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Do atoms generally exhibit superposition when not isolated?

  1. Feb 6, 2014 #1
    Do atoms "generally" exhibit superposition when not isolated?

    Do atoms "generally" exhibit superposition when not isolated from the enviorment?I know that electrons and photons do but are regular atoms in the real world "generally" in a state of superposition without all the scientific technices and devices that isolate that atom for wave function experiments. I read somewhere that because most atoms are constantly interacting with each other this collapses their wave functions. Is this correct?
     
  2. jcsd
  3. Feb 6, 2014 #2
    You know they will never have exact positions.
     
  4. Feb 6, 2014 #3
    Why not? I think they can but they are constantly moving so....but thats completley diffrent from being in superposition. Im asking this question because Nugatory said in another thread that atoms rarley exhibit superposition in the human body but I dont know the why.
     
    Last edited: Feb 6, 2014
  5. Feb 6, 2014 #4

    atyy

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    Basically all quantum states exhibit superposition. If a quantum state has a definite position, then it is in a superposition of many different states of momentum.

    When a quantum system is measured by a classical apparatus, the quantum system collapses into one of a set of possible states. The set of possible states depends on the measuring apparatus.

    If you treat the quantum system, apparatus and environment together as a large quantum system, an effect known as decoherence can choose the possible states that the system will collapse into when measured. For this reason, decoherence is sometimes said to represent apparent collapse. However, true collapse is still needed, as a quantum system evolving by itself can never collapse.
     
  6. Feb 6, 2014 #5
    so WHAT EVERYTHING is in superposition, the classical world doesent exist Im currently typing this in china while simulatneously being in canada. This is nonsense.
     
  7. Feb 6, 2014 #6
    That's Quantum Mechanics for you.
     
  8. Feb 6, 2014 #7
    I believe Nugatory was talking about a degree of superpositioning that would challenge molecular bonds. For example, the topological position of a carbon atom within a DNA molecule is, as a matter of practicality, fixed. It's position relative to your center of gravity is plenty precise enough to keep that DNA intact. Still, there is some superpositioning.
     
  9. Feb 6, 2014 #8
    Do not try to build the world from bottom up and set a more realistic and modest goal. That's my modest advice.
     
  10. Feb 6, 2014 #9

    atyy

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    The simplest way of thinking is to say quantum mechanics is only for calculating the results of experiments when a classical measuring apparatus probes a quantum subsystem of the universe. We treat the subsystem as quantum, and the measurement results are classical. Of course this is not entirely satisfactory, as it requires common sense to say what a classical measuring apparatus is and what a quantum subsystem is. But it works.

    If you want quantum mechanics to "fully" represent "reality" in the common sense way, you have to use an interpretation like de Broglie-Bohm theory or many-worlds.
     
    Last edited: Feb 6, 2014
  11. Feb 6, 2014 #10

    bhobba

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    You mean aside from the fact the classical world obviously exists. Its obvious some explanation for the emergence of a classical world is required.

    A lot of progress has been made in figuring that out, but a few issues do remain that most think is simply crossing the t's and dotting the i's stuff but one never knows.

    To be specific decoherence explains how a superposition is transformed to an improper mixed state (without detailing exactly what that is), which can be interpreted as the system really is in one of the states prior to observation and exists independent of us. Many moons ago Von Neumann showed the cut between classical and quantum could be placed anywhere, but with the modern understanding of decoherence it's obvious these days it should be placed just after decohrence.

    The few issues that do remain are related to things like showing that exactly the same thing results it doesn't matter how you decompose the system called the factoring problem. You will find a discussion of the current state of play, at a level that can reasonably be understood without knowing the math and technical details, in Omnes book:
    https://www.amazon.com/Understanding-Quantum-Mechanics-Roland-Omnès/dp/0691004358

    To the OP - get a hold of Omnes book by all means. But the answer to your question is that when not isolated (or equivalently using objects like photons that interact only weakly) decoherence prevents superposition being exhibited unless isolated - it gets converted to the mixed state mentioned previously which is effectively (apparent collapse is another word used) the same as collapse.

    Thanks
    Bill
     
  12. Feb 6, 2014 #11

    bhobba

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    No its not. But research is ongoing and progress has been made.

    Exactly what will eventuate isn't of course known, but many think its simply a matter of resolving a few technical issues like the so called factoring problem. And we have interpretations like Bohmian mechanics that are perfectly fine without further ado - the only issue is figuring out how to experimentally test them.

    Thanks
    Bill
     
  13. Feb 8, 2014 #12
    I really suggest reading the thought experiment in "Sneaking a Look at God's Cards" plus the thought experiment in David Albert's "Quantum Mechanics & Experience". Both contradict what von Neumann supposedly showed. He was wrong about hidden variables not existing, could he also be wrong assuming that the cut could be placed anywhere?
     
  14. Feb 8, 2014 #13

    bhobba

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    Feel free to post the full details of the argument and exactly how it contradicts this well known fact. I have read "Sneaking a Look at God's Cards" and can't recall such an earth shattering revelation anywhere - but its been a while since I read it.

    He may have made a mistake - but until its actually shown to be a mistake I think it's more likely you have misunderstood something.

    Thanks
    Bill
     
  15. Feb 8, 2014 #14

    atyy

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    http://arxiv.org/abs/quant-ph/9712044 ?
     
  16. Feb 9, 2014 #15

    bhobba

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    Had a quick look.

    Interesting paper.

    Its conclusion however is of zero concern:
    'However, as shown in Sect. III, a “bad” choice of apparatus is incompatible with a classical description (more precisely, the semiclassical results do not coincide with those predicted by quantum theory, though they may asymptotically be the same for large r).'

    All its saying is a bad choice of apparatus is incomparable with a classical description.

    Its final remarks says it all:
    'Plain orthodox quantum mechanics and classical statistical mechanics correctly reproduce all statistical predictions that can be verified in experiments, provided that the measuring apparatus satisfies suitable conditions, such as those discussed above. If enough care is exercised, no inconsistency shall arise'

    In fact it strengthens my position that measurement is once decoherence has occurred independent of measurement apparatus, and one can place the Von Neumann cut right there if one so desires.

    I often explain to people QM is a generalized probability model about marks left in the common sense classical world of everyday experience. They, once they think about it a bit, correctly say - that needs to be a firmer definition - and I agree. It practice its perfectly OK - but we are speaking of issues of principle - not practice - and something better is required.

    Saying measurement occurs just after decoherence resolves the issue.

    Thanks
    Bill
     
    Last edited: Feb 9, 2014
  17. Feb 9, 2014 #16
    Nope.

    I will attempt to explain the thought experiment and how it contradicts the von Neumann notion that you can place collapse anywhere along the chain, however it'll take me a while and it'll be a good idea to have the book at hand as I'm not use to Latex and can't write equations from the book in that format to save my life.
     
  18. Feb 9, 2014 #17

    atyy

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    Nope as it it isn't the argument you were referring to, or you don't think it supports your point (I thought it did)?
     
  19. Feb 9, 2014 #18
    Nope as in it doesn't align with the thought experiment I'm referring to.
     
  20. Feb 9, 2014 #19

    bhobba

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    I have had a look at my copy of Sneaking a Look at God's Cards, specifically 15.4 on the Von Neumann chain. Its entirely conventional. Say your measurement gives the position on a pointer. We need to conduct a measurement to determine that position with some apparatus in a regress until we reach the final destination - a conscious observer. The conclusion was the only thing manifestly different was at the consciousness of the observer, so that's when he placed it and you end up with this conscious observer created stuff.

    But there is nothing - zilch - in that chain that says we cant say the pointer behaves classically and hence from that point on everything is classical. Its exactly the same nonsense people carry on about in Schrodinger's cat. The particle detector is where the quantum weirdness happens - everything is common sense classical from that point on. And in fact this is precisely what Copenhagen does, which is why the Von Neumann regress is a trivial concern in that interpretation. It assumes the existence of a common sense classical world and that assumption makes all these issues disappear. Its also what the statistical interpretation does.

    The cost however is in explaining how that world emerges. That's where decocherence comes in. If you put the selection of a particular outcome just after decoherence by interpreting the improper mixed state as a proper one then you can explain this classical world and there is no issue. Other issues such as the so called factoring problem appear, but they are under active research, and we will need to await what comes out of it, however many think its simply tying up loose ends.

    Copenhagen's minor blemish is thus fixed:
    http://motls.blogspot.com.au/2011/05/copenhagen-interpretation-of-quantum.html

    'Is that true? Is that a sign of a problem of the Copenhagen interpretation?

    It is surely true. It's how the world works. However, one may also say that this was a point in which the Copenhagen interpretation was incomplete. They didn't quite understand decoherence - or at least, Bohr who probably "morally" understood what was going on failed in his attempts to comprehensibly and quantitatively describe what he "knew".

    However, once we understand decoherence, we should view it as an explicit proof of this fifth principle of the Copenhagen interpretation. Decoherence shows that the states of macroscopic (or otherwise classical-like) objects whose probabilities are well-defined are exactly those that we could identify with the "classical states" - they're eigenstates of the density matrix. The corresponding eigenvalues - diagonal entries of the density matrix in the right basis - are the predicted probabilities.'

    As assertion that you can't place the point a specific outcome is selected just after decoherence would be akin to saying an improper mixed state is not formally the same as a proper one. If that was the case it would be big news with many interpretations such as GRW, MW, Decoherent Histories, Ignorance Ensemble all going down the gurgler - not to mention the formalism itself would be in jeopardy.

    In fact the G in GRW is Ghirardi, the author of Sneaking a Look at God's Cards. There is a very close connection between GRW and Decoherence:
    http://arxiv.org/pdf/quant-ph/0701218v1.pdf

    It seems highly unlikely to me his book proves his own interpretation is in trouble - but I suppose one never knows.

    This is why I am VERY confident such a proof is in error.

    Thanks
    Bill
     
    Last edited: Feb 9, 2014
  21. Feb 9, 2014 #20
    I'm referring to the thought experiment on pages 373 - 376.

    If collapse occurred at the macroscopic detector, then we would get 1/4 probability |X>|45>, |Y>|45>, |135>|X>, |135>|Y> instead of the predicted 1/2 |45>|X>, |135>|Y>.
     
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