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Is there a size limit to QM?

  1. Jun 9, 2012 #1
    Is there a maximum size an object can have above which it won't follow the laws of quantum mechanics anymore? e.g. could the double slit experiment in theory work with macroscopic objects?
  2. jcsd
  3. Jun 9, 2012 #2

    Vanadium 50

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    Quantum mechanics always works, but you will have a hard time building a 100 micron slit that will pass a 10 ton truck.
  4. Jun 9, 2012 #3

    you ask, when loses the linearity ? macroscopic object superposition, where is the limit ?

    maybe at 10 14 atoms.
  5. Jun 9, 2012 #4
    Although I have yet to find an explanation of the double slit issue from QM, the interference pattern effect does continue for larger objects, but the effect diminishes as the objects get larger. Buckyballs have been used and they produced a very tiny but distinguishable interference pattern effect (or so they tell me).
  6. Jun 9, 2012 #5
    actual limit, 430 atom molecule interference.
  7. Jun 9, 2012 #6
    i dont think theres a limit. Only the limit on what you can observe. The bigger the object the closer the wave function gets to 1, where 1 is a single state, but as i understand no object actually reaches 1. But reaches close enough to 1 that you will only see a single state unless you observe for more time than the universe is likely to exist.
  8. Jun 9, 2012 #7
    As others said, QM applies at all scales; however, as objects become larger, heavier and
    more complex, they tend to interact more intensely with their environment, in a way that
    tends to destroy many of the typical quantum mechanical effects, like interference.

    Look up 'environmentally induced decoherence' for a more in-depth explanation for this
  9. Jun 9, 2012 #8
    Here's an post of mine (from a previous thread) that you may find interesting:
  10. Jun 9, 2012 #9
    i say actual experimental limit.

    there are planned experiments on bigger objects, that way they test quantum mechanics versus macro-realistic theories (if macroscopic objects obey macrorealism, or whether QM prevails).
    Last edited: Jun 9, 2012
  11. Jun 9, 2012 #10
    not proved yet.
    Last edited: Jun 9, 2012
  12. Jun 9, 2012 #11

    Vanadium 50

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    Of course it was - way back in the 20's, or possibly 30's. These are called correspondence theorems, and they demonstrate that the predictions of quantum mechanics approach those for classical mechanics as systems get large. Paul Ehrenfest did a lot of work on these.
  13. Jun 10, 2012 #12
    but the maths doesnt exclude it.
  14. Jun 10, 2012 #13
    You're right about the correspondence theorems, but those don't really describe the emergence of classicality from quantum theory. In fact, that emergence is still considered a mostly unsolved problem (despite all the advances in decoherence theory and related subjects).

    Ehrenfest's theorem and other correspondence theorems only show that it is sensible to assume that classical behavior can be generated by quantum systems, in one way or another. That mostly refers to classical trajectories of certain quantities. But for explaining real classical behavior you have to explain the lack of interference (decoherence, in the 70s and 80s mostly) and the the uniqueness of classical properties (i.e. the measurement problem)

    While the lack of interference is understood quite well, the measurement problem is still not understood fundamentally.
  15. Jun 10, 2012 #14


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    Quantum behavior was observed with oscillating cantilevers of ~50µm size - big enough to be visible by eye (but not during the experiment, of course, as the required light would disturb it).
    It is just an experimental limit, and the limit becomes bigger and bigger.
  16. Jun 10, 2012 #15
    I can't wait until a proper test is done on the Leggett-Garg inequality. I know somewhere (might be the US), there is a plan to put a 40kg mirror into superposition.
  17. Jun 11, 2012 #16
    Right stevie

    this one with a kg mirror.
    The LIGO Science Collaboration Experiment, New J. Phys. 11 073032 2009.

    Large Quantum Superpositions and Interference of Massive Nanometer-Sized Objects
    O. Romero-Isart, A. C. Pflanzer, F. Blaser, R. Kaltenbaek, N. Kiesel, M. Aspelmeyer, J. I. Cirac
    6 Jun 2011.

    ...this includes experiments in a hitherto unachieved parameter regime where collapse theories predict quantum mechanics to fail, or even more general tests of quantum theory against full classes of macrorealistic theories...
    ...we shall discuss the application of using this experiment to test theories beyond quantum mechanics that provide an objective collapse of the wavefunction for sufficiently large objects.....

    http://arxiv.org/PS_cache/arxiv/pdf/1103/1103.1236v1.pdf [Broken]

    ...Another motivation to consider the possibility that quantum physics is only an approximation to a deeper underlying theory...

    Last edited by a moderator: May 6, 2017
  18. Jun 12, 2012 #17
    If you're curious, here's an experiment from 2002 which does just that: Matter-wave interferometer for large molecules.

    And here's one from 2011 using molecules made up of 430 atoms: Quantum interference of large organic molecules.
    I think that's what yoda jedi was talking about.

    It looks like experimentalists are in a long-term race to see who can send the biggest object through a beamsplitter and still show de Broglie interference.:biggrin:
  19. Jun 18, 2012 #18
    Last edited by a moderator: May 6, 2017
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