Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Quantum-classical correspondence?

  1. Apr 23, 2009 #1
    I don't know why scientists investigate the quantum-classical correspondence?

    I think it can not help us solve any problem in quantum mechanics.

    Any comment is welcome!

    Thank you!
    Last edited: Apr 23, 2009
  2. jcsd
  3. Apr 23, 2009 #2
    We dont have what is called bird's view: we see only a shadow on the wall, trying to guess a shape of 3D object.

    That is why Quantum Decoherence is learned: to be sure that our vision of the quantum world (obtained purely from the observation of the classical things: indicators, cameras, voltmeters etc) is valid.

    And yes, it is not supposed to give anything new regarding the High Energy, for example.
  4. Apr 23, 2009 #3
    Thank you for your reply!
    But I am still confused!
  5. Apr 23, 2009 #4


    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor

    How would you know it doesn't?

    The fact that the classical world and the quantum world are SO different, BEGS the question on the nature of the transition between those two. And considering that our devices are getting smaller and smaller, at some point, we WILL bump against this mesoscopic scale where both of them will either mix, or fluctuate, between each other. How is such information not useful?

  6. Apr 23, 2009 #5


    User Avatar

    > I think it can not help us solve any problem in quantum mechanics.

    What "problems" are you thinking about?

    I think some of the problems of understanding this correspondence become even more acute when one tries to ponder how a coherent framework that incorporates both GR and QM might look like, because one seems to run into self-observations and regress, and then it becomes difficuly ot insist on a static line between classical and quantum. Any boundary seems to be ambigous or subjective.

    There seems to be different roads here. Either you picture some external birds view, representing some kind of quantum reality, or "gods view", which you can picture as an abstraction of the laws of physics, and from this picture the inside views are "computed/prediced".

    Or you think that makes no sense, and instead turn this around, and ask how an apparently stable objective reality, can emerge as seen from an inside observer.

    So either the gods view, is use to derive the frogs view. Or the frogs view somehow must explain the "coincidences" that all frogs tend to be synchronized to consistently describe different projections of some "effectively objective abstraction quantum reality".

    I subscrive to the latter.

    One version of the former, quite common, is to have a realist view of these symmetries. And thus, these symmetries neatly escapes scientific questioning. They are considered objective ontologies of reality, just for us to discover, thus their status as observable or not is a non-issue. In this view, it's like the scientists take the place of God.

    The latter idea, instead must explain how the symmetries, as subjective after all, in the classical limit, are likely to produce objectivitiy. I think of this as evolving and emergent symmetries. In this view, the scientist proving space is no different than an atom probing it's environment. We are about as lost and ignorant. The problem for this approach, is to argue how come, given this denial of objective universal law, very stable laws, are nevertheless observed.

    Somehow both approaches address a similar problem, but from different starting points.

  7. Apr 23, 2009 #6
    The more important problematics in modern physics are more or less indirectly related with QM.

    A correct interpretation of QM in terms of classical dynamics could allow, for instance, the solution of quantize gravity. More practically, it could be useful in the the renormalization procedure of Feynman diagrams avoiding cancellation between infinities, virtual particles or stuff like that. (How far can we go calculating more and more loops order in diagrams?)

    An example of how powerful it could be is given by the AdS/QCD correspondence which allows to calculate non-perturbative QCD using classical configurations of the fields in an AdS metric.
  8. Apr 23, 2009 #7
    Looking at the same problem with the different views.

    But I think the quantum-classical correspondence can be valid only in a small and special scale.
  9. Apr 23, 2009 #8
    So, the quantum-classical correspondence that you have in mind is a partial correspondence. Now think to a complete correspondence, think to have a classical theory that originate QM.
  10. Apr 23, 2009 #9
    Yes, we can think classical theory orginates QM.
    In my opinion, this subjects to quantum to cassical transition.

    Maybe I am confused between "quantum-classical correspondence" and "quantum to cassical transition".

    Any comments are welcome!
  11. Apr 23, 2009 #10


    User Avatar

    It seems you have specific proposals in mind, that are at variation with some "standard views" of the correspondence such as quantum expectation values obeying classical laws. If that's your point, I agree, but that is a disucssion of its ow.

    I interpreted your original post as you objecting not to specific views of the correspondence, but on the correspondence beeing researched at all.

  12. Apr 23, 2009 #11
    Should there be a quantum-classical correspondence in the first place? I know quantum physics keeps trying to find interpretations which create a causal and logical transition from the microscopic to the macroscopic, but that strategy appears to have failed.

    Once again the heart of this problem lay in the measurement problem: its like all roads lead to Rome. Had any of the interpretations actually provided a genuine foundational solution we would not still have a classical-quantum paradox..as we do.

    Interesting how the majority of threads on this quantum physics forum directly or indirectly allude to measurement problem phenomena.
  13. Apr 23, 2009 #12


    User Avatar

    I agree there.

  14. Apr 23, 2009 #13

    The measurement problem had been solved about 10 years ago.
  15. Apr 23, 2009 #14


    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor

    Don't be so sure. One could also arrive at the classical scenario not by invoking decoherence, but simply due to coarse-grained measurement[1].


    [1] J. Kofler and C. Brukner, Phys. Rev. Lett. v.99, p.180403 (2007).
  16. Apr 23, 2009 #15
    Is it available for free? I did not find any free sources. And I dont want to pay for reading what I am not agree with :)
    Or could you explain the ganaral idea?
  17. Apr 23, 2009 #16
    Perhaps you did not read the whole of the wiki article you referred to:

    "However, decoherence by itself may not give a complete solution of the measurement problem, since all components of the wave function still exist in a global superposition, which is explicitly acknowledged in the many-worlds interpretation. All decoherence explains, in this view, is why these coherences are no longer available for inspection by local observers. To present a solution to the measurement problem in most interpretations of quantum mechanics, decoherence must be supplied with some nontrivial interpretational considerations (as for example Wojciech Zurek tends to do in his Existential interpretation). However, according to Everett and DeWitt the many-worlds interpretation can be derived from the formalism alone, in which case no extra interpretational layer is required."

    So even Zurek, one of the founders of decoherence interpretation admits its not a foundational solution as such.
  18. Apr 23, 2009 #17
    Yes, I read it many times and I, as Many Worlds fan, really like the part you quoted.
    So step 1 is to accept the Decoherence
    After that the only logical possibility is to accept Many Worlds.
  19. Apr 23, 2009 #18
  20. Apr 23, 2009 #19
    I accept both decoherence and many-worlds as interpretations; neither solves the "measurement problem".
  21. Apr 23, 2009 #20
    Hm, could you explain why?
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook