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Double slit electron experiment inquiry

  1. Mar 11, 2010 #1
    I have been trying to understand the result of the double slit electron experiment
    I’m still not clear abt the result of the experiment.

    I have seen the Dr. Quantumm - double slit experiment video(). Its very simple explained.

    it says when an measuring device is placed to look through which slit the electron passes when electron are shoot one at a time, it then doesn’t created an interference pattern but two band of lines but I have also read that when electron fired one at a time they randomly strike at the screen and produces the same interference pattern like a wave.
    So which is correct???

    Also, does a electron travel in a straight path like a bullet when acts like a particle or it travel randomly when fired one at a time?

    Last edited by a moderator: Sep 25, 2014
  2. jcsd
  3. Mar 11, 2010 #2


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    The Dr Quantum animation is misleading in that it gives you the impression that you can "just look" at the electron and find out where it is. A position measurement is an interaction between the electron and a measuring device that changes the state of the electron. So the "thing" that goes through the slits isn't exactly the same in the two experiments. When you keep that in mind, the results seem a bit less crazy.

    In QM (or at least the standard version of it), it's definitely a mistake to think that a particle has a definite position that you can find out by measuring it. Particles aren't localized until you perform a position measurement. The interaction between the measuring device and the particle is what localizes it to the region where the interaction took place. In general particles don't have positions. They are "smeared out" over some region, and a position measurement is an interaction that makes that region smaller.

    Your last question is only meaningful if we think of QM as a description of reality instead of as just a set or rules that tells us how to calculate probabilities of possibilities. I think it's probably the latter. But you will at least be near an accurate description if you think of the particles as "smeared out" over some region which can be made smaller or larger by interactions with other systems.
    Last edited: Mar 11, 2010
  4. Mar 11, 2010 #3
    thanks for the answer.

    does it mean,even if a single electron is fired it give into many electron over the probability region.

    also, if we position measure simultaneously over the probability region, would we find as many as we measure.

  5. Mar 11, 2010 #4


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    If you send a single particle into a region with lots of detectors, at most one of them will detect a particle. A photographic plate can be thought of as "lots of detectors", so what I just said means that a single particle can at most cause one dot on a photographic plate.
  6. Mar 17, 2010 #5
    thx for ur reply, I was reading more abt the experiment but didn’t find the result on this.

    As the pattern formed on the wall depend upon whether we have the information through which the particle has passed even if there is a detector at the slits(delayed choice - i.e if no one is observing the measurement made by the devices, the wavefunction doesn’t collapses).

    So what will be the pattern on the wall, if the decision to see the information on the detector is not yet decided?

    i.e if the decision to see the information or erase it is made only after we see the pattern.
    Last edited: Mar 17, 2010
  7. Mar 17, 2010 #6
    I'll start from the beginning:

    Basically in Quantum Mechanics (as we mere mortals understand it so far), particles having mass, like the electron, as well as with no mass, such as the photon, travel in what is called a "quantum wave function".

    In this "wave function", the location of the particle at any point through it's traveling process is dictated by a wave equation that can ONLY describe the PROBABILITY of the particle's existence at any given point and never locate it EXACTLY.

    As mentioned above, only when the particle hits something (like the wall) and stops moving, can it's definite position then be measured. Until then, it can only be inferred that the "wave function" passed through both slits, even though no single classical individual particle ( like one electron) could pass through both slits.

    It's frustrating to figure out I know, but its seems physics on a quantum scale seems to act very differently then what you or I would consider to be normal at our classical level. Perhaps the wave function inherent in all matter, as De Broigle suggested, is based on the fact that matter and energy travel in different dimensions that you or I cannot perceive or detect yet. But we do not know this for sure.....there could be a myriad number of reasons why "wave-particle" duality exist.
    Last edited: Mar 17, 2010
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