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Two-slit & n-slits?

  1. Nov 27, 2007 #1
    I'm trying to be 100% sure if I get this experiment right and it's complications as it obviously is a base to quantum physics.

    During the experiment and when electrons are fired one by one, immediately leaving the gun, an electron has an infinite number of choices/paths until it hits the screen.
    The electron gun makes all possible choices/paths to collect on one place which is the very straight path up to the screen as the momentum it gives to electrons requires so. Otherwise the electrons would spread out to universe equally, statistically.
    Electron is everywhere, not just on the screen where we have more likely to find than in any other places after it leaves the gun.
    Electron gun's mere function is to concentrate them on the straight path, again statistically. If we do not use ANY slits at all what we see is the wave collapse function of infinite possibilities but mostly and statistically on the very straight path. What seems as a chunk of electrons displayed on the screen is actually collapsed wave functions of infinite electrons, no patterns. As a matter of fact it's not "no-patterns", it's the collections of "n" wave patterns.
    When we use two slits, on the other hand, we eliminate all the other possible wave functions but allow these two to reach to the screen.
    That's why it doesn't matter if we fire them one by one or in bulk, their collective wave function is pre-determined, what we see is what we "choose" to see. If we choose to see only TWO of the "n" wave collapses, we use two slits, if three then so on.
    It's not that electron leaves the gun as a "single" entity but when it comes near to the slit it splits into two. It leaves the gun as "n" entity all following a different path and the two possible routes we choose to observe and these two paths' overlap we see on the screen.

    Am I right or am I right ? :)
  2. jcsd
  3. Nov 27, 2007 #2

    The dispersion of the electrons is not an innate part of the beam being generated by the electron gun that “collapses” if we “do not use ANY slits at all” by not even putting up the barrier. The dispersion is created by the barrier being in place with one of two slits, it does not matter the dispersion remains the same. What changes is that within the fixed ‘dispersion’ pattern of the photons we see another ‘interference’ pattern of light and dark spaces appear within the same area of dispersion.

    To get a better idea of the concept, suggest you work with the tool at: physics.northwestern.edu

    A description like:
    Is an attempt to put the issue into commonsense or semi-realistic terms that we can understand classically but cannot be verified by direct observation of what the electron is doing as it goes through the slit(s). Any observational measurement of it near either slit destroys the ‘interference’ pattern. Which is why QM in the “Copenhagen” view considers itself “complete” and attempts to describe what is happening between slit(s) and screen should be abandoned.
    Sometimes put as “Shut Up and Calculate” meaning just calculate QM HUP statistics of screen probabilities based on slit configurations as the best explanation that cannot be improved upon by any other means (and has not for 80 years).

    Although there are some of us like Einstein that hold out hope that a Local & Realistic description can still be found. Local Realists sometimes considered scientific “nuts”. Einstein was not called “nuts” but often considered as wasting his final decades in a vain attempt to discredit the Non-Local view as in QM as the final answer.

    So don’t expect to be 100% sure; the accepted and popular view is you need to “just calculate” without trying to use a “classic sensible” description, even an unrealistic multi pathed one.
  4. Nov 30, 2007 #3
    In the case of the double slit experiment, we do not know whether the electron passed through the left slit or the right slit, so we assume that it passed through both slits simultaneously. Each possibility being a "state", and because the electron fullfills both possibilities it is said to be in a "superposition of states."

    So MWI claims that the electron has two definite choices-either it passes through the left slit or the right slit-at which point the universe divides into two universes, and in one universe the electron goes through the left slit, and in the other universe the electron goes through the right slit. These two universes somehow interfere with each other causes the intereference pattern. So whenever an object has the potential to enter one of several possible states, the universe splits into many universes, so that each potential is fulfilled in a different universe.
  5. Dec 1, 2007 #4
    Well, I for one did find the OP difficult to read..

    I think he [the OP] was right.. he basically seemed to be giving the same story that Zee recounted about Feynman, in the first chapter of QFT in a nutshell.
    Last edited: Dec 2, 2007
  6. Dec 2, 2007 #5


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    Staff: Mentor

    I've split off a discussion about Afshar's experiment into this thread.
  7. Dec 4, 2007 #6
    I disagree, the Feynman questions about adding new holes and new screens are meaningless unless you have dispersion that spreads the likely locations of electrons across those holes. Thus the source of electrons Feynman refers to is from ether of the two slits or both. However, not the original beam exiting from the electron gun.

    So the answer to the OP is still NO.

    (jtbell - Good choice to split this thread)
  8. Dec 4, 2007 #7
    I have a problem with that statement.

    In the book "Introduction to the Quantum Theory" - David Park, Chapter 10, paragraph 10.1 he analyzes the double slit experiment and in the subsequent problem 10.1 he asks to consider what would be the pattern on the final screen if we detected, with "magical detectors" behind the slits, the simultaneous passage of the wave in both slits; with "magical detectors" he means detectors which would detect the passing of a particle without changing its wavefunction at all (he computes the case of one only "magical" detector in the paragraph).

    I have made that computations and my result is that the interference pattern vanishes.
    So, maybe we can't even say that the electron passes through both slits.
  9. Dec 4, 2007 #8
    The act of detecting or observing which slit the electron goes through forces the electron to go through one or the other slit, which is why the interference pattern vanishes.
  10. Dec 4, 2007 #9
    Ok. Now my question is: if you don't detect the electron in a slith or both, can you say that, however, the electron has passed through both slits? How would you prove your assertion?
  11. Dec 4, 2007 #10
    The only proof is by examining the interference pattern that is created and comparing it to quantum theory.
  12. Dec 5, 2007 #11
    Ok, but this proves that a wave has passed through both slits, not a particle.
  13. Dec 5, 2007 #12
    So when unobserved it is a wave and when observed it is a particle
  14. Dec 5, 2007 #13
    Yes, but we should be more precise, since you "observe" it even in the final screen when there is the interference pattern: you have detectors and "clicks" even there. So, where exactly is the difference? It's the fact that in the first case you put detectors so near the slits that you can establish which slit the particle has passed through, while in the second case you can't. So, it's not the fact that we have "observed" the electron, but that we have the information on its position near the slits. It's only this information that creates the particle behaviour, isnt'it?
  15. Dec 5, 2007 #14
    Exactly, until we go to detect which slit it has gone through, it is not even a matter of its location and which slit it goes through. There are four logical possibilities for the electron to go through: A, B, BOTH, NEITHER. You can systematically go through each route and eliminate each one. Design an experiment to test each route, put a total of nothing box in route A, and find that is has an effect on the particle, bout total of nothing boxes don't have an effect on things that pass through them so that eliminates that route. You can eliminate route B for the same reason. To see if it went through BOTH routes stop the experiment in the middle and will find that it is either on one side or the other but not both. Block both routes and nothing gets through so that eliminates that possibility too. You can eliminate each possible route it went through one by one, but that is assuming that it even makes sense to ask which route it has gone through. The question of asking which route it went through is just an inappropriate question to even ask.
  16. Jan 4, 2008 #15
    I have a slightly different question: What exactly is involved when we say the electron (or photon for that matter) is detected at one slit or another? How exactly does the apparatus interact with the electron or photon? Is there a kind of absorption and reemission of the detected wave/particle at the slit? Can anyone describe, in laymans terms? Careful, I'm a student of the arts, not of science!!
    Last edited: Jan 5, 2008
  17. Jan 5, 2008 #16
    I take it you are new to the forums here, welcome.
    Be sure to read the “sticky” posted threads at the top of each forum to be sure you understand what is expected in each forum by the mentors that monitor them.
    As to using the posting features, you can fix things like ‘absorbtion’ in #15 by using “edit” on your posting no need to waste a new post. I think you have 24 hours to make such edits. You can then even go to your #16 and edit it be deleting it ( I think my post #17 will turn into #16 I’m not sure) But you do have the chance to clean up typos etc. but only within the first full day.

    As to an art student layman description you wanted.
    Consider testing a thousand balls thrown at a wall and know that only 50 each will make it though any hole of a given size. With two holes you know a total of 100 go though to hit a target behind the wall and you cannot “see” which hole is traversed by any one ball. You set up vertical light beams above each hole and detect for a shadow below.
    NO absorption and reemission required which would ruin the test but even the impact of the light on the balls might effect the out come – even if small we cannot discard it as it could also ruin the test.
    SO we only put the beam across one of the two holes, and carefully record the results for each and every ball that hits the target screen behind the wall.
    And we exclude any 50 target hits that happen when a shadow is detected the expected 50 times. Leaving 50 target hits that had to come through the other hole where now light beam test existed to possibly interfere with the test.
    You keep track of each of the thousand throws to be sure they are not thrown quickly enough to allow two balls to go through at the same time, that would also ruin the test.
    Now you have kept track of 50 balls that could not have been interfered with as they go through that one hole.

    I think you already know the rest about how the electron and photon versions of these balls proceed to hit the dark spots in the pattern they used to create before the detection was turned on over the hole they are not going through.
  18. Jan 5, 2008 #17
    Thanks for your reply. I can see how such a test would work with balls, but photons and electrons are much different (aren't they?). The beam of light detector would definitely ruin a test with quantum phenomena. What's more, if I understand the double slit experiment correctly, if photons and electrons are sent through the slits one at a time, they still interfer with each other. Balls wouldn't do that. I suppose what I need is to understand (perhaps in more technical terms) how the photon or electron detector does its detecting. I have been reading Greene, Feynman, Rosenblum, Kuttner, Smolin and Oerter on the subject of the double slit experiment. All of them describe variations on the classic experiment with some of the variations become quite tortuous. All of this is done to prove the point that the act of observation collapses the probability wave. Since observation seems to be the critical influence in these experiements, I want to understand everything I can about those observations (in this case detection at the slits). So I guess what I'm really asking is how does an electron or photon detector work? I haven't been able to find anyone who could answer this question.
  19. Jan 5, 2008 #18
    With a QM understanding of the situation is it consistent to use the description that the electron or photon "passes through" the slits at all? (I'm not asserting that it's not accurate to describe it that way, I'm really asking.)

    For example, with a modern understanding can this maybe be seen as the same thing as when a "wave" of light "passes through" the solid matter of a prism?
    Last edited: Jan 5, 2008
  20. Jan 5, 2008 #19
    If what I've been reading corresponds with QM (and what I've read is the limit of my understanding on the subject), QM is not as clear cut as that. When the quantum particle/wave is observed the probability wave collapses, it becomes a detected particle and the interference pattern disappears. When the wave passes throught both slits unobserved, the probability wave persists and the interference pattern is observed. This is true whether quanta are released in the direction of the slits in large quantities or one quantum at a time. I may well have misunderstood Brian Greene, Rosenblum and Kluttner, or perhaps they were dumbing it down sufficiently that I carried away a false impression. Perhaps someone can set me straight.
    Last edited: Jan 5, 2008
  21. Jan 6, 2008 #20
    In my personal opinion, the 2 slits experiment shows that:

    1. The particles producing the interference pattern, be them electrons or photons or what else, cannot have (transverse) dimensions less than the slits distance, because they cannot be spatially localized better than that distance (otherwise the interference pattern would vanish).

    2. The very act of detecting them in one slit or another gives them dimensions less than the slits distance, because they can now be spatially localized better than that distance; so you can now say the particle has passed through one only slit, and this makes the interference pattern vanish.

    The personal conclusion of this is: quantum objects do have dimensions but these depend on the quantum object and on the experimental setting.
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