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Confused about Quantum Mechanics and Causality

  1. Aug 11, 2010 #1
    Hi all,

    The following is probably a very stupid question, but could anybody please tell me what they think? It has been bugging me for quite a while.

    I'm a bit confused about the probabilistic interpretation of Quantum Mechanics. I can't see how it necessarily follows from the experiments I know about. Now, I know you will probably all start answering about how the double-slit experiment and the EPR paradox prove the probabilistic interpretation, but bear with me. I will explain why I am still not convinced.

    I am also confused about the idea of a wave-particle duality. I actually do not understand where the evidence for fundamental 'particles' is. It seems to me (at my small level of understanding) that all the evidence points to matter having soley a wave structure.

    The reason why I am so confused is rather complex, but I will try to explain it bit by bit.
    I would love it if any of you could prove me wrong! I am sure that I could not be correct. If I was correct it would mean many of the greatest minds of physics are wrong, and I don't believe that. But I would like to improve my understanding, and get my head around QM.

    Now, for the details.

    1. I don't see how the double-slit experiment proves any 'particle' nature of matter
    During the photon's (or electron's) interaction with the screen, it clearly goes from being spread, as a wave, over a large area to being localised to a much smaller area. But the electron is by no means reduced to a 'point particle'.
    And even if the photon is absorbed to cause an excitation in an electron in the screen, the electron in the screen is also a wave. So there would be no reason for the photon to have to collapse to some kind of 'particle' before being absorbed.

    • Thought Experiment: Double-slit experiment on electrons with the screen replaced by a conductive metal
    In conductive metals, the potential felt by the electrons is regular in all directions, and therefore the electrons (the outer ones, at least) exist not as matter-waves that are each localised about one nucleus, but as matter-waves spread through the whole material.
    If an electron matter-wave with a large initial spread hit a metal, there would be no reason for it to 'collapse'. How could it possibly collapse when the electrons already in the metal are not 'collapsed'?
    Similarily, if an electron from the double-slit experiment hits a regular crystal, it would not be collapsed. This is seen by the experiments that have been done in scattering electrons using crystals. The electron would be scattered by the crystal (wave phenomenon) rather than localised to a smaller area.
    Therefore, in order to understand why the electrons are seemingly 'collapsed' by the photographic screen, one needs to look at the properties of that screen.

    This question is taking me a lot longer to write than I though it would. I have been thinking about all this for quite a while. I will come back to finish it tomorrow.
    Last edited: Aug 11, 2010
  2. jcsd
  3. Aug 12, 2010 #2


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    What do you mean by matter having a pure wave nature? Pure waves dont have mass, while the mass of many particles have been experimentally measured and found.
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