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B Wave-particle Duality Help

  1. Jun 29, 2017 #1
    After reading on the basics of this & watching videos, here are some questions I can't help asking.

    When the wave is travelling in the room that is filled with air, will the photon interact with all the air
    molecules continuously collapsing the wave aspect?

    As I understand measurement/interaction collapses the wave aspect of the photon. So wouldn't the actual wall itself with the slits do this preventing the wave passing through both slits?
    Looking at the videos of the double slit they show some kind of wave that is spread out across the whole wall with the slits & yet interfere with itself on other side much like a water wave in a pond. In the case of the water wave the whole wave looks to literally interact with the wall & slits just as a beach wave strikes a long stretch of barrier. But with the photon, the wave behavior is on a scale comparable to the two slits.

    Even so, the middle of the slits is solid matter & looks to cut the wave, and surely the edges/boundary of the wave & the slits themselves are not precise but peter out. If so I have a hard time seeing how the wave picture alone can account for interference. Shouldn't the edges of the wave interact with the edges of the slit?

    How does a buckyball of 60atoms act like a wave? As wave collapse happens by interaction, like when locating an electron with a photon. Yet aren't such interactions continuously happening with & within the molecule itself?

  2. jcsd
  3. Jun 29, 2017 #2


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    Not every interaction causes collapse of a quantum state, and even when it does, there can be many ways how the collapse can happen depending on what observable you're measuring. If you assume that collapse happens in every interaction, you'll get absurd results, i.e. if a nitrogen molecule in air were to go into a position eigenstate (state with definite position) after every collision with other molecules, you'd quicly end up with ##N_2## molecules that would have very large kinetic energies (large enough to break energy conservation) due to Heisenberg uncertainty principle.
  4. Jun 29, 2017 #3


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    It can display interference/diffraction patterns in experiments, see e.g. this page: Diffraction and Interference with Fullerenes.
  5. Jun 29, 2017 #4


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    Conceptually, what kinds of interactions do cause collapse vs don't?

    Is the discussion now interpretation dependent? Collapse itself is an interpretation of an experiment as opposed to a direct experimental result, is that right?
  6. Jun 29, 2017 #5


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    Any discussion involving collapse is necessarily interpretation-dependent, because collapse itself is an interpretation.
    Not quite. The opposite of "interpretation" is "part of the mathematical formalism", not "seen in experiments".
    Last edited: Jun 30, 2017
  7. Jun 30, 2017 #6
    "As I understand measurement/interaction collapses the wave aspect of the photon."

    If by `collapse' you mean that measurement projects the wavefunction onto an eigenstate, then we have entered the realm of interpretation. One can use quantum mechanics to make predictions without having to suppose that such collapse of the wave function ever happens.

    But what is true is that, the more the photon interacts with its environment, the interference effects will become smaller and smaller, very quickly becoming negligible. As interference effects become smaller, the less are the "wave-like" the system becomes. This process can be explained and described without the need for a special class of collapsing interactions called `measurement' that do not obey the Schrodinger equation. Decoherence explains and tells us what it is about the environment that so quickly supresses interference effects.

    These days, some working physicists use `collapse' as a name for this process rather than as the novel and distinctive dynamical process that von Neumann introduced. Empirically, decoherence and von Neumann collapse are, for all practical purposes, equivalent. Is decoherence enough to solve the measurement problem? That's unclear -- but it's enough to get the right physical results without the need to posit interactions that are not described within quantum mechanics -- so von Neumann collapse becomes merely an interpretive issue.
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