that it is still very peculiar?  On that we can all agree. It is what makes physics fascinating”

There are tons more but I hope this whets your appetite

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  • MadRocketSci  On May 14, 2020 at 5:38 am

    I’ve been rereading some of the lectures myself recently. I do have to say something about Feynman’s treatment of paramagnetism and diamagnetism.

    Feynman repeats Bohr’s argument about the impossibility of classical diamagnetism. The Bohr van-Leeuwen theorem supposedly rigorously proves this to be the case, which would be news to any plasma physicist. The problem is Bohr is *wrong*: he made a mistake in his derivation by using a test-particle accounting for the particle motion (particles move in response to only the applied field), and failed to account for the self-energy of the particles moving in response to the induced field. I think there’s a paper by Hanno Essen that points this out. (What’s worse: contemporary physicists *knew* he was wrong, but their objections were forgotten in the chaos and successes of early QM.)

    But more importantly, he’s wrong empirically, and for a structural/logical reason! Empirically: A plasma is diamagnetic, and lab plasmas are hot enough that they’re classical: The De Broglie wavelength of the charge carriers are submicroscopic, and everything quantum about them is averaged out. Any accounting of their diamagnetism has to work on a classical level. (And can be accounted for in ways standard in plasma physics.)

    Likewise, a box of marbles with magnetic gyroscopes in them is classical and paramagnetic in exactly the way the classical theory of paramagnetism accounts for.

    The structural reason is the following: The semiclassical theory of electromagnetism that everyone actually uses when deriving their quantum explanations copies the electrodynamic part from Maxwell’s equations. Your electrons might be doing quantum things in this picture, but Maxwell’s equations are right there: Most of the quantum results have, and must have, a dual classical analogue explanation. (There are things touched on, such as the flux quantization and Josephson junction behavior that are legitimately quantum effects, but it’s important to keep what is legitimately quantum and what is a disguised semi-classical effect straight.)

    I do love Feynman for his treatment of the classical electron (in Vol 2 I think?): He’s *honest* about where physics stands on that problem, and why it’s dynamically important. It’s also important, because an analogous problem pops up in quantum field theory every time you encounter a renormalization problem! Imposing cutoff lengthscales, dialing in free parameters like “bare charges” and “bare masses” – exactly the same sort of issue! So many other textbooks simply declare classical EM “inconsistent” (even though we know it can’t be!) and tell us to avert our eyes and sweep things under the quantum rug!

  • luysii  On May 14, 2020 at 4:59 pm

    MadRocketSci thanks for commenting, but it’s at a much higher level of physics that I ever reached. Perhaps one of the other followers will say something

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