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Insights Why Is Quantum Mechanics So Difficult? - Comments

  1. Aug 12, 2014 #1
    Last edited: Aug 24, 2016
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  3. Aug 12, 2014 #2

    bhobba

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    Agree entirely.

    The mathematical formalism is required to understand the concepts.

    That is exactly the process taken in my favourite QM book, Ballentine, and is much more rational than the semi historical approach usually taken.

    The only problem with Ballentine is it is at graduate level.

    I have always thought a book like Ballentine, but accessible to undergraduate students, would be the ideal introduction.

    In particular it would have a 'watered' down version of the very important chapter 3 that explains the dynamics of QM from symmetry. Its a long hard slog even for math graduates like me - definitely not for undergraduates. But the key results and theorems can be stated, and their importance explained, without the proofs. I think its very important for beginning students to understand the correct foundation of Schroedinger's equation etc from the start - if the not the mathematical detail.

    Thanks
    Bill
     
    Last edited: Aug 12, 2014
  4. Aug 12, 2014 #3
    I don't think there's one best way. Some learn better using the approach advocated here. Others learn better the other way around.
     
  5. Aug 12, 2014 #4

    WannabeNewton

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    Honestly I think at the undergraduate level QM is the easiest physics class one has to take. It is just a cookbook on calculations. Every book is uninspired and my class was certainly uninspired. It is an incredibly boring subject at this level. So I don't think difficulty is the issue. It is simply the lack of physical concepts and a healthy dose of philosophy that is avoided when teaching QM at the undergraduate level. Indeed one of the professors I know basically called Griffiths' book a cookbook in differential equations. A good book can go a long way. For me the saving grace was Landau and Lifshitz. It is the sole reason I started liking QM. Seriously the way undergrad QM is taught really isn't fun for the students. Boredom from a lack of intellectusl stimulation really isn't how a physics class should be.
     
  6. Aug 12, 2014 #5

    atyy

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    Hmmm, I still can't derive the Stefan-Boltzmann whatever - chills down my spine. How is that easy?
     
  7. Aug 12, 2014 #6

    WannabeNewton

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    What o_O
     
  8. Aug 12, 2014 #7

    atyy

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    Is it easy?
     
  9. Aug 12, 2014 #8

    WannabeNewton

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    Im actually not sure what youre referring to. Are you talking about the Stefan Boltzmann law of radiation? Im not sure what that has to do with undergrad QM apart from historical impetus but there is a particularly lucid derivation in section 9.13 of Reif if youre interested. It's more of a statistical mechanics derivation. Which is good because statistical mechanics, both classical and quantum, is actually extremely interesting at the undergrad level.
     
  10. Aug 12, 2014 #9

    bhobba

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    That I agree with.

    I gave it away for health reasons no need to go into here. But I did enrol in a Masters in Applied Math at my old alma mater that included a good dose of QM. When mapping out the course structure with my adviser he said forget the intro QM course - since you have taken courses on advanced linear algebra, Hilbert spaces, partial differential equations etc it's completely redundant. Other students with a similar background to mine were totally bored. He suggested I start on the advanced course right away.

    Really I think it points to doing a math of QM course before the actual QM course where you study the Dirac notation etc - basically the first and a bit of the second chapter of Ballentine. You can then get stuck into the actual QM.

    And yes - I like Landau and Lifshitz too. Their Mechanics book was a revelation; QM, while good and better than most, wasn't quite as impressive to me as Ballintine. But like all books in that series it's, how to put it, terse, and the problems are, again how to put it, challenging, but to compensate actually relevant.

    Thanks
    Bill
     
    Last edited: Aug 13, 2014
  11. Aug 12, 2014 #10

    atyy

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    Actually, I only dimly remember what it is, although it was very exciting. It sounds right that it should be in a stat mech book, because the whole point IIRC was that classical thermodynamics was able to derive all sorts of completely correct things about blackbody radiation, yet classical stat mech could not. Then miraculously when one switched to quantum stat mech everything fell in place with classical thermo. I remember the narrative, but none of the calculations except Planck's. The text we used was Gasiorowicz, and I think his chapter 1 is all about this.

    Apart from the Stefan-Boltzmann law, the other amazing derivation was Wien's displacement law. IIRC, these were all from classical thermodynamics, with no quantum mechanics, yet they are correct!
     
    Last edited: Aug 12, 2014
  12. Aug 13, 2014 #11

    vanhees71

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    Well, the historic approach is bad. You are taught "old quantum mechanics" a la Einstein and Bohr only to be adviced to forget all this right away when doing "new quantum mechanics". I've never heard that it is a good didactical approach to teach something you want the students to forget. They always forget inevitably most important things you try to teach them anyway, but in a kind of Murphy's Law they remember all the wrong things being taught in the introductory QM lecture.

    You see it in this forum: Most people remember the utmost wrong picture about photons, and it is very difficult to make them forget these ideas, because they are apparently simple. The only trouble is they are also very wrong. As Einstein said, you should explain things as simple as possible but not simpler.

    Concerning philosophy, I think the healthy dose is 0! Nobody tends to introduce some philosophy in the introductory mechanics or electrodynamics lecture. Why should one need to do so in introdutory QM?

    If you want to rise interpretational problems at all, you shouldn't do this in QM 1 or at least not too early. First you should understand the pure physics, and that's done with the minimal statistical interpretation. If you like Landau/Lifshits (all volumes are among the most excellent textbooks ever written, but they are for sure not for undergrads; this holds also true for the also very excellent Feynman lectures which are clearly not a freshmen course but benefit advanced students a lot), I don't understand why you like to introduce philosophy into a QM lecture. This book is totally void of it, and that's partially what it makes so good ;-)).
     
  13. Aug 13, 2014 #12

    bhobba

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    Abso-friggen-lutely.

    And to make matters worse they do not go back and show exactly how the correct theory accounts for the historical stuff and students are left with a sort of hodge podge, not knowing whats been replaced and what changed or the why of things like the double slit experiment.

    Thanks
    Bill
     
  14. Aug 13, 2014 #13
    May be QM is primarily predictive. Quantum mechanics construed as a predictive structure. After we try to interpret it with épistemic or ontological human sense.

    For example "The debate on the interpretation of quantum mechanics has been dominated by a lasting controversy between realists and empiricists" : http://michel.bitbol.pagesperso-orange.fr/transcendental.html

    Patrick
     
  15. Aug 13, 2014 #14

    bhobba

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    I think philosophers worry more about that sort of thing more than physicists or mathematicians.

    An axiomatic development similar to what Ballentine does is all that's really required, with perhaps a bit of interpretational stuff thrown in just to keep the key idea behind the principles clear.

    And I really do mean IDEA - not ideas - see post 137:
    https://www.physicsforums.com/showthread.php?t=763139&page=8

    It always amazes me exactly the minimal assumptions that goes into QM and what needs 'interpreting'.

    Thanks
    Bill
     
  16. Aug 13, 2014 #15

    stevendaryl

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    When discussing the best approach to teaching something like quantum mechanics, I think you really have to consider the purpose in teaching it. Some of the people studying quantum mechanics are going to go on to become physics researchers, but my guess is that that is a tiny, tiny fraction. A small fraction of those who learn QM go on to get undergrad physics degrees, and a small fraction of them go on to get postgraduate physics degrees, and a small fraction of them go on to get jobs as physics researchers. So for the majority (I'm pretty sure it's a majority) who are not going to become physics researchers, what do we want them to know about quantum mechanics?

    I'm not asking these as rhetorical questions, I really don't know. But I think that if we want people to be able to solve problems in QM, there might be a best way to teach it to get them up to speed in solving problems. If we want them to understand the mathematical foundations, there might be a different way to teach it. If we want them to be able to apply QM to problems arising in other fields--say chemistry or biology or electronics--there might be another best way to teach it.

    So when people say things like "You shouldn't bring up X, because that will just confuse the student" or "The historical approach, with all of its false starts and blunders, is just not relevant to today's students", they need to get clear what, exactly, they want the student to get out of their course in QM. And I think that the answer to that question isn't always the same for all students.
     
  17. Aug 13, 2014 #16

    stevendaryl

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    I assume you mean the idea expressed by the sentence:
    I would say that that's a single sentence, but I'm not sure I would call it a single idea. There are many other ideas involved in understanding why we would want basis-independence, why we are looking for probabilities in the first place, why we want the outcome probabilities to be determined by [itex]E_i[/itex] (as opposed to depending on both the system being measured and the device doing the measurement), what is an "observation" or "measurement", why should it have a discrete set of possible results, etc.
     
  18. Aug 13, 2014 #17
    probably not theory, but the people :

    Erwin Schrodinger : Mind and matter - What Is Life? - My View of the World - ...
    Werner Heisenberg : Physics and Philosophy: The Revolution in Modern Science - Mind and Matter - The physicist's conception of nature - ...
    ...


    Patrick
     
  19. Aug 13, 2014 #18

    bhobba

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    Know both those books - but they are old mate.

    These days the following is much better at that sort of level:
    https://www.amazon.com/Understanding-Quantum-Mechanics-Roland-Omnès/dp/0691004358

    But of relevance to this thread you will get a lot more out of that book if you know some of the real deal detail.

    Thanks
    Bill
     
    Last edited by a moderator: May 6, 2017
  20. Aug 13, 2014 #19
    To understand the quantum theory in terms of mathematical language, we have in "France" some good free lecture like this one from "Ecole polytechnique" : http://www.phys.ens.fr/~dalibard/Notes_de_cours/X_MQ_2003.pdf

    on the other side there is not a unique look on its interpretation.

    Patrick
     
  21. Aug 13, 2014 #20

    atyy

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    In fact Landau and Lifshitz introduce philosophy early and correctly in their QM book, which is what makes it so wonderful.
     
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