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Double Slit Experiment Misinterpretation

  1. Aug 13, 2014 #1
    For some time now I have been absolutely fascinated with quantum mechanics. Unfortunately for me, I am well aware that what I know of QM comes from over-simplifications specific to educational reading material and videos where a core goal of publication was to avoid the actual mathematics as much as possible, as to not turn off 99% of the viewers. These simplifications have resulted in me making what I think is a serious 'mistake' while thinking over the double slit experiment, the results, and their implications. I am hoping that someone here can put me in the right direction.

    Up until now, I have interpreted that during this experiment as follows:
    If observed, a particle will pass through 1 slit or the other, and no interference pattern will show on the backdrop of the experiment which maps the 'landing' point of the photons or electrons. If, however, the particle is not observed, it appears to pass through both slits at the same time (like a wave) and interfere with itself , thereby causing the interference pattern.

    Obviously the above is a complete simplification of what is really happening, but it also seems completely incorrect and impossible. This, however, is actually how it's explained in alot of descriptions of the experiment online.

    What made me question this, aside from the description obviously conflicting with our current view of the behavior of matter at our own scale, was a more refined version of the experiment which i read about. In this version, particles were fired one-by-one at the 2 slits. Each particle left only a single mark on the backdrop, but if it was not observed which slit the particle passed through and enough were fired the same interference pattern would emerge. Once again, if observation device were installed the interference pattern would cease. But if we keep observers at the slits out of the picture, thereby retaining the interference pattern, we still only have 1 particle hitting the backdrop at one place. Its only when an enormous amount of particles are fired and the overall pattern of contact with the backdrop is analyzed that the interference pattern is noticed. This brings me to my question.....We know what were seeing is identical to an interference pattern, but what is actually interfering with what? It seems more reasonable now, after seeing the experiment where particles are fired 1 by 1, that the particle is not interfering with itself but the waves of probability which describe which path the particle will LIKELY take are actually interfering with themselves to cause a pattern to emerge in the reality of where these photons land. This takes me away from the idea presented in so many videos I've seen, that matter exists as a wave of probability that interferes with itself and only solidifies when observed by a conscious being (which always seemed wrong to me), and draws me to the idea that 2 or more potential futures can actually interfere with each other. I envision that at the moment the particle leaves the source, also emitted are the waves of probability for which path the particle travels and the particle travels along these waves. Since the waves interfere, so the particles show it in the pattern that emerges only if enough particles are fired. Aside from the question of can probability travel faster than light, Does anyone even know what's interfering with what? The whole part where observation collapses the wave function is a secondary problem from this perspective which I cannot contently ponder over until I know that answer :) Thanks in advance for your time!
    Last edited: Aug 13, 2014
  2. jcsd
  3. Aug 13, 2014 #2
    The wave isn't really a wave of probability. Probability is always positive, so that wouldn't create any cancellations. All we really know about the wf is that it's a pattern that assigns a complex number to each point in space at each time.(also to each momentum etc.) If you try to detect the particle at some point, the probability of it showing up is the square of the modulus of the wf value for that point.(modulus: if your complex number is a+bi, you position it on a plane, a units to the right of the origin and b units up. The modulus is the total distance from the origin, the hypotenuse of the right triangle) Why this weird mathematical trick works in real life, and how to describe the particle during the time when you're not measuring it, and whether the wf corresponds to anything real, are not part of the theory. There are all sorts of suggested models and interpretations, but they are all speculative, so it's important to keep them separate from QM itself, which does nothing but predict probabilities for measurements- and does that with incredible success.
    Many scientists don't even care what QM actually means. Either all they want is a tool to work with, or they think that trying to interpret QM is hopeless. I think that's a great tragedy. I see science as man's quest to understand his world. That's the only reason people care deeply about it. We have no right nor ability to abdicate that quest.
    So here are the names of the leading interpretations, in order of popularity among physicists(I think): Copenhagen interpretation (comes in many versions, but sorry I can't sort them out for you), Everett many-worlds interpretation, Bohmian Mechanics, objective reduction models (Grimaldi-Rimini-Weber's, or Penrose's). Go ahead and figure out what makes the most sense. But don't forget that what we actually know is just the math.
  4. Aug 13, 2014 #3


    Staff: Mentor

    The explanation of the double slit is explained by the formalism of Quantum Mechanics.

    Things like the above, such as it behaves sometimes like a particle and sometimes like a wave are wrong - or rather what is said to students just starting out so they can get a bit of a grasp to begin with.

    That is then used to motivate the formalism of QM, but what they then should do is go back and see how that formalism explains the things that motivated it like the double slit.

    When you do that you see wave particle duality etc really has nothing to do with it.

    To that end have a look at a correct analysis of the double slit:

    The math may be beyond what you know, but hopefully you can get a feel for what the real explanation is.

    I believe for starting out in QM its better to begin with its conceptual core that the usual semi historical path of the double slit, wave particle duality etc:

  5. Aug 13, 2014 #4


    User Avatar
    Science Advisor

    Sure. Actually you can think of the particle interfering with itself in both the single and double slit cases. As you point out below, it is more accurate to say that the particle is represented by a "probability wave " (as maline points out, it is even more accurate to say the "square root" of a probability wave), and it is that wave that interferes with itself. The particle does not have a definite position or momentum at every point between the slit and the screen, and is only observed to have a definite position when the measurement is made.

    In fact these two ideas are not opposed. The wave interfering with itself, and only producing a definite position when it is measured is the more general picture. However, one can also loosely and usefully think of the paths intefering among themselves - this is Feynman's path integral picture.
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