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Correct interpretation of double slit experiment?

  1. Jan 30, 2016 #1

    Another me trying to grasp some physical experiment. I'm working myself through a Dutch popular science book "Snaartheorie" (String Theory) by 'our' professor Marcel Vonk: it's meant to give the reader a maths-less impression of the theories behind string theory.

    Vonk starts with an explaination of quantum principles: after giving an argumentation to prove the particle-wave duality, he moves on to trajectories in the quantum world. He explains that when we know the start and end position of a particle, we can't know its trajectory (he explains it without argumentation - I'm assuming none exists).

    Vonk then explains how a double slit experiment (which was previously covered) in which one particle at a time would still cause a pattern of interferention. This is how I think his reasoning works:

    When the particle arrives, we do not know what trajectory it took: there are equal possibilities of left and right slit. One could therefore interpret, since the trajectory cannot be recovered, that the interpretations of the particle traveling to both slits are equally likely, thus equally valid truths (which reminds me of the different valid truths of SR). The particle therefore travels through both, and interferes with itself.

    My questions are these:
    1. Is my interpretation correct and a valid explaination of what happens when we send individual particles towards the two slits?
    2. Other threads on PF show signs that this phenomenon cannot be explained - I have assumed it can be since Vonk makes an attempt. What about this?
    3. If I want to get a better grasp on this phenomenon, are there any terms or phenomena I should check out on wikipedia or youtube? Other threads speak of quantum superposition but so far that's just another word for me.

    Thanks for thinking along!
  2. jcsd
  3. Jan 30, 2016 #2


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    Physics cannot answer "why" questions a fundamental level. Even the existence of a fixed trajectory would be in disagreement with observations. And you cannot know something that does not exist.
    What exactly would be an "explanation"?
    Quantum mechanics in general. Wave functions and their resulting effects are quite fundamental in quantum mechanics.
  4. Jan 30, 2016 #3
    What I'm generally looking for is some more fundamental principles than just the observation made during the experiment (like when most people learn SR, they start with the postulates and work out the rest from thought experiment). Some people in this thread https://www.physicsforums.com/threads/how-does-a-particle-interfere-with-itself.14254/ made it appear like there is no reason why particles would interfere with themselves.

    In summary, is this statement true?

    "As quantum mechanics makes it impossible to determine what slit the particle travels through, and the odds are equal, we must assume that it travels through both simulteanously."
  5. Jan 30, 2016 #4


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    There are no odds that could be equal. It goes through both slits.

    The observation that wavefunctions can describe particles doesn't have a known deeper explanation. Interference with itself follows from this fundamental concept.
  6. Jan 30, 2016 #5


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    This is not universal explanation as photons from two independent lasers can interfere as well.
  7. Jan 30, 2016 #6
    Yeah I get that concept, and I understand how experimental evidence leads us to believe in the wave-partical duality.

    But how could one take that 'law' (that particles can be interpreted as wavefunctions) and describe how a particle may interfere with itself? How would you, for example, write a text/thought experiment explaining that a particle interferes with itself, making use of a few fundamental concepts of quantum mechanics without taking this self-interference as a fundamental concept?
  8. Jan 30, 2016 #7


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    The evolution of the wave function automatically includes things that are called "interference".
  9. Jan 30, 2016 #8


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    I think you will find the following enlightening:

    Then you can read:

    It's not not quite what the popularisations say. But you need to reach that conclusion yourself.

    The interesting thing is once you become familiar with it starts to make a sort of sense:

    You may even come to believe, as I do, it's not weird unless present as such.

  10. Jan 30, 2016 #9


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    Or not ascribe any property such as travelling through slits unless you observe such. That's the real key to QM - don't ask classical questions or make classical assumptions. You go down a rabbit hole if you do.

  11. Jan 31, 2016 #10


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    That was Bohr's attitude. "There is no quantum world. There is only an abstract quantum physical description. It is wrong to think that the task of physics is to find out how nature is. Physics concerns what we can say about nature." Of course, that doesn't mean it didn't strike him as strange. He also said, "Anyone who is not shocked by quantum theory has not understood it." Whether or not something is "strange" depends on the perceiver.
  12. Feb 1, 2016 #11
    There is a lecture given by Feynman that explains this quite nicely. It's part of the 'Charatcer of Physical Law' lectures he gave at Cornell.
    The link: .
    It's also worth watching the rest of the series. They're absolutely brilliant.
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