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Two-fold question about wave function

  1. Dec 21, 2014 #1
    So let's say I shoot an atom from A to B. If it interacts with other atoms, electrons or photon's, the wave fucntion collapses right? And the particle exists in our 'normal' world, no quantum tunneling etc. And it looks as if it was a normal particle all along right?

    But let's say it goes from A to B. It would go through the slit in the double slit experiment. And it interacts with a photon. Will it spread out in a wave function again if it does not interact with anything between the interaction and the screen?

    And what causes a particle or photon to behave as a wave? What conditions have to be present? I know what makes a wave function collapse, but what conditions have to be met to create a probability wave function in an electron/atom etc.

  2. jcsd
  3. Dec 21, 2014 #2


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    Staff: Mentor

    That's not really right, although you'll see it described that way in books that are trying to spare you from the math.

    A quantum object always has some wave-like characteristics and some particle-like characteristics; which one you see more of depends on the details of the interaction you're observing. (Note that I am not saying that the particle is both a wave and a particle - it is neither, it is a quantum object with some characteristics of both. You wouldn't say that a sheep is both a table and pillow and discuss sheep in terms of "table-pillow duality" because a sheep has four legs like a table and is fuzzy like a pillow, so you shouldn't say that a quantum object is both a wave and a particle and talk about "wave-particle duality" either).

    Wave function collapse doesn't eliminate the waviness, even for a moment. It just changes the wave function in a particular way, one that is consistent with the measurement that caused the collapse. The wave function continues evolving from there according to Schrodinger's equation.

    (Also, you should be aware that although wave function collapse is a common way of describing what the mathematical formalism of QM is telling us, it is not actually part of that formalism. It is possible to do QM without ever thinking in terms of wave function collapse, and indeed that's the easiest way of attacking some QM problems).
  4. Dec 21, 2014 #3
    I supose I should really start learning all the math behind it then.

    But as you say, when something is measured, it basicly still is a wave function? Except we call it a particle because it is a very narrow wave? Is it still the same wave?

    Is that the reason that the uncertainty principle still counts for large objects?

    And if i want to understand this properly I probably have to understand the whole integral vector thing really well? And trigonometry as well?
  5. Dec 21, 2014 #4


    Staff: Mentor

    Everything is a wave function because everything is quantum - but its better to call it a quantum state rather than wavefunction - the distinction will become clearer when you learn the detail..

    If it wasn't destroyed by the observation, the observation is what is known as a filtering type observation which is a state preparation procedure. You have prepared the particle in a different state. If it was a position measurement then you have prepared the particle is a state of definite position etc. These days state and state preparation procedure are pretty much synonymous. Whether such a view resolved anything is another mater - but it certainly is a lot clearer.

    It counts for all objects, large or small, it doesn't matter, because everything is quantum. We simply don't notice it here in the macro world.

    I suggest 3 books:

    There are also associated video lectures:

    Last edited by a moderator: May 7, 2017
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