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Layman's question on Quantum decoherence

  1. Sep 4, 2011 #1
    Why doesn't it occur constantly and only when 'interacting" with another larger scale phase space? Why does an electron for example require a large scale human readable detector in the way to interact that way with the environment when the environment is there anyway? Air, Dust, etc.
    --
    Also, how can decoherence be 'delayed' as in the more complex eraser experiments?
     
    Last edited: Sep 4, 2011
  2. jcsd
  3. Sep 4, 2011 #2

    xts

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    Practically, the electron decoherences, as it encounters a grain of dust. Or, as it ionises some air molecule, while travelling through Wilson's chamber.

    Of course, the decoherence may be delayed. As much as you want, as long the information do not reach your, solipsistic mind. Cat did not decorenced the experiment. It was Schrödinger, who had to spot dead cat, to conclude decoherence.
     
  4. Sep 4, 2011 #3
    I don't think that's correct. The world doesn't depend on humans' existence.
     
  5. Sep 4, 2011 #4

    xts

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    Decoherence or collapse is not something really happening in the world. They are just measures of our knowledge about the world state. From cat's perspective decohernece/collapse occurs when the machine emits either poison or whiskas. From Schrödinger's perspective it occurs when he opens a cage.

    You can't say that some wavefunction is collapsed or not in the same meaning, as you may say, that glass of water is liquid or frozen. That's not a property which may be checked if it happened or not. It is just a mental construct, allowing us to simplify further calculations.
     
    Last edited: Sep 4, 2011
  6. Sep 4, 2011 #5
    This is according to many sources outright incorrect.
     
    Last edited: Sep 4, 2011
  7. Sep 4, 2011 #6

    xts

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    OK. So let me repeat:

    'practically' (and that is the most common understanding) decoherence occurs, when something thermodinamically irreversible happens (drop in Wilson's chamber forms around ionised molecule).

    'philosophically' - you may defer it as far as you like. Calculations (if you are able to perform them) in the views:
    - decoherenced state + classical apparatus;
    - entangled state of your particle and apparatus.
    lead to the same result.
    If your detector (apparatus) is very simple (e.g. it is single electron scattered by the photon you measure) - the second approach is also feasible computationally.
     
  8. Sep 4, 2011 #7

    xts

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    Oh! You should tell it to Niels Bohr!
    Nature of the world probably does not require an observer. Nature of QM, which is just a description of the world behaviour, also doesn't. But any particular calculations require some edge assumptions, which are based on experimentalist's knowledge about the world. Collapse is just a trick in computations, elliminating all those previously possible branches of further evolutions, which may be excluded using the knowledge we just possesed.
     
  9. Sep 4, 2011 #8
    Decoherence is different from collapse. Decoherence rejects collapse and explains it as something that creates the appearance of collapse.
     
  10. Sep 4, 2011 #9

    xts

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    Uuuch? May you elaborate on it a little bit, please!

    If you mean 'decoherence' as in Żurek's "DECOHERENCE, EINSELECTION, AND THE QUANTUM ORIGINS OF THE CLASSICAL" http://arxiv.org/abs/quant-ph/0105127v3
    that is just what I said: decoherence occurs when our state goes entangled with so many other elements of the world, that:
    1. it becomes unfeasible to track those entanglements in QM calculations;
    2. they are thermodynamically irreversible .
    3. Thus: the approach of 'collapse' is an only feasible (and statistically valid) way to continue with our model.

    The electron (leaving visible track) in a Wilson's chamber is a good example of such practical, "real", and feasible approach.
     
    Last edited: Sep 4, 2011
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