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Quantum Behavior As Extreme Classical Behavior

  1. May 19, 2013 #1
    Why can't quantum behavior be explained as an extreme version of classical behavior?

    For instance, the idea of a small quantum particle being in superposition could be explained by that particle switching between 2 or more states at an extremely high frequency. How high a frequency? Well, on the order of a Planck Length or Planck Unit.

    The only addendum to classical behavior that would be required would be non-locality or tunneling (ie. macroscopic objects are too big to tunnel, but quantum-sized objects are small enough to squeeze through the cracks)
  2. jcsd
  3. May 19, 2013 #2
    How would you explain the Stern-Gerlach experiment as extreme classical behavior?
  4. May 19, 2013 #3
    Hmm, so I just did a quick read on Stern-Gerlach, and it showed that atoms have spin. So what's the problem? How is that irreconcilable with classical mechanics? In large macroscopic objects, the tiny quantum-scale forces would not be large enough to impart an apparent spin. In quantum-scale objects, those forces would be large enough to impart spin.

    A leaf blowing in the breeze can be spun around easily with all the currents and eddies. But a large airplane is too big to be spun around like that so easily. So why can't classical mechanics offer an adequate explanation for this?
  5. May 19, 2013 #4


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    Check out Susskinds lectures - he explains it masterfully and clearly:

    But basically you have things like Bells theorem:

    It shows if you assume classical behavior (in this case the very basic property of local realism ie properties are only influenced by local surroundings) then it leads to predictions at variance with what experiments show - but QM is in full agreement with them. The lectures above gives the full detail.

    There are other things as well. For example, classically if you have two particles, say particle 1 and particle 2, you can tell them apart so that if you exchange them so you have particle 2 and particle 1 then that is different than before the exchange. If you assume this very obvious classical rule then you can derive a property of gasses called the Maxwell Boltzmann distribution:

    But weirdly this doesn't work for quantum particles - when you exchange them there is no difference - you get either the Bose-Einstein distribution:

    Or the Fermi-Dirac distribution:

    Last edited: May 19, 2013
  6. May 19, 2013 #5
    I still don't see why locality and non-locality can't be differentiated for through classical mechanics.

    I can't walk through a screen door, but certainly the breeze can move through it. Classical mechanics can explain that just fine.

    By the same token, I as a macroscopic entity can't tunnel-jump to some remote location, but a small quantum-scale object can.
    I as a macroscopic entity can't influence any other object at a non-local distance, but a small quantum-scale object can. It can do so just like the air particles can pass through the screen door while I can't. I don't see anything in this that inherently thwarts classical ideas, just as Special Relativity doesn't screw up Relativity.
  7. May 19, 2013 #6

    Doc Al

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    What's interesting about the Stern-Gerlach experiment is that the spin values are quantized. That's what you would need to explain classically.
  8. May 19, 2013 #7


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    But can classical mechanics explain these electrons behaving like this?

  9. May 19, 2013 #8
    Yeah, I know, I've seen the double-slit explanation countless times, and it still puzzles me as much today as it did 25 years ago when I first saw it. Here's more kid-friendly video for laymen like me:


    So they say that the results of the experiment changed when they introduced a "detector", but they don't say exactly what the detector is or what its mechanism of detection is. Obviously the introduction of the detector changed the experiment. But even before they introduced the detector, weren't they likewise "observing" the electrons when they saw the fringe pattern?

    To me, the presence of wave behavior while firing particles would intuitively (classically) indicate that those particles are traveling through a medium. If you fire a bunch of projectiles through a medium - even one at a time - the projectiles will interact with the medium to produce waves.

    It's still not clear to me what the detector was doing exactly. Was it intercepting electrons at some point?
  10. May 19, 2013 #9


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    Duplicate post
  11. May 19, 2013 #10


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    Susskinds lectures I gave the link to explains this in detail - please view them.

    The detector becomes entangled with the particle localizing it so it goes through one hole only hence you do not get an interference pattern.

    In fact its this weird phenomena of entanglement that is the rock bottom thing that distinguishes classical from quantum behavior:

    Last edited: May 19, 2013
  12. May 19, 2013 #11
    Okay, so here's an explanation with a little more detail on what the detectors were doing:


    So leaving the speaker's hokey "consciousness" claims aside, the explanation still says that the electrons "knew" when they were going to end up as a magnetic recording or not, and changed their behavior accordingly. To me, that intuitively implies something like a circuit (ie. electric current doesn't flow unless there's a full circuit path available ahead of it, so in that sense the electrons "know" whether to flow or not)

    So why can't a circuit be used as a classical analogy here?
  13. May 19, 2013 #12
    Then you end up with the De Broglie-Bohm pilot wave theory, which is still definitely not classical in any way.
  14. May 20, 2013 #13
    Last edited: May 20, 2013
  15. May 20, 2013 #14
    There is a medium involved. As mentioned in my earlier post, quantum mechanically this would correspond to the pilot wave theory. Unless somehow you think that the idea of photons and electrons emitting guiding waves is classical, I still don't see this "proves" that the double slit experiment with photons and electrons can be explained classically.
  16. May 20, 2013 #15


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    In addition to spin, entanglement, Bell and the double-slit experiment (which are good examples):

    The uncertainty principle(s) (relation(s)) is also a nonclassical feature, quite counterintuitive. Here's a nice clip demonstrating it.

    I'd also say that antimatter, annihilation & pair production are nonclassical features. Two massive particles (e.g. electron + positron) can annihilate into two photons - who would have expected that in classical physics? :smile:
  17. May 20, 2013 #16

    Quantum Physics from Classical Physics with an epistemic restriction

    and briefly
    physics as an interface to underlying structure
  18. May 21, 2013 #17
    Alright, so the rules governing quantum behavior are markedly different than those governing classical behavior. But it's all still happening in the same universe, so it's not like the "quantum world" is truly separate from the "classical world" is it?

    So is there a sliding scale of transition between quantum rules of behavior and classical rules of behavior? Or does it transition abruptly? Can we say that classical behavior is an emergent behavior arising from quantum behavior? Is it reasonable to think that what happens on the macro scale is the aggregate result of what happens on the small scale?
  19. May 21, 2013 #18
    Another thing about the double slit experiment that puzzles me.

    When the electron(s) passes through the double slit, it only interferes with itself but does not "collapse" by interacting with itself. It's when the electron(s) interact with a detector that its wave "collapses"

    You say this collapse is due to entanglement? So no entanglement means no collapse?

    And electrons automatically interfere with other electrons, right? So in that case, entanglement is unavoidable? Is there any circumstance where an electron can collapse another electron's wave?
  20. May 21, 2013 #19


    Staff: Mentor

    Everything is quantum - the classical world emerges from entanglement and decoherence.

    For example all quantum particles of the same type are indistinguishable - the reason atoms etc can be distinguished at our classical level is entanglement. At a low enough temperature you have what are called Bose Einstein Condensates where the atoms have totally lost their individuality and they behave as one single giant atom with very strange properties. Raising the temperature means you are entangling it with the environment and that's when it starts to behave classically and the constituent atoms/molecules become distinguishable.

  21. May 21, 2013 #20
    Alright, but if I shoot a basketball at a double slit, it's not going to create an interference pattern. How large an object can I use, and still get the interference pattern?

    I've read that it's possible to use C-60 buckyballs and still get the interference pattern.
    I've read they've even gone larger than that with even bigger molecules and gotten the interference pattern.

    So is it a gradual transitioning of wave propagation to particle model?
  22. May 21, 2013 #21


    Staff: Mentor

    A few issues here. First the very existence of collapse is interpretation dependent - some interpretations have it - others don't. What all interpretations have is when an observation occurs (here observation means some kind of record appears here in the macro world) then the probability of that is given by the Born rule. It is entirely up to how you want to interpret the math if a wavefunction collapse occurs - see for example the Ensemble Interpretation:

    In modern times many believe decoherence (decoherence is a form of entanglement) has something to do with it. But you have to understand what decoherence does. To understand that you need to understand a mixed state:

    What decoherence does is transform a pure state into an improper mixed state. Here improper means mathematically and observationally it's the same as a proper mixed state but it has not been prepared the same way. A proper mixed state is a pure state that has been randomly selected. If the improper mixed state of decoherence was prepared that way then vola - no measurement problem - what you observe is there prior to observation - its the state that has randomly been selected and you observe it - no change in the state - no collapse - nothing - everything is sweet:
    http://en.wikipedia.org/wiki/Quantum_mind%E2%80%93body_problem [Broken]
    'Decoherence does not generate literal wave function collapse. Rather, it only provides an explanation for the appearance of wavefunction collapse, as the quantum nature of the system "leaks" into the environment. That is, components of the wavefunction are decoupled from a coherent system, and acquire phases from their immediate surroundings. A total superposition of the universal wavefunction still exists (and remains coherent at the global level), but its fundamentality remains an interpretational issue. "Post-Everett" decoherence also answers the measurement problem, holding that literal wavefunction collapse simply doesn't exist. Rather, decoherence provides an explanation for the transition of the system to a mixture of states that seem to correspond to those states observers perceive. Moreover, our observation tells us that this mixture looks like a proper quantum ensemble in a measurement situation, as we observe that measurements lead to the "realization" of precisely one state in the "ensemble".'

    To get around the issue that it just gives the appearance of collapse an extra interpretive assumption is required. Different assumptions lead to different views. Just as an example check out Decoherent Histories:

    But other ways of handling it are possible - eg Many Worlds. And still others like the Ensemble interpretation claim its not even required.

    Welcome to the weird and wacky world of different quantum interpretations.

    Last edited by a moderator: May 6, 2017
  23. May 21, 2013 #22


    Staff: Mentor

    That's an area of active investigation - there are outstanding issues. If you want to get the up to date view of it you need to read the modern literature:

    Last edited by a moderator: May 6, 2017
  24. May 22, 2013 #23
    A wave function is supposed to be fully continuous, and thus infinitely divisible, right? The double slit experiment utilizes Huygens Principle to create the interference pattern, as if produced from 2 point sources. I guess we can assume that if you put 3 slits after the double slit, and then 4 after that, and 5 after that, etc, that conclusions from Young's experiment will still hold true.

    But a particle isn't supposed to be infinitely divisible.

    As a wavefront spreads out while propagating, its amplitude falls off with distance squared. But particles don't fall off with distance squared. In principle, the particle is still supposed to stay the same no matter how far it travels, right?
    I assume that any detector at one of the 2 slits would still detect an electron at full strength, and not some fractional faded out electron.

    Furthermore, for a true wave, all points on its wavefront are supposed to be equal. But observation of quantum particle waves shows that's not true, since the wave function can be collapsed to a particular point.
    So is that wave being observed really a true wave? How can all points be equal, and yet one point is more equal than others? Why should any collapse be possible at all?

    A particle moves in 1 dimension, a straight line. But a wave from a point source moves symmetrically in all dimensions. So if my double slit wasn't directly in front of the electron gun, but was off to the side, or above, shouldn't it still show the same interference pattern?

    I'm used to thinking of ballistic travel in terms of cartesian coordinates of x,y,z. But in the case of a wave that collapses into a ballistic trajectory, it seems easier for me to visualize this in terms of polar coordinates, because the wave is advancing along the r coordinate and it's only when it collides with something that the angular component collapses into the particular value. Hmm, so it's only the angular component that's indeterminate during propagation...
    Last edited: May 22, 2013
  25. May 22, 2013 #24


    Staff: Mentor

  26. May 22, 2013 #25
    Thanks - so just improving my thoughts here...

    The difference between quantum wave motion trajectory and ballistic motion trajectory is that for the former, the radial and angular components are both determined by collision, whereas with the latter just the radial component is determined by the collision. Fair enough?

    So isn't that then what separates the quantum world from the classical world? For the trajectories of quantum objects, angular indeterminacy is just as apparent as radial indeterminacy, but for trajectories of classical objects there is no angular indeterminacy but only just radial indeterminacy for ballistic motion.
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