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Quantum mechanics for big things?

  1. Sep 23, 2013 #1
    From my understanding QM deals with small things in the universe I use the term "small" loosely when I refer to small I'm talking about sub atomic particles. Anyways back on to the question here it goes.

    Why can't QM be applied to bigger objects? I know that we have General relativity for planetary masses and galaxies to describe their behavior. Could the two be interchangeable, say we could use general relativity for things that are small and QM for stuff that is big?
     
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  3. Sep 23, 2013 #2
  4. Sep 23, 2013 #3

    DrClaude

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    QM applies to all objects. It is just that, beyond a certain size, the effects tend to be so small that they are not noticeable. But there are observable effects in "big" objects, such as big molecules. Also, you can't explain things like neutron stars without QM.
     
  5. Sep 23, 2013 #4

    ZapperZ

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    Size isn't a factor here. We have seen large molecules, the size of fullerines, and a large conglomerate of particles made up of up to 10^11 electrons exhibiting quantum effects. It will get larger.

    The issue here is the ability to maintain coherence so that these quantum effects can be clearly evident on our scale. Maintaining coherence gets progressively more difficult with size and with time! So it just isn't size here. I may be able to get something the size of an elephant be in a coherent state, but it is of no use and not easy to observe if it does that only in the first 10^-15 second before environmental decoherence sets in.

    Zz.
     
  6. Sep 23, 2013 #5
    Quantum mechanics was initially invented to describe black-body radiation curves, a macroscopic phenomenon.
     
  7. Sep 24, 2013 #6
    There's a wiki article for this, try searching.

    One thing I find especially interesting are quantum vortices. When we make swirls in a superfluid, the vortices don't act randomly, but in arrange in cool geometrical shapes. I also like how the wavefunctions (probability density) smoothly become particle/mass density.

    This convinces me that quantum phenomena may have some more familiar macroscopic interpretations. If we try to split the Schrodinger's cat's wavefunction, then we don't neccessarily get a zombie cat (half-dead, half-living). Instead it might be: very cold cat, or a cat rotating over its axis, or a cat a sound wave travels through or something else.
     
  8. Sep 24, 2013 #7

    DrClaude

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    This is non-sensical.
     
  9. Sep 24, 2013 #8

    Vanadium 50

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    180,000 gallons of liquid helium shows quantum effects. Big enough?
     
  10. Sep 24, 2013 #9

    bhobba

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    Exactly.

    Getting macro sized objects to display quantum effects aren't easy - but its not impossible. And when you do some very strange things emerge:
    http://physicsworld.com/cws/article/news/2010/mar/18/quantum-effect-spotted-in-a-visible-object

    Thanks
    Bill
     
  11. Sep 25, 2013 #10

    meBigGuy

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  12. Sep 25, 2013 #11


    What is the quantum mechanical explanation for the liquid helium behavior and why would that be a qm effect?
     
  13. Sep 25, 2013 #12

    DrClaude

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    Maybe you can start with Wikipedia: Superfluid helium-4.
     
  14. Sep 25, 2013 #13


    There seems to be a contradiction. It could be me(wouldn't be the 1st time anyway) or it could be that there are wrong claims in papers and textbooks on quantum theory(or possibly with the qm explanation on superfluidity which according to what I've read is still an ongoing process).

    One of the 1st things one learns from high-quality books is that a wavefunction can never be observed, even in principle. And it seems that most quantum mechanical explanations on superfluidity center around the idea that at temperatures close to absolute zero, the internal random motion of atoms stops and they start behaving as a giant wavefunction, which in turn is routinely directly observed in experiments since the 1930's and filmed in videos.
     
  15. Sep 25, 2013 #14

    DrClaude

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    What is the contradiction?

    There are always details to clear out, but nobody doubts that superfluidity is a QM effect. Landau won the Nobel prize way back in 1962 in part for that.

    What kind of books are you talking about? Popular science, textbooks, or monographies?

    Not everyone agrees with that statement: Direct measurement of the quantum wavefunction

    Are you saying you have a problem with that statement?
     
  16. Sep 25, 2013 #15


    That it is possible to directly observe a wavefunction. This should be news to a lot of folks here.





    Yes, I do. It invalidates all interpretations that posit that the wavefunction is only a mathematical tool and that includes the standard interpretation found in textbooks.
     
  17. Sep 25, 2013 #16

    DrClaude

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    Putting aside the question of whether you can observe a wave function, my problem with your initial statement is that the explanation for superfluidity is quantum mechanical. And when you see superfluid helium flow, you are seeing a QM effect even if you are not observing a wave function.
     
  18. Sep 25, 2013 #17
    It still doesn't feel right that you can have a very large body that displays directly observable quantum behavior in daylight, in front of cameras... Even one photon was supposedly sufficient to trigger massive decoherence in less than a thousand of a second and a rapid return to classical behavior.
     
    Last edited: Sep 25, 2013
  19. Sep 25, 2013 #18
    The wavefunction itself cannot be measured. It is a complex function and the magnitude of that complex fuction can be measure. may be that's the source of confusion? The problem of many introductory texts is that they violate the principle that statements should be made as simple as possible but not simpler.
     
  20. Sep 25, 2013 #19


    No, every measurement on the wavefunction forces the quantum state to become one of the eigenstates of the operator corresponding to the measured observable.

    What is not clear is why a large body would display quantum behavior in broad daylight in front of recording equipment without observable decoherence setting in with a rapid return to classicality(see post 17) - esp. since the fluid is in contact with macroscopic objects like the fluid container?

    My primitive explanation is that(perhaps contrary to commonly adopted phrasing) liquid helium is a new entirely classical behavior due to quantum effects, but not a quantum behavior in and of itself. It's still confusing as all macroscopic behavior should be due to quantum effects.
     
    Last edited: Sep 25, 2013
  21. Sep 25, 2013 #20

    bhobba

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    The behavior of liquid helium is complex and difficult to explain all its features even with QM.

    But some features are easy to see. For example that it flows without friction is a consequence of the fact its in its lowest energy state - if it had friction it would loose energy which is not possible. You can do an internet search for explanations of other weird aspects - but they are all based on QM.

    Of course that's a superficial explanation - the correct one is much more difficult and deeper eg
    http://cds.cern.ch/record/808382/files/p363.pdf
    'Putting it in another way, we can say that the destruction of superflow would require a transition that takes a macroscopic number of atoms from one state to another simultaneously, and such a process has very low probability.'

    Thanks
    Bill
     
    Last edited: Sep 25, 2013
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