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Picturing wave/particle duality

  1. Dec 27, 2004 #1
    I need some help understanding the nature of the wave in terms of QM. Many articles and papers seem to use the metaphor of a wave in water, though admiting its inadequacies. And I've seen models from victorian times that showed the wave in terms of a leading edge following a helix like pattern. This latter is obviously inadequate to explain youngs double slit results in terms of achieving interference patterns. So the water one is more useful but it does have the problem that its medium is kind of two dimensional (removing time for the time being), and the wave function itself provides the extra dimension, whereas the reality being described has, from 'our' perspective, a three dimensional medium, and the wave itself creates the fourth dimension we call EM. So we can adapt our imagination and see waves emanating from a point almost as if that point is a planet 'radiating' its atmosphere away in a pattern thats almost like breathing. Of course breathing requires an inward breath and so thats only really a good visualisation for something like the Transactional Interpretation.

    But lets forget about dimension for now, after all the maths seems to suggest there are more than space and time, and focus on that old victorian idea of a particle being the leading edge of the wave. We are imediately confronted with a problem. The particle does not go through both slits. When you see the sea, its not uncommon for waves to "break" along a lateral line, it always seems to be right to left but thats probably just the beaches I've been to. But a wave that is breaking has far more effect on a body it meets than one that is just a swell. So in a detector placed at the slits, if the wave in water analogy holds this far when ignoring dimensions, then the detector would only detect the part of the wave that is "breaking".

    I realise these analogies are simplistic, in fact I'm working backwards from the fact light doesn't experience time and the concept of extra dimensions, so simplistic is the best I can do in terms of a way to think about these things. It really gets exciting when you consider Neutrinos that travel as the speed of light and so don't experience time, yet still change as they move.

    Of course there is something about symetry that seems plain wrong with the idea of a rolling wave front. But if you accept the possibility of extra dimensions, and how crude the visualisations presented even in the best papers are on the actual mechanics of whats happening in something as simple as the double slit experiment, then I'd prefer to think that the macro is full of clues about the micro.

    Now we must bring time back in. For EM of any type it doesn't exist. So why do all QM interpretation rely on it so much ? To be fair the Transactional Interpretation is bold enough to push time's arrow out of the picture in considering the whole concept of advanced and retarded waves. And Bohm didn't seem to concerned with making his theories relativistically invariant. But surely a theory on something that doesn't experience time should start at that point, rather than our own perspective where time exists ? Then we can work towards a way of seeing things that do experience time in a fuller way.

    We know that the maths, the quantum formalism, is good. And we know that all the interpretations have no real explanatory power in terms that einstein could explain to his waitress. Why are people so attached to the copenhagen interpretation that has spawned so many "New Age" misinterpretations, when those misinterpretations are mainly due to the fact it doesn't explain what happens in the double slit experiment. Observation in terms of physical measurement is an input into the system - of course. But maybe even when the "particle" is detected at one of the slits, its 'wake' also travelled through the other slit ?

  2. jcsd
  3. Dec 27, 2004 #2
    You can't picture it, ever. They have all the properties of waves until you analyze it and then it behaves as a particle. The closest you'll ever come is to visualise it as a packet of waves spread over a finite volume. The point is the wave/particle duality means it either a particle OR a wave. It can change between them but it's not both at the sametime.
  4. Dec 27, 2004 #3
    No wonder you support Fulham :)
  5. Dec 27, 2004 #4


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    i must chime in and say i think it is extremely difficult (especially for the western mind) to imagine an object that holds two contrasting properties simultaneously. it is much easier for us to view things as 'either, or', not both.
    i think this is key when trying to visualize electromagnetic waves.
    the problem i have when trying to do this is i see the waves only projecting from a 2d level.
    ex. imagine my body is a transmitter and i hold my arm out at 90degrees and this is where the waves emanate. i know this is not the case, and in fact (i suppose) the waves are projected at all angles into space .
    its very tricky
  6. Dec 27, 2004 #5
    Hi Et

    That was why I refered to Einsteins waitress. Surely its mainly "tricky" because we don't understand it ?

  7. Dec 27, 2004 #6
    i think I must agree with Fulhamfan, but I would like to add that it is in the state that you want to observe it in. This doesn't mean that it's both, it just means that it is whatever you are looking for. If the problem is seeing it as both, don't try.
  8. Dec 27, 2004 #7
    Quid pro quo

    Hey you physics people are getting really deep and cosmic sounding man!

    If the problem is trying to avoid understanding things, then don't try.
  9. Dec 27, 2004 #8
    It's not that we're trying to avoid it. It's that we do understand that we don't understand.

    You could take the view of string theory where all particles are vibrating strings of pure energy. That would explain alot about why a partcle would diffract but doesn't explain to me about why the other slit would affect.

    Or there's another view where it's interacting with particles in another universe. I think I heard that Stephen Hawking takes this view seriously.

    Then there's feynman's interpretation. where a particle not only goes through both slits but also take an infinite number of other paths. Meaning a particle going through one slot is as plausible as a particle going to the moon and back. All paths cancel out except for the one you observed for the experiment.

    I think the most plausible is feynman's where if you think of it as a wave that propagates everywhere, all paths cancel out except one.
  10. Dec 27, 2004 #9


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    hi SimonA

    Neutrinos don't travel at the speed of light. They have mass.

    It is in the nature of a localised particle that it travels at a
    speed less than [itex]c[/itex]. As you know, noncommutativity of
    observables is fundamental to QM. For example, between
    [itex]\mathbf{x}[/itex] (static) and [itex]\mathbf{p}[/itex]
    (dynamic). It is therefore quite tricky to describe this without
    putting time in.....

    ....but as it happens, there is a way to interpret things where
    quantum information 'generates its own spacetime'. Consider the
    qubit: this is simply [itex]\mathbb{C}^{2}[/itex] as a Hilbert
    space with basis [itex] | 0 \right\rangle[/itex] and [itex] | 1
    \right\rangle[/itex]. According to:

    A modular functor which is universal for quantum computation
    Michael Freedman, Michael Larsen, Zhenghan Wang

    one can associate to the state space of a quantum computer built
    on k qubits, given by [tex]( \mathbb{C}^{2} )^{\otimes k}[/tex], a
    disc with 3k marked points, on which there is naturally an action
    of the braid group [itex] B_{3k} [/itex].

    In other words, one can use Jones polynomials....the same knot
    invariants that get used to describe so-called pure quantum
    gravity in three dimensions (that's one time and two spatial

    Now recall that the 'wave' is to be interpreted as a probability
    density. If we can remove the space on which the wavefunction is
    specified, then the probability density does not rely on the
    background a priori, but defines the geometry that we observe.

    If we have two slits, we should expect to see evidence that the
    geometry is 'wavelike' because that is a constraint that we have
    put on our observation.

    Don't know if this helps.

  11. Dec 27, 2004 #10

    You are a gentleman (or woman), and a scholar :)

    This is great. I hope to understand it better soon!


  12. Dec 27, 2004 #11
    Hi FulhamFan3

    Does "weak measurement" change your fundamental adherence to CI at all ? And where exactly is the observer relevant except in terms of the detector made of matter ?

  13. Dec 27, 2004 #12
    I don't think the wavefunction of a particle and the physical wave of a particle is the same thing, right?
  14. Dec 27, 2004 #13
    What is "the physical wave of a particle " ?
  15. Dec 27, 2004 #14
    What do you mean by "weak measurement"? If the experiment is setup as particle source aimed at 2 slits then the observer is at a screen past the slits taking note of where those particles land.

    Do you want another interpretation because you think you can predict the result otherwise?
  16. Dec 27, 2004 #15
    Ever seen this ?

  17. Dec 28, 2004 #16
    Although it is not necessary to have a picture or model for everything in physics, still it is better to have one. It only helps in its progress. Noharm in trying to visualize things. And there's nothing that can't be visulaized.

  18. Dec 28, 2004 #17

    Hi All:

    What helped me to obtain a satisfactory mental image of this conundrum was remembering Einstein's remark that matter is just lumpy bits of spacetime, the lessons in Feynman's book, " The Strange Theory of Light and Matter", the concepts of loop quantum gravity and the shapes of the "curled up" dimensions of M Theor and Born's interpretation of the Schroedinger/Heisenberg equations ( the wave packet is the particle). Bearing in mind from the teachings of field theory that the wave equation's expanse is infinite, albeit asymtotically diminishing, that a particle can "smell" the area surrounding it (Feynman) becomes more comprehensible. This view also provides a model for understanding non-locality events. Of course, this all requires that we abondon the zero dimensional concept of a point, but nothing ventured, nothing gained!

    Dennis Plews
    Sarasota Fl
  19. Dec 28, 2004 #18
    The "Wavefunction Colapses" when we do a measurement.. what exactly is a measurement?

    If in the double slit experimet we just turn off the Wich way detectors at the slits, but we leave them there, the interference patterns disapear too??

    another question, inside of a living cell, what dominates? quantum or clasical physiscs???????
  20. Dec 29, 2004 #19
    Thanks Mr Plews for your thoughts.

    Burnsys - "a measurement" is what is required to detect a property of the "particle(s)"

    On your second point, apart from "weak measurement" which I believe is still controversial, detectors are matter which will absorb the "particle" whether or not they are "turned off". Its a good point though in terms of some of the interpretation of CI you get.

    On your last point, both quantum and classical processes are at work. And in the middle are molecules such as proteins that move around a cell like the way cars move around a city. We have no idea really what drives them to do so.

    Someone recently introduced be to Shroedingers Negentropy -> http://www.i-sis.org.uk/negentr.php

    Last edited: Dec 29, 2004
  21. Dec 29, 2004 #20


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    OK, I just can't help but ask this: Aren't you the least bit uncomfortable or even weary, that you are basing your confidence of your ability to explain away this problem on an area that (i) are still FAR from being in an accepted stage (ii) are still not verified empirically (iii) are still undergoing major changes that is still nowhere near a final, coherent form? I mean, REALLY! It's one thing to read about loop quantum gravity and M-theory and "curled-up" dimensions, but to actually use that to comfortably explain away what is essentially a non-problem? That's astounding!

    There is no such thing as "duality" of light in QM. Don't believe me? Open a QM text and find where light is described in two different formulations. This duality arises due to our inherent NEED to catagorize something either as a wave, or as a particle, base on the classical wave and particle behavior. You are trying to fit a square object into a round hole. When it doesn't fit, you blame the hole, rather than look at the fact that what you're holding is not MEANT to fit into that hole. Once we all realize this, then we automatically see that there's no "duality" or any problem here!

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