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Quantum behavior in a classical system?

  1. Jul 30, 2013 #1
    So there's a new paper out by Yves Couder's group that observes quantum mechanical-like wave dynamics by averaging the long-term dynamics in a purely classical system. In other words, it's hidden variables.


    While the paper focuses purely on the classical aspects of this, the promotional material and accompanying video make a connection to quantum mechanics and promote hidden variables in a not-so-subtle way.

    It seems to me that the connection is rubbish and entirely ruled out by Bell's theorem (unless you are willing to posit superdeterminism, which could be a plausible explanation). Anyone have any thoughts?
  2. jcsd
  3. Jul 30, 2013 #2
    interesting. Could be the closest represention of wave and its mechanics.. Excitation of a field etc.

    It still has to accept non locality. Which they probably do.

    their hypothesis/interpretation seems to assume both -

    1. Hidden variables
    2. Non locality
    Last edited: Jul 30, 2013
  4. Jul 30, 2013 #3


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    Well, if they really managed to replicate the double-slit experiment with a ‘classical droplet’ the result would not have been on MIT News, but on the front page of New York Times.


    This is nonsense. Even a layman like me can understand that you will never get interference from one droplet going thru one slit.
    Last edited by a moderator: Sep 25, 2014
  5. Jul 30, 2013 #4
    DevilsAvocado: Not so fast; the 2006 experiment involved two slits, and did indeed observe something akin to double-slit diffraction. I can't shake the feeling, though, that there must be some fundamental aspect of it that is not being reproduced.

    San K: That's what bothers me. The dynamics of the 'walkers' appears to be quite local. The walkers ride on their own excitation wave, and the wave is only nonzero around the walker.
  6. Jul 30, 2013 #5
    The only non-local property I can think of is the phase of the oscillator, and I'm not sure that really qualifies.
  7. Jul 31, 2013 #6
    It seems they are surprized that the dynamics of a drop of liquid mimic those of quantum waves. I just checked and it's 2013 not 1905 so rediscovering the wave particle duality should probably not shock anyone. Why is this supposed to be surpizing? Sorry if I missed something.
  8. Jul 31, 2013 #7


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    Well... the fundamental aspect is that it did not happen as in “used this system to reproduce one of the most famous experiments in quantum physics: the so-called “double-slit” experiment”.

    Check out the video @2:12.
  9. Jul 31, 2013 #8


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    :smile: :thumbs:
  10. Jul 31, 2013 #9
    I came here to be skeptical of this research, but it seems I'm the one defending it...

    Maui: You mean to say that in 1905 people demonstrated quantum behavior with **droplets** (not waves!!) of fluid?

    DevilsAvocado: Are you referring to their 2006 work? http://phys.org/news78650511.html (ignore the photo at the top of the page, it just shows a single slit. Experiments with double slits were also done).

    The droplet approachs the two slits, then appears to randomly move about (in a very strange way) and 'pick' one of the two slits at random. It then goes throug the slit and, afterwards, is guided by the interference wave. This is basically the same idea as the 'pilot wave' theory of de Broglie. There's nothing special about the wave interference. What's special is that the wave is *generated* by the droplet, and further, the droplet remains stable and propapagates through the slits to the other side. Also, the trajectory of the droplet is chaotic, so even if started from nearly the same initial conditions it will wind up on different points on the screen.
  11. Jul 31, 2013 #10


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    Nope, dBB needs two slits to produce interference:


    (and you already know what happens if we measure/block one slit)

    And why are not the interference pattern presented? Because there is none!
    You can run any single and very chaotic droplet you want, but *one* droplet will never produce this:

  12. Jul 31, 2013 #11
    Devils: I'm aware that it needs two slits to work. That's why I referred you to that page. If you read it (and also check out the videos they made), they very clearly show an interference pattern. And yes, the interference pattern is produced by one droplet. They make it quite clear that what is happening is that:

    1. The wave generated by the droplet goes through both slits and produces interference.
    2. The droplet itself only goes through one slit but is guided by the wave it has generated.

    Both of these are in accordance with the double-slit experiment. A single photon will interfere with itself (check) but will never show up in more than one detector (check).

    Now I'm not sure what would happen if the separation between the two slits were large. In their experiments it seems to be small. I'm not sure if an interference pattern would be produced if the separation between slits was much larger than the wavelength. That might be something worth investigating.
  13. Jul 31, 2013 #12


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    I must be missing something here... on this page you have this picture & video:



    And on this page you have these pictures:



    The last page discusses SINGLE-particle interference, which is something else and perfectly expected for one droplet guided by a water wave. We can very easily remove the redundant droplet, and get exactly the same result with plain water:


    A great cry and little wool, quoth the Devil when he sheard the hog. :wink:

    Really? Did you watch the video @2:12??

    That’s not correct, is it?
    Last edited by a moderator: May 6, 2017
  14. Jul 31, 2013 #13
    Yeah, that physorg.com article doesn't explain it very clearly I suppose, it was just the only non-technical summary of it I could find. You can check out the original paper (published in Physical Review Letters):


    They do both single-slit and double-slit experiments and clearly show both the graph for single-slit and double-slit interference. FTA:

    "The interference fringes are clearly observed and well fitted by this expression. It can be noted that a given droplet is observed to go through one or the other of the slits. However its associated wave passes through both slits and the interference of the resulting waves is responsible for the trajectory of the walker."
  15. Jul 31, 2013 #14


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    Bell's theorem rules out local hidden variable theories, not all hidden variable theories. There's no superluminal stuff going on with these corralled waves, so the results could be deterministic yet still match quantum statistics. Indeed, superdeterminism feels like eactly the right interpretation here; this little system has existed and been closed for quite long enough for subluminal causation to determine its entire state.

    I'm finding myself thinking that this is a macroscopic analog to the deBroglie-Bohm model plus superdeterminism.... Kinda interesting even if there's no compelling new insight - those are few and far between.

    (The tone of the MITNews piece sets my teeth on edge, but that's not the fault of the researchers).
  16. Jul 31, 2013 #15
    As acknowledged by the authors, those experiments are still very far from QM for the following reasons:
    Nevertheless, it is interesting that their set-up does generate space and time non-locality via "path memory". See slides:

    A macroscopic-scale wave-particle duality
  17. Jul 31, 2013 #16
    Nugatory: Hrm that sounds about right.

    bohm2: Yup, all those 5 points are valid, but the connection is still tantalizing :)
  18. Aug 1, 2013 #17


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    As far as I understand it, this whole thing has nothing to do with quantum mechanics at all. It's just a classical wave phenomenon which has a mathematical analogy with the wave structure of the quantum mechanical equations of motion for probability amplitudes. This is, of course, interesting but not very surprising.

    Mathematical analogies are important in theoretical physics, because one can use methods developed in some field of research in another and it provides a certain form of intuition. E.g., wave equations in fluid dynamics are similar as wave equations in electromagnetism although they describe completely different phenomena, but the mathematical picture provided by the solutions are similar.
  19. Aug 1, 2013 #18
    I agree. The most interesting part is the non-locality as summarized in this recent paper by Y. Couder et al. where they discuss the 2 different models proposed by Bohm versus de Broglie's theory of the Double Solution with reference to the diffraction of bouncing droplets:
    Probabilities and trajectories in a classical wave-particle duality

    As a side note, Gerhard Grossing's et al. group has tried to use some of the insights gained from the bouncing/walking droplets in the experiments of Couder's group to model certain QM phenomena:

    "Systemic Nonlocality" from Changing Constraints on Sub-Quantum Kinematics

    Most of this group's work can be found here:

    I also started a thread on the topic a few years ago with updates:

    Wave-particle duality at Macro scale?
  20. Aug 2, 2013 #19
    A little while ago I posted a question


    A number of objections were posted to the idea which I think might be relevant to this discussion:

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