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Interpretations of Quantum Mechanics

  1. Feb 1, 2004 #1
    I was wondering if an expert in quantum physics can clarify a couple of things for me.

    I read this in the Wikipedia:

    This seems to suggest that are at least five different interpretations of quantum mechanics, each one the result of eliminating one of the five properties listed above.

    My questions:

    (a) Do any of these interpretations conflict with what is physically observed in the quantum realm?

    (b) Do any of these interpretations hinder the practical application of the theory and its equations?
  2. jcsd
  3. Feb 2, 2004 #2
    (a) If they conflict with reality, then I don't think you can call it quantum mechanics. We'd have to give it to the mathematicians :p

    (b) I think that some do, although I only know Copenhagen. I don't think many-worlds would be appropriate for say a chemist.
  4. Feb 4, 2004 #3


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    The "practical" interpretation of quantum mechanics is the one given by the "Copenhagen school", and first laid out precisely by von Neumann. It comes down saying that at the end of the calculation, you obtain probabilities to observe phenomena. This is what can be compared to experiment, and this is what fits.
    All the other issues deal with meta-physical or mathematical issues on the foundational level, and have no influence what so ever on the relation of quantum mechanics with experiments, so they are scientifically all equivalent (and even to a certain point irrelevant).
    So why do people do this ?
    The main problem with quantum mechanics in the Copenhagen interpretation is that the mathematical rules for "system evolution" (deterministic and unitary) and "observation" (irreversible and stochastic) are different ; how can this be if an "observation" is a physical process just as another one ?

    In practice there is no problem, because any realistic observation contains so many degrees of freedom that it isn't tractable as a quantum system anyway (you very quickly couple to the whole universe !). So in practice you'll never be able anyway to contradict the stochastic evolution with a unitary evolution during an observation because the mathematical problem of the unitary evolution is overwhelming.
    But on a philosophical level, there is this issue. And that's where the different interpretations take different forms.

  5. Feb 7, 2004 #4
    Nicely summerised.Two thumbs up.
    So You find the fault in QM and our modeling to be an artifact of the process of the observation and quntifacation rather then a fundamental flaw in the general theories of QM ?
  6. Feb 8, 2004 #5


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    where's the fault ?

    if I really knew where the "fault" was, I would be famous :-)

    There are 3 different approaches. As I said before, the experimentalist one is:
    "shut up and calculate". Indeed, as I tried to explain, the Copenhagen interpretation is perfectly all right for comparing experimental observations to theory. And the point is, it works very well.

    The second approach is: "QM is fundamentally correct, and it is our desire to talk in classical concepts that gives rise to all these interpretational problems".
    Decoherence and the Everett interpretation go in that direction (it is my personal favorite).

    The third approach is: "QM is fundamentally wrong, something is missing". Some people try to add for instance small nonlinearities in the Schroedinger equation in order to force collapse. The difficulty here is that QM works so very well experimentally, so it is not easy to modify it and to still have agreement with all experimental results.

  7. Feb 9, 2004 #6
    Re: where's the fault ?

    Bohmian mechanics does not seem to fit in any of them.
    According to BM everything is nice with QM predictions,
    but particle trajectories are missing. It is a classical
    deterministic theory.
  8. Feb 10, 2004 #7


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    And Bohmian mechanics is not causal. But I agree that it is a very nice viewpoint. I plan to learn more about it at some point.

    The interpretational issues are not relevent for the vast majority of all practical applications, but there are some esoteric places it can come up. For example, the "freezing in of quantum fluctuations" during cosmological inflation is not really agreed upon in a rigorous way. Its hard to assign a measurer outside to the universe to collapse its wavefunction! Other cosmological applications as well as quantum gravity need to concern themselves with the measurement problem.
  9. Feb 10, 2004 #8


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    Yes, I know that Bohm doesn't really fit into the 3 interpretational schemes I proposed, but in fact, Bohm doesn't give an interpretation ! It is just a different way of formulating QM. The pilot wave in _configuration space_ is a very strange concept when you have many particles.
    To me, Bohm's theory is a bit to quantum mechanics what Lorentzian Ether theory is to special relativity.

  10. Feb 10, 2004 #9
    Re: bohm...

    I agree that Bohm is to QM what the Lorentz ether is to SR. (This connection is indeed very close, a relativistic version of BM requires a preferred frame.)

    Nonetheless, once it gives (in equilibrium) the same predictions as QM, why not name it an interpretation? At least if you describe various interpretations it seems necessary to mention BM too.

    If you find a pilot wave in configuration space strange or not is a personal opinion about the interpretation, but does not change its character as an interpretation. Functions on configuration space we have in classical mechanics too: The Lagrangian.
  11. Feb 27, 2004 #10
    I offer more than one interpretation of quantum mechanics at my website, below.
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