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Double slit experiment - with molecules?

  1. Nov 10, 2009 #1

    DaveC426913

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    I was browsing about the double slit experiment and found the following tidbit on Wiki:

    (emphasis mine)

    I knew that electrons could interfere with themselves - I understand how an electron can act like a wave. Even an atom I can understand.

    But a molecule?

    This isn't merely suggesting that the molecule behaves wavelike, or has a resonant frequency, it is suggesting that the molecule itself is somehow passing through both slits simultaneously.

    This is difficult to absorb.
     
  2. jcsd
  3. Nov 10, 2009 #2

    Born2bwire

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    Yeah, I have a paper by Art Hobson that I posted in the other thread where he states that they have been able to do the double slit experiment with buckeyballs.

    Man, is there anyway that a buckeyball can be even more awesome?

    EDIT: Here is the reference that he gives:
    Olaf Nairz, Markus Arndt, and Anton Zeilinger, “Quantum interference experiments with large
    molecules,” Am. J. Phys. 71, 319-325 (2003).

    I can't read it right now cause I'm at home. In fact, it's bedtime.
     
  4. Nov 10, 2009 #3
    Only if you think the wave function provides a complete description of a quantum system. Any simple hidden variable interpretation of QM, such as de Broglie-Bohm, gives a trivial explanatory picture of double slit experiments with molecules. See, there are molecules, which pass through one slit, and an accompanying wave, which passes through both. The wave develops an interference pattern, and the molecule - which is propelled or guided by the wave - ends up being pushed into the regions of constructive interference where the wave amplitude is big and away from the regions of destructive interference where the wave amplitude is small. Repeat a million times, and you have a set of interference fringes made of little dots where the molecules hit the screen.

    As Born2bwire said, they can even do this with fullerene molecules now (60-atom buckyball cages) so fullerenes are definitely quantum particles. And - according to p. 18 of http://www.tcm.phy.cam.ac.uk/~mdt26/PWT/towler_pilot_waves.pdf" [Broken] - since attaching fullerene molecules to the feet of fruit flies causes them to lose their ability to climb walls, that just about demonstrates that molecules are still there even if you don't look at them. Unless the fruit fly is keeping a very firm watch on its own toes. Ha ha, very funny.

    I even read someone is planning to do this with a virus. Now that really would be amusing.

    It is becoming increasingly apparent that Bohr has no clothes, as the fairy story said.
     
    Last edited by a moderator: May 4, 2017
  5. Nov 10, 2009 #4

    DaveC426913

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    I've never heard it described this way. So you're suggesting that the molecule only passes through one of the slits, but is pushed into one of the "bright areas" by the wave.

    But wait - the bright areas are determined after-the-fact. Whereas the molecules chose a slit before-the-fact.
     
  6. Nov 10, 2009 #5
    The de Broglie-Bohm pilot-wave interpretation gets discussed endlessly around here (though I admit that if Demystifier and I were run over by buses it would be considerably less). Do you mean that whilst writing over 8000 posts, you've never noticed? I don't know why I bother.
    That's right.
    Not so - think about it. Repeating the single-molecule experiment one million times with identical state preparation means that each time the system has the same wave function. The point about hidden variables is that they make each member of the ensemble different (if you think the wave function is all there is, then you mistakenly think they're all the same). The molecules - the positions of which are the hidden variables - are distributed initially according to the square of the wave (don't worry about why for the moment). They thus have a range of starting trajectories available to them, hence the random scatter when they hit the screen. Though initially the strikes on the screen appear to be random, over time there are more strikes in the region where the bright fringes will form - because the molecules are being guided by the wave field - and the pattern becomes apparent.

    I mean have you never seen a video of the pattern forming over time? Towler (the guy who wrote the lecture I referred to in my previous post) has a copy of one of these on his pilot wave http://www.tcm.phy.cam.ac.uk/~mdt26/pilot_waves.html" [Broken] (albeit for electrons rather than buckyballs, but the principle is the same).
     
    Last edited by a moderator: May 4, 2017
  7. Nov 10, 2009 #6

    DrChinese

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    Hopefully this will not become a thread about dBB.

    Regarding your question: I have seen papers showing all kinds of atoms (as well as buckyballs) can be sent through the double slit apparatus. Similarly, they have demonstrated entanglement and other quantum phenomena with atoms. Obviously, all nuclei heavier than hydrogen will be composite systems and in that sense similar to a molecule. I think the point is that you can see quantum effects like those if you know how to set up the experiment given the characteristic wavelengths.

    Do you need more references? The buckyball one should pretty well be convincing.
     
  8. Nov 10, 2009 #7
    This is not a thread about deBB. I am simply using deBB to answer the guy's question. It's not illegal is it?

    And note that I did answer it. On the contrary you merely repeated what he said (effectively "yes it is possible to do double slit experiment with molecules"). I'll let him draw his own conclusions.

    I'm actually quite surprised you would say this. You know perfectly well that Copenhagen forbids answering conceptual questions on principle, so when a guy asks how the reality of a quantum process can be understood, I am in effect forced to use deBB. Do you know any other viewpoint which can give a conceptual explanation of such processes?
     
    Last edited: Nov 10, 2009
  9. Nov 10, 2009 #8

    Vanadium 50

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    Why is a molecule - a composite object - any harder to comprehend than an atom - also a composite object? Or a nucleus - also a composite object? Or a proton - also a composite object?

    We're surrounded by composite object, and the same laws of mechanics apply to them as to elementary objects.
     
  10. Nov 10, 2009 #9
    It isn't any harder to comprehend. But the point is that in Copenhagen and similar viewpoints you can't comprehend interference processes involving either elementary objects or compound ones. In deBB you can comprehend both.

    There is also the question of where the dividing line is between the Copenhagen classical objects and quantum ones. Pollen grains? Bits of dust? Viruses?
     
  11. Nov 10, 2009 #10

    DaveC426913

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    I am well-aware of the double-slit experiment and the interference pattern created. Also well-aware how it forms over time. I know it can be done with electrons. (I can accept that an electron is both a particle and a wave.)

    I had just never realized that the principles could be extended to an object of comparatively macro scale and complexity.



    Getting my head around it. I guess I assumed that the wave-particle nature would be washed-out when constrained by a bunch of other atoms.
     
  12. Nov 10, 2009 #11
    Fine. Then I don't understand your objection:

    "But wait - the bright areas are determined after-the-fact. Whereas the molecules chose a slit before-the-fact. "
     
  13. Nov 10, 2009 #12

    DaveC426913

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    I can deal with the idea that an electron can be considered as a diffuse wave packet, whose wavefront can pass through both slits.

    I'm having trouble conceiving of a fixed, structured molecule being smeared out.
     
  14. Nov 10, 2009 #13
    I know you are, which is why I'm telling you that there is an alternative viewpoint - perfectly consistent with all experimental data and with the predictions of quantum mechanics - in which the molecule is just a bunch of discrete particles. It isn't smeared out at all. The reason you do not see the two classical clumps of spots immediately behind each slit - but rather, a 'set of interference fringes' - is just that the accompanying wave exerts an additional force on the molecule so that its trajectories are not the usual rectilinear Newtonian ones.

    Wave-particle duality now has the simple meaning that there is a particle and a wave.

    And if you now say, well this is all pointless unprovable metaphysics and one can never really tell whether particles are there if you don't look at them, then just look at how you and Born2bwire started this thread, with gee-whiz statements such as:

    "the molecule itself is somehow passing through both slits simultaneously"

    "Man, is there anyway that a buckeyball can be even more awesome?"

    Fine, but be aware that this is a choice you are making. The existence of de Broglie-Bohm theory is an explicit counterexample to the orthodox insistence that nature must be 'intrinsically probabilistic' and 'weird'. 'Molecules', in the normal human sense of the word, do not have to go through both slits. It is an interpretational artefact.. With a simple shift of perspective, QM can be reformulated as a dynamical theory of particle trajectories rather than as a statistical theory of observation, and whatever you think about the metaphysics of that, it's important to be aware that this is possible. Particularly as the answer to your question is then very clear.
     
  15. Nov 10, 2009 #14

    DaveC426913

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    Cool. Food for thought.
     
  16. Nov 10, 2009 #15
    It would be interesting to do double slit experiments on the simplest molecules and increasing their complexity until some sort of critical mass is reached where they stop interfering and behave as classical particles.
     
  17. Nov 11, 2009 #16

    Vanadium 50

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    Waht, it doesn't work like that.

    Quantum mechanics is, first and foremost, mechanics. It describes and explains the laws of motion (or, if you want to be a little more pedantic, the time evolution of systems) for all bodies - big and small, simple and complex. There is no scale at which QM "takes over" - instead, classical mechanics is an approximation that becomes progressively more and more valid as bodies get larger and larger.
     
  18. Nov 11, 2009 #17
    So why does Bohr have to presuppose the existence of a classical world containing measuring instruments?

    It is only in deBB where one can formally derive the classical limit from quantum mechanics (with the classical Hamilton-Jacobi dynamics emerging naturally out of the Schroedinger equation as the wave component of matter becomes passive).

    You cannot deduce a classical theory of matter from a statistical theory of observation i.e. from any solution of the Schroedinger equation in any limit, even well-localized ones (packets) that approximately remain so over time. You have to supplement the pure theory of linear fields by a physical postulate (like in pilot-wave theory) or you can't claim a material object is at definite x independent of measurement as in classical mechanics.
     
    Last edited: Nov 11, 2009
  19. Nov 11, 2009 #18

    Vanadium 50

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    I have no desire to get into a philosophical discussion. I am here to discuss physics.
     
  20. Nov 11, 2009 #19
    Then I'm here to help you. It's only Bohr and friends who requires philosophy to talk about the classical limit. With the deBB approach you can do it with er.. precisely defined mathematics. Isn't that kind of the point?
     
  21. Nov 11, 2009 #20

    Demystifier

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    The problem with this statement is that it is not known where exactly the borderline between physics and philosophy is, especially when one considers fundamental questions.

    For example, your own statement "Quantum mechanics ... describes and explains the laws of motion ... for all bodies" is very vague. What is a "body"? Is body a wave function? Or is body an object with a well defined position? If quantum mechanics (QM) only gives probabilities, then is it really correct that QM describes and explains - MOTION?
    You probably think that such questions are philosophical, but the fact is that without giving a clear answer to these questions, your own (allegedly physical) statement has no clear meaning. That's why you need some "philosophy" in physics.
     
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