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Can quantum decoherence be described as a causal interplay?

  1. Nov 22, 2015 #1
    If yes, then how is it any different from other facts of causality, and why is there a puzzle? Can one not simply view quantum particle changes as the causal interaction between subatomic and macroscopic objects, yielding certain processes of behavior under physical law, depending on the physical scale of the interactors with each other?

    Can quantum decoherence be described as yet another form of causality, or is this an inaccurate way of putting it?
     
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  3. Nov 22, 2015 #2

    Simon Bridge

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    What do you mean by "causal interplay"?
    What do you mean by "quantum particle changes"?

    I think the descriptions you gave are too general ... everything that we can make happen can be considered something to do with causality in some way. The trick is working out which way. The quantum mechanical model is successful at describing everything statistically and making something happen is a bit like rolling dice.
    That does not mean that a deterministic model may not emerge later but so what? Come up with one, then we can talk.

    If you are thinking that the statistical nature may be an emergent result some hidden determinism - like with the dice: we have incomplete knowledge so the result appears random? This sort of "hidden variables" idea is well examined since Bell's Theorem - it is the way that quantum statistics work that prevents such theories taking hold.

    But I can't really tell what you are talking about.
     
  4. Nov 22, 2015 #3
    I am comparing it with causality in everyday life between macroscopic objects. Why can't quantum decoherence be defined as yet another form of cause and effect? Just as there are chemical properties producing cause and effect in chemistry?

    I am referring to the observer effect in quantum mechanics.
     
  5. Nov 22, 2015 #4

    Simon Bridge

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    In terms of everyday interactions - you do something and cause something to happen.
    That is how decoherence is described. You did something to the system, causing decoherence to happen.

    The devil is in the details - nobody knows why the details are like that, the Universe just happens to work that way.
     
  6. Nov 22, 2015 #5
    If we gain (as we have done) a greater appreciation of the fact that microscopic and macroscopic objects influence each other causally, I see no mystery. The reason we can't relate to why different objects affect each other, is simply because quantum decoherence kills any potential of quantum phenomenon to occur in everyday life. I don't find it paradoxical at all that subatomic particles can be at two places at once, given that it's not a macroscopic object, and is not be bound by the laws of gravity in the same way as macroscopic objects are.
     
  7. Nov 22, 2015 #6

    Simon Bridge

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    When does something get big enough that it is "bound by gravity" the same way as macroscopic objects ... i.e. when does something become a "macroscopic object"?
    Can an object be "bound by gravity" in such a way in some situations and not others?

    Or are you asking "why do people find this weird?"
     
  8. Nov 22, 2015 #7
    Easy. The threshold is when the object is no longer in the realm of quantum phenomenon of double slit experiment and other things.
    The theory of gravity has been proposed by the mathematican Roger Penrose, and seem to make sense of it all, if you insist on an explanation why there is a difference between the laws for micro and macroscopic objects in the world.
     
  9. Nov 22, 2015 #8

    Simon Bridge

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    It's more about the trouble with identifying the divide between the macro and micro.
    For instance, you have mentioned "in the realm of the double slit experiment" ... OK - so how big is that?
    What is the biggest object that can show quantum interference at slits do you think? Have you checked?

    Note: "the theory of gravity" was proposed by Newton. You can make all kinds of true statements by being vague - I am trying to help you understand by forcing you to be specific.

    Anyway, I'm getting confused. You are saying you have no trouble with micro and macro worlds obeying different rules, but also that macro causality can work for the micro world.
     
  10. Nov 22, 2015 #9
    This is physically and mathematically determined. Wheter we know the exact line or not, the universe clearly has a line, or the quantum crazyiness would be a part of our everyday life, and it isn't. So it's decided by the objects constitution in some way shape or form. I don't find that muddy at all. And it's just as intuitive as the fact that there are some creatures who are tall and some who are dwarfes.
     
  11. Nov 22, 2015 #10

    Nugatory

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    That is not true, and indeed I find myself wondering how much you've looked at the math behind decoherence. According to that math the split between the macroscopic and the microscopic world is no more a clear line than the division between classical systems that are treated according to the ideal gas law (many many many molecules of gas) and classical systems that are treated by specifying the position and momentum of each molecule. The distinction is a matter of degree, based on our ability to obtain a complete specification of the system state and our willingness to consider statistical fluctuations negligible.
     
    Last edited: Nov 22, 2015
  12. Nov 22, 2015 #11
    But there is a distinction, and that is all you need to know.
     
  13. Nov 22, 2015 #12

    Nugatory

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    But you're not saying that there is a distinction, you're saying "whether we know the exact line or not, the universe clearly has a line". That's a very different claim, and one that is completely unsupported by the mathematics of decoherence.
    (That claim does appear in some interpretations, including some variants of Copenhagen, but decoherence is interesting and important precisely because it neither supports nor requires that claim).
     
    Last edited: Nov 22, 2015
  14. Nov 22, 2015 #13

    bhobba

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    Indeed. Some interpretations like Decoherent Histories do not even have observations. This macroscopic microscopic divide is a furphy because everything is quantum. One of the big advances in modern times is a definition of observation that is purely quantum.

    Thanks
    Bill
     
  15. Nov 22, 2015 #14

    Simon Bridge

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    Put another way: there is no such divide between macro and micro in Nature.
    If you think there is one, then show it to me. What is your evidence that there is one?
    Show me it is not just a semantic convenience like the apparent division between energy and matter?
    Show me something other than word play.

    It looks like you know this topic mostly from pop science shows and books... is this correct?
    ie. Do you have a reference for the Penrose theory of gravity? Penrose has written a bit on quantum gravity but, afaik, has not produced a working theory. If we have some idea of your background we can pitch answers better.
     
  16. Nov 25, 2015 #15
    This is coming from a guy who claims QM is indeterministic. The wave function evolves deterministically. If you don't know such basics, It's no use discussing.
     
  17. Nov 25, 2015 #16

    bhobba

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    State evolution is deterministic. Observation on the state aren't. Because of that context shift a bit of care is sometimes needed in understanding what is being said.

    The main point needs to be emphasised. Everything is quantum, there is no classical quantum divide. We now have ways of viewing QM that eliminates the need for an implied classical world.

    Thanks
    Bill
     
  18. Nov 25, 2015 #17

    DrChinese

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    Quantum effects are seen in much larger objects than might be expected, indicating there is no such line.

    http://qudev.ethz.ch/content/courses/phys4/studentspresentations/waveparticle/arndt_c60molecules.pdf

    Nobody as of yet really can describe decoherence well in PHYSICAL terms (any more than many other quantum effects). There have been mathematical treatments, including analysis of the speed of decoherence. The classical world consists of many quantum objects - some sharing degrees of entanglement, some not - whose quantum effects tend to average out in everyday observation.

    Trying to think of decoherence as a causal phenomena - per the OP - is really "interpretation" dependent. As I alluded, no one can explain why states evolve in a particular way when there is decoherence.
     
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