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The double slit experiment question?

  1. Mar 18, 2014 #1
    Hello,

    Very recently i learned about the quantum theories and the double slit experiment.. honestly i was startled from what i have seen and heard of the quantum laws known for us at this time..

    Well i can't help it but my mind keeps asking zillions of questions and it's not stopping.. :P

    Before asking anything theorethical i first want to be certain about the results of the experiment.

    Question 1:
    What actually happens when the electron hits the screen behind the slit?
    First case- when no one is observing the experiment the electrons should be in waveform like.. right?
    Forming disturbance pattern on the screen behind the slit.
    Second case- when observation is present the electron hits the screen as it was particle like..
    Third case- observations were performed just before the waveform hits the screen (not before when the electrons were fired from the electron gun but right before the moment of the hit)- and than suddenly the electron transformed back to particle alike back from waveform like, and as i am informed the electron that was observed in that given moment had particle alike properties just like if it was observed from the very beginning, as if the time turned back and changed it properties as if it was observed from the beagining when the electron gun fired it..

    So.. what actually happens here? How did we actually measure the second case scenario?
    How do we get wave pattern alike on the screen? Becase as i understand the experiment.. when we observe the experiment the electrons are with particle like properties.. so how do we get the wavelength disturbance pattern on the screen? When actually we don't see it??
    Or the screen is somehow sensetive? So when we dont look at it the electrons are leaving marks on the screen so thats how we know there is waveform pattern??

    I need this question..confirmed.. and i may have a interesting theory :)

    thanks in advance!
     
    Last edited: Mar 18, 2014
  2. jcsd
  3. Mar 18, 2014 #2

    bhobba

    Staff: Mentor

    QM is basically a generalised probability model about the outcomes of observations. When it hits the screen you are observing its position, just like when you flip a coin and look to see if its heads or tails. The difference is we know what the coin is doing before it lands heads or tails - its spinning around - what its doing in QM is anyone's guess - the theory is silent about it - we don't even know if its a question that needs answering - nature may simply be like that. Note a key point. Observations are 'marks' that occur here in the common sense classical macro world. They exist independent of us regardless of if its being observed or not. The assumption is, exactly as it is in everyday use, if a tree falls in a forest and there is no one to hear it then it makes a sound in exactly the same way if someone heard it ie the observer has nothing to do with it.

    It makes no difference if anyone observes it or not. The only difference is if what hole its going through is observed - then you get no interference effect.

    This is one of the issues we have in the usual way QM is taught, and presented in popularisations. The so called wave-particle duality, which is actually not true, is used to motivate QM, but once that's done QM is not then used to explain things like the double slit experiment that motivated it.

    IMHO its better to go right to the central core of QM first:
    http://www.scottaaronson.com/democritus/lec9.html

    Basically its an extension of ordinary probability theory. Such extensions go under the name of Generalised Probability Models and is an area of active research in math these days. QM is basically the most reasonable such model that allows continuous changes and that turns out to be equivalent to entanglement. But explaining all of that will require some deep math, however for completeness here it is:
    http://arxiv.org/abs/0911.0695

    Here is the correct Quantum Mechanical explanation of the Double Slit Experiment:
    http://arxiv.org/ftp/quant-ph/papers/0703/0703126.pdf

    The above should be done after the full machinery of QM is available in textbooks where QM is taught - but unfortunately usually isn't, and sometimes leads to misunderstanding even amongst those who have studied it in detail.

    Thanks
    Bill
     
    Last edited: Mar 18, 2014
  4. Mar 19, 2014 #3
    Of course if you measure the system just before it hits the screen (I'm thinking along the lines of measuring the system in this instance in a style mentioned in a book I don't have on hand at the moment - using an electron I think), you will see the system behave as a particle.

    This will determine that the particle takes one path from gun, determining an UNDETERMINED past which before was the system taking both paths (not classically though).
     
  5. Mar 19, 2014 #4

    bhobba

    Staff: Mentor

    First physical continuity demands if you measure its position then measure it immediately after you will get the same position. So yes, in the sense it has a definite position it appears particle like, but at no time does it behave as a particle in the classical sense.

    At all times it behaves like quantum stuff. One of the properties of the quantum objects you can do the double slit experiment with its position is an observable.

    What its doing when you are not observing it QM is silent about.

    Thanks
    Bill
     
    Last edited: Mar 19, 2014
  6. Mar 19, 2014 #5

    DrChinese

    User Avatar
    Science Advisor
    Gold Member

    We already have an interesting theory, it is called Quantum Mechanics. :smile: By the way, welcome to PhysicsForums!

    Seriously, the double slit rule is relatively simple to express: if you know - or could have known, in principle - which slit a particle passes through, then there is no interference pattern. There are numerous experimental setups which all confirm this. (Looking at the final results, as far as anyone knows, does not change the outcome. By definition, this can never be confirmed for any experiment.)

    I might recommend that you learn about these a bit more *before* you develop your own "theories". Please keep in mind that a theory which reproduces the predictions of existing theory is sometime called "ad hoc". And in the context of quantum theory, it is called an "interpretation".

    There are a number of quantum mechanical interpretations currently, none of which make any predictions different than the other. In practical scientific terms, there is no utility or value to those at this time.
     
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