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Probabilistic breakdown of quantum mechanics?

  1. Sep 30, 2012 #1
    When does the probabilistic nature of QM breakdown?

    Is it just as a the system gets larger, it's less probabilistic?
    Last edited: Oct 1, 2012
  2. jcsd
  3. Oct 1, 2012 #2


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  4. Oct 1, 2012 #3
    Do note that decoherence is "for all practical purposes" (FAPP). In principle, at least according to QM, no transition to a classical world (i.e. definite state) occurs. We still have a measurement problem on our hands.

    We need to wait and see the results of various experiments taking place, whereby putting a heavy mirror (I think 40kg) - definite a large mirror in size, so macroscopic, into a superposition of two distinct macroscopic states (in this case the observable position). There is a book reference I cannot find now, but there is notes on making clear that we are seeing distinct MACROSCOPIC states, conforming to certain (minimum) standards, as to ascert e.g. between a live and dead cat. I will try find it later, and if I can, will post it in this thread.

    Do note, even though we find macroscopic superposition occurs, we still need to rule out macrorealism - even using the Leggett-Garg inequality, WITHOUT the use of coarse-grain measurements.
  5. Oct 1, 2012 #4


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    As I said:
    Suppose there's a spin 1/2 particle with with +1/2 and -1/2 states. Decoherence explains the emergence of a classical probability distribution 50% - 50% for +1/2 - -1/2, but it does not explain why a one-particle state collapses either to +1/2 or to -1/2
  6. Oct 1, 2012 #5
    right and more succintly;
    how does a diagonal density matrix turn into a definite outcome.

    decoherence does not answer that.

    Last edited: Oct 1, 2012
  7. Oct 2, 2012 #6
    I must apology. Generally I am really good, but unfortunately I haven't time yet to look for the references to macrosopic distinct states. Only logging on here briefly, then onto other things.
  8. Oct 2, 2012 #7
    a good parameters at:

    Phys. Rev. Lett. 106, 220401 (2011)
    Quantification of Macroscopic Quantum Superpositions
    http://prl..org/abstract/PRL/v106/i22/e220401 [Broken]

    ...we propose a novel measure to quantify macroscopic quantum superpositions. Our measure simultaneously quantifies two different kinds of essential information for a given quantum state in a harmonious manner: the degree of quantum coherence and the effective size of the physical system that involves the superposition. It enjoys remarkably good analytical and algebraic properties. It turns out to be the most general and inclusive measure ever proposed that it can be applied to any types of multipartite states and mixed states represented in phase space...
    Last edited by a moderator: May 6, 2017
  9. Oct 3, 2012 #8
    Thanks for the links, I've never heard of quantum decoherence (or if I have, I forgot).

    I will study up on the links, and post any questions in this thread!
  10. Oct 3, 2012 #9
    Yes thanks from me as well.

    And I think this is the above (original link gave me problems):
    Quantification of Macroscopic Quantum Superpositions within Phase Space
    Chang-Woo Lee, Hyunseok Jeong
    Last edited by a moderator: May 6, 2017
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