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Quantum Superposition

  1. May 15, 2012 #1
    Hey there! I'm still a beginner with many physics theories, so this may be an obvious question. I understand that a specific particle will exist in all of its possible states simultaneously, but once measured or observed, it will take only one of these possible positions (Please correct me if I'm wrong here). I was wondering, does this apply on a large scale? I keep hearing some statements regarding superposition that basically state that all objects that aren't being measured or observed also exist in all of its possible states simultaneously.

    But, this just doesn't seem valid to me. If I understand correctly, quantum superposition only occurs on the atomic level, and does not apply on a larger scale due to the incredibly large number of atoms congregated together. Am I correct about it not applying on the larger scale?
     
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  3. May 15, 2012 #2

    K^2

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    It applies on large scale. There are a number of macroscopic states that rely on quantum effects. Superconductivity and superfluidity are good examples. You can take a look at Heisenberg Uncertainty Principle to try and figure out why the effects tend to be far more noticeable on small objects.
     
  4. May 16, 2012 #3
    Really? Wow thats interesting. I'll definitely take a deeper look into Heisenberg's Uncertainty Principle. So basically, lets say that there is no one in my basement and no measures currently being taken down there. Do all of the objects in the basement exist in their superposition until I physically observe them? Sorry if thats a stupid question.
     
  5. May 16, 2012 #4

    Nugatory

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    Not a stupid question at all - this question of what quantum mechanics "really means" has been actively debated since the beginning, and still is. The various answers are called "interpretations", and there are easily a half dozen or more respectable ones.

    Arguing among interpretations is at least as much a matter of philosophy as of physics. You cannot design an experiment that will answer your question about unobserved objects in your basement without somehow knowing what's going on in your basement - and that's an observation so it can't tell you anything about unobserved objects.

    Try searching here and on Google for "Copenhagen interpretation", "Quantum mechanical interpretations", "Schrodinger's cat", "Is the moon there when nobody looks" and you'll get a pretty good starting survey of this stuff.
     
  6. May 17, 2012 #5
    You should read about Bells theorem. John bell talks about using entangled electron positron pairs and then measuring their spin states. You assume that the particles are always in a spin state and you get a contradiction. Pretty neat stuff.
     
  7. May 18, 2012 #6

    bhobba

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    Yea it can occur on the large scale but most of the time because of a phenomena called decoherence it doesn't - stuff like superconductivity is the exception.

    Thanks
    Bill
     
  8. May 18, 2012 #7

    Matterwave

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    You don't actually get a contradiction (in the logical sense), you just get a different set of predictions (probabilities) than with standard QM. Tests show standard QM's predictions to hold. =]
     
  9. Oct 20, 2012 #8
  10. Oct 20, 2012 #9

    bhobba

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    Superposition is a fact of QM - its incorporated right into its foundations. The only issue is one of interpretation - just what it means. To the best of my knowledge no experiment has ever been able to distinguish one interpretation from another - if so it would be big news - big news indeed - if some experiment was able to do it. I am pretty sure all the the above experiment does (and I cant bring up the link) is confirm QM regardless of interpretation.

    Thanks
    Bill
     
  11. Oct 21, 2012 #10
    I think the issue comes down to the interpretation of the word measurement in its relation to quantum mechanics. Observation is sometimes used interchangeably here also and I think that breeds confusion. The question of whether the moon really exists or not if no one is looking at it seems absurd on its face, and I personally think it is, although popular physicists like to throw that visualization around like a Halloween party trick.

    The idea of measurement, on the other hand, is, I think, used in a difference sense by serious quantum theorists. They say that something is “measured” when it comes into contact with or interacts with another physical system. That is very different than the concept that a sentient human being has to be involved in the matter (no pun intended :redface:).

    Therefore, the objects in your basement exist as you might imagine they do when no one is looking at them because the air molecules in the room are colliding at all times with those crumpled beer cans. In fact, I think the principal obstacle with building a quantum computer is somehow preventing the quantum “cores” from interacting with other matter inappropriately which may erringly force an unwanted collapse of a solution prematurely.

    I may be wrong on this analysis so some comments would be appreciated. I am a brain-consciousness researcher and have to deal physicist-consciousness researchers at conferences, almost all of whom propose some kind of consciousness collapses the wave function kind of solution to the issue of consciousness-sentience. I argue against that position for the reasons outlined above, so it would be nice to get more opinions here in this forum.
     
  12. Oct 21, 2012 #11

    bhobba

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    If consciousness causes collapse is very interpretation dependent and is not implied by QM. Wigner was in that camp but after seeing some early work of Zurek on decoherence realised it solved the issue - specifically it allows you to assume the system is in the observed state prior to observation so no need for conscious to do anything. Check out:
    http://arxiv.org/pdf/quant-ph/0312059v4.pdf
    'The reduced density matrix looks like a mixed state density matrix because, if one actually measured an observable of the system, one would expect to get a definite outcome with a certain probability; in terms of measurement statistics, this is equivalent to the situation in which the system is in one of the states from the set of possible outcomes from the beginning, that is, before the measurement. As Pessoa (1998, p. 432) puts it, “taking a partial trace amounts to the statistical version of the projection postulate.”'

    Next time one of those guys claims QM implies consciousnesses causes collapse you might like to point them to the paper above and in particular the quote I gave.

    Thanks
    Bill
     
  13. Oct 21, 2012 #12
    It has been my experience that "popular physicists" are usually unpopular physicists.
     
  14. Dec 14, 2012 #13
    This is a good question, but I must point out that most of the standard 'mainstream' QM interpretations do not succeed in explaining why we don't see macroscopic superpositions. Decoherence does not really solve this problem. I provide an interpretation that straightforwardly specifies where the micro/macro cut is and how it comes about. For introductory and preview material, see: rekastner.wordpress.com.

    For the full version see my new book,

    http://www.cambridge.org/us/knowledge/discountpromotion/?site_locale=en_US&code=L2TIQM

    Ruth Kastner
     
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