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Wave(ing) Particles?

  1. Apr 18, 2005 #1
    Concerning one of the fundamental quantum fysical difficulties about wave / particle duality I came to some questions...

    Following the result obtained from Youngs Two slit experiment, photons create an interference pattern if no measuring apparatus is used at the slits. Also electrons or neutrons form an interference pattern if shot at the two slits, thus proving they also must be made out of waves.

    If ! you use a measuring tool to detect the passing parts at the slits, and gather the information at a useful way, you will see that no interference pattern will build up at the detection screen. Thus, following QM, the wave function has collapsed and formed a particle due to measurement at the slits...

    What I don't understand is that this wavefunction is a propability function, not a function that defines the path of an actual existing part, thus by quantum mechanics, implying quantum superposition. In other words, roughly said, if not measured, the photon or other particle is in a state of superposition where it effectively is smeared out as a possibility, and NOT as a fysical existing part of matter, anywhere.

    This was the context, now my question;

    Why can't particles be fysically existent all the time, travelling not by straight vectors( what is implied if assumed that light acts as a particle), but following their wave function? In my eyes, this would explain both particle and wave properties, and avoid the abstractness and weirdness of actual superposition.

    And Why is it , that if a measuring apparatus is used at the slits, that imply a collapse of the propability wavefunction, and creation of the particle (cause no interference is seen anymore), the real particles STAY real particles till they have reached the detection screen?
    In other words, why cant the particles formed out of the collapse of wavefunction at the slits be converted automatically back into waves once they left the measuring apparatus on their way to the detection screen and althus create an interference pattern (which doesnt happen)???

    Hope I asked some clear questions,
    Many thanks,
    Last edited: Apr 18, 2005
  2. jcsd
  3. Apr 19, 2005 #2


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    I no doubt could use a refresher course in this area, but I think you are concluding something that is not true. What evidence is there that the particles STAY particles between the one slit and the screen? Even if you force a photon through one slit, the diffraction pattern on the screen is evident. In wave theory, that pattern is developed by considering the propegation of wavelets originating at all points within the slit propegating in all directions and interfering with one another. It is slightly different mathematics, but is the same basic analysis as two slit interference.
  4. Apr 19, 2005 #3
    You (DMuit) must realise that these objects (electrons, photons, in fact everything) are all quantum particles; and quantum particles are neither the classical wave or the classical particles. The rules quantum particles obey are given by the rules of quantum mechanics.

    Stop regarding the quantum world classically!
  5. Apr 19, 2005 #4
    In the particle wave duality , particle means finite amount of energy. In QM a particle does not need to have finite spatial boundaries, i think you know why ? :wink: Read the Introduction to QM of Bransden and Joachain. They clearly explain why a particle cannot be seen as an entity with finite spatial boundaries like in classical mechanics. That is a common misconception and i recommend this book if you want a clearer view. Moral of the story : a QM 'particle' is no classical 'particle'

  6. Apr 19, 2005 #5
    Well, after reading some books on quanta physics and QM, which go on and on with various experimental results and analysis. My understanding is: matters are particles, but they obey the wave mechanism to move.
  7. Apr 19, 2005 #6
    They don't even need wave mechanisms to move - all objects can be described quantum mechanically, which has aspects of classical particle and classical wave descriptions, but is neither. All matter is quantum-mechanical -- and this is not the classical wave nor the classical particle.
  8. Apr 20, 2005 #7
    Well. Your answer is QM is QM. But in fact we have to understand it from a viewpoint which is not itself. And we did.
  9. Apr 20, 2005 #8


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    That is essentially Bohmian mechanics.

  10. Apr 20, 2005 #9
    Do we have to understand it from a viewpoint which itself is not? That's like saying understand reality from a viewpoint which is not itself real. What's the point in that?
  11. Apr 20, 2005 #10
    Sorry for my poor English. It is hard for me to express the ideas.
    For example. If you ask the question: What is force? The answer force=ma helps nothing. So do QM=QM. The way to understand QM is comparing it to what we HAVE known. And what we have known are particles and waves, whcih are our standpoint to investigate.
    Of course, QM itself is QM.
  12. Apr 20, 2005 #11


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    The standard answer to your question is: you must look at the formalism of QM. QM is a theory which is properly expressed mathematically, as is General Relativity. The words used to describe the theory are approximations which are sometimes useful shortcuts; "wave" and "particle" are such words. So it is easy to get caught up in the words when the formalism is what matters.
  13. Apr 20, 2005 #12
    This is precisely the kind of point I was trying to get across, but alas, I failed.
  14. Apr 20, 2005 #13
    Yes. QM has its own formalism. But it's different form GR, which is beat all but not impenetrable.
    At the same time, "wave" and "particle" are not just shortcuts. What are shortcuts? Definition is maded by words, all words are "shortcuts". QM is not a mathematical system. A mass of matrixes, bras and kets say nothing. I think at the beginning of your studing QM, you explored it from its formation but not its formalish.
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