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Is it the fallacy of my thought?

  1. Apr 14, 2012 #1

    dpa

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    quantum mechanical phenomena are microscopic phenomena and exactly anti intuitive.
    Is it not wrong to imagine as in schrodinger's cat case that cat is in superposition of different states. Living and dead.
    Cat is macroscopic and has no such thing as quantum state to exhibit superposition.

    I know its just an example but sometimes people introduce macroscopic examples which make QM more anti intuitive and difficult to believe.

    Is not it wrong to interpret it that way?
     
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  3. Apr 14, 2012 #2

    jambaugh

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    Yes, but it isn't just a matter of microscopic vs. macroscopic. One could in principle demonstrate superpositions with macroscopic objects. (The biggest thing I recall reading about were whole silver nuclei but that was a long time ago.)

    Another important conceptual point is this. Every "state" is a "superposition of different states". It is a matter of how one has decided to resolve observables and a basis. So the "is a superposition" property is not applied to the object but to the choice of observables themselves. With that in mind, and having decided to resolve the cat in "alive" vs "dead" you can ask if one can see the effects of a cat previously in a superposition of the two.

    To see any quantum superposition phenomena we would need to see interference. This will require two things: we must product many identical systems to build up a statistical interference pattern, and to keep them "identical" we need to cool the systems down to nearly absolute zero. Imagine a beam of frozen cloned cats.

    These requirements are complementary to the chosen observables of "alive" vs. "dead". (a beam of frozen cats would also be a beam of dead cats!) So fundamentally the inability to observer Schrodinger's cat in a superposition of alive vs dead is a matter of Born's complementarity. It is impossible in the same way it is impossible to observer simultaneously position and momentum.
     
  4. Apr 14, 2012 #3

    mfb

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    I wouldn't consider a small cantilever "microscopic" - you can see it by eye. And yet it can be measured that it shows non-classical behaviour.
     
  5. Apr 14, 2012 #4

    jambaugh

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    Ah yes, I'd forgotten about the more recent cantilever systems. (My recollection of the silver ions is decades old.)

    For that matter you could say (the beam of) a regular laser pointer is a perfectly valid macroscopic quantum system in superposition (if it is vertically polarized it is a superposition of oblique "states", or equally a superposition of L-circular and R-circular states.) The fact that it also has a description in terms of classical superpositions of waves does not negate this fact.

    That may seem like a stretch but it is really a demonstration that superposition is NOT a system property but rather a relative property of the observables (the acts of observation). The classical and quantum superpositions are the same. The "weirdness" of systems when considered under superpositions of observables is their lack of a classical state based description. In classical theory we can pretend that superposition is "about the system" when it is in fact "about the measurement process". In quantum mechanics we cannot avoid this fact.
     
  6. Apr 14, 2012 #5

    Demystifier

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    The problem with such an interpretation is to say where exactly the boundary between microscopic and macroscopic is. New experiments constantly shift that boundary toward bigger objects, so it seems more likely that a fundamental boundary does not exist at all.
     
  7. Apr 14, 2012 #6

    vanhees71

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    As far as we know there is no such boundary between macro and micro cosmos. Of course our understanding of the quantum nature of gravity is incomplete yet, but as far as we can tell quantum theory is the comprehensive theory describing all phenomena in nature correctly.

    The "classical" behavior of macroscopic systems, i.e., systems of a lot of microscopic degrees of freedom, relevant to its behavior under the given state of this system, is emergent and can be understood (at least in principle) from the underlying quantum dynamics of its microscopic constituents.
     
  8. Apr 14, 2012 #7

    bhobba

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    That depends on what particular interpretation you hold to - each has a bit of a different take on it. For example the Many Worlds Interpretation would say the alive and dead cat exist simultaneously whereas the Consistent Histories interpretation would say long before it reaches the classical level of the cat it would decohere so that it is no longer in a superposition but either alive or dead with a certain probability.

    The fallacy of your thought is in thinking this issue has not been thoroughly examined. What you need to do is become acquainted with the details of it before forming a view.

    Thanks
    Bill
     
  9. Apr 14, 2012 #8

    bhobba

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    That's true - but there are a number of theorems about that show in the world of everyday experience the troublesome off diagonal elements of the density matrix are very small and quickly go to zero so quantum weirdness is well and truly hidden.

    Thanks
    Bill
     
  10. Apr 14, 2012 #9

    dpa

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    I definitely dont know much but then just like wavelength of any object's wave from wave particle duality decrease with increase in mass, can we not conclude that degree of other quantum mechanical phenomena shown decreases in similar fashion? And hence no clear demarkation exists.
     
  11. Apr 15, 2012 #10

    zonde

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    Wikipedia says:
    "Schrödinger did not wish to promote the idea of dead-and-alive cats as a serious possibility; quite the reverse, the paradox is a classic reductio ad absurdum."

    So I would say that sometimes people are so confused that they mix up examples that are meant as explanations with examples that are meant to demonstrate ambiguity of concept.

    About superposition - it is not property of objects. Superposition arises when you have two different histories leading to the same result. In that case you have interference effect where result changes depending from phase between two histories. So we can say that superposition is property of the whole setup that includes measurement and two different paths leading to this measurement.
     
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