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Double Slit Experiment

  1. Jan 7, 2009 #1
    I have a problem with the outcome of the Double Slit experiment , in that it was concluded that when under observation the electron particles behaved differently to when not directly under observation - conclusivly stating that it was the ACT of observing that changed the electron behaviour in the end . Why has it not been questioned or put forward that it was the detection/observing apparatus/equipment/device itself that interferred in the behaviour? Its not like we can see them with the naked eye! I just havnt seen anyone ask this question before....
     
  2. jcsd
  3. Jan 7, 2009 #2
    You're 100% right. The observing equipment observes the electron by bouncing light off it. This light must be of a high enough frequency to resolve a "tiny" electron. Such a frequency of light has the effect of imparting some momentum onto the poor electron, ruining any interference it may have been having with it's self in private in it's own bedroom with the lights off and a box of kleenex handy. How rude.
     
  4. Jan 7, 2009 #3
    THANK YOU Gendou2 :smile: Finally some clarification, knew someone must have wondered that before.... :smile:
     
  5. Jan 7, 2009 #4
    PS. Thanks for the laughs - made my afternoon :smile:
     
  6. Jan 7, 2009 #5

    ZapperZ

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    Er.. this is silly.

    I don't need to bounce photons off electrons to detect them,. The moment electrons pass through a loop of wire, I can detect its signal, per Faraday Law.

    Now it doesn't mean that, in some way, this doesn't effect the electrons. But the fallacy that I have to detect electrons ONLY via destroying its path via photon collision is faulty. It relegates the superposition principle to nothing more than an instrumentation issue, rather than something inherent to the system as indicated by QM.

    Even with photon detection, one can show that even with non-destructive measurement, the system STILL collapses via QM's description. This clearly shows that it has nothing to do with the instrumentation of what we use, but rather it is an inherent property of the system.

    Zz.
     
    Last edited: Jan 7, 2009
  7. Jan 8, 2009 #6
    Hey, maybe you can answer this question I've long had: in any of these multiple contingent path scenarios, what counts as detection? What are the characteristics of a detector or of the detection interaction that distinguish it from, say, a photon going through a half-silvered mirror and hitting a rock (which is stimulated in some way, though not in a way constructive to waveform breakdown, no?).

    Thanks for helping.
     
  8. Jan 10, 2009 #7

    That article says... "For the first few measurements the answers are evenly distributed from zero to seven. This means that not enough information has been gathered to ascertain the number of photons, since the system is still in a superposition. But after many measurements the cumulative distribution of answers begins to centre on a particular number, revealing that the system is collapsing into a well-defined state."

    This is averaging. Isn't this a property of mathematics calculating averages rather than a property of the system?

    I am still wondering how one has determined that the act of measuring something changes it's state and why the device measuring it is not responsible for it.

    As far as I can tell the interference of the measuring device is a predictable interference based on the methods of measurement.



    How many different methods are there for measuring the quanta directly? Can it even be done or is it entirely inferred interpretation?
     
  9. Jan 10, 2009 #8

    ZapperZ

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    I have no idea what you are saying here. Even if it is an 'average', why isn't this a physical property of the system since it is the system itself that is giving you the average value. What is <x> if it isn't the average value of the a system that you actually measured? Is <psi|x|psi> a property of mathematics, or the physical system?

    You should read the actual paper (the exact references is in the article) rather than rely on a report of the paper.

    Zz.
     
  10. Jan 10, 2009 #9
    Faraday's Law is true because of the photon - the carrier of the electromagnetic force. A Feynman diagram of your loop of wire will show photons carrying a signal between the loop of wire and the electron. Right?
     
  11. Jan 11, 2009 #10

    ZapperZ

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    yeah, but you were talking about REAL photons, not virtual photons that are responsible for EM interaction. Do you think these are the SAME thing as in what you described earlier? I mean, did you had to send in a photon of a certain wavelength to detect the electron that had just passed through a loop of wire? No you didn't. You have no control over that.

    It is still besides the main point, which is that the HUP has nothing to do with the instrumentation. I can make as accurate of a measurement as I want the single measurement of position and momentum based on the technology available. The uncertainty of each position and momentum measurement that I got has nothing to do with the HUP, no matter how I measured them.

    This is the most common misconception of the HUP (try googling it).

    Zz.
     
  12. Jan 12, 2009 #11
    ZapperZ, you probably know better than I, but it has been my experience that comprehending the uncertainty principal involves explaining why all experiments confirm it's existence. So, in this case, the reality of the virtual photons wasn't "besides the main point" but it was the main point. Why do people call virtual photons different from regular photons? I'm for photon equality. Is there some way to tell wether a given photon is virtual or not? Do they have to wear an upside-down triangle pin?
     
  13. Jan 12, 2009 #12

    ZapperZ

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    Yes. I hear they have some on sale at Macy's this week.

    Then show me how the virtual photon actually affect the the trajectory of the electrons in the SAME way as photons that you use to collide with those electrons to detect its location or momentum. Go on, do it. You insisted that these are the SAME effect. So now show it before a misinformation infraction falls onto that upside-down triangle pin.

    Zz.
     
  14. Jan 12, 2009 #13
    I don't insist. I just thought there ought to be some sameness, even if by analogy.
    Any way you measure the electron in the double slit experiment, you got to interact with it.
    I don't know how to show that the virtual photon interactions of a loop of wire. I'm not smart.

    On the other hand, it occurs to me that when you pass an electron through a loop of wire, there's an induced current and thus a magnetic field, effecting the momentum.
     
  15. Jan 12, 2009 #14
    I have a question on this experiment as well, if the detector was placed at some point beyond the double-slit then would the outcome be the same? Would there still be no interference pattern? The reason I ask is because if it is placed beyond the double-slit then wouldn't it have already interfered with itself by the time it reached the detector?
     
  16. Jan 12, 2009 #15

    ZapperZ

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    Yeah, so? How does this destroys the original state of the electron the same way that you scatter off a photon? It is NOT the same, and in fact, this measurement using electrons can be made with very little disturbance to its trajectory. Is there some disturbance? Sure! I mentioned this already in the very beginning. But it is NOTHING like a photon collision as in Compton scattering! It is NOT the same!

    The HUP is something INTRINSIC to the system. It isn't about the instrumentation. If this is something you cannot comprehend already by now, then you are welcome to continue in believing what you believe. I have no more patience to continue.

    Zz.
     
  17. Jan 12, 2009 #16

    ZapperZ

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    A "screen" is a detector. After the slit, it no longer matters anymore if you detect it or not. The superposition of path through the slit is preserved.

    Zz.
     
  18. Jan 12, 2009 #17
    Im sorry I didn't quite get your answer. Would there be an interference pattern or would there be two lines?
     
  19. Jan 12, 2009 #18

    ZapperZ

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    That would be an interference pattern. It is AFTER the slit, which is the "normal" way to detect the interference pattern, isn't it?

    Zz.
     
  20. Jan 12, 2009 #19

    DrChinese

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    To follow up on ZapperZ's answer:

    a. Interference pattern on the screen (which is a position detection instrument) results when we do NOT know which slit the particle passed through.

    b. Two bar pattern on the screen results when we DO know which slit the particle passed through.

    There are a lot of ways to convert from a. to b. For example, you could put a barrier down the middle of the apparatus so that a particle going through one slit cannot pass over to the other side. You can place crossed polarizers so that you can determine from the spin which slit the particle passed through. Or you can do it the old fashioned way, and simply cover up one slit or the other. Any way you choose to "look" at the path taken, that will result in the two bar pattern and not the interference pattern.
     
  21. Jan 13, 2009 #20
    ZapperZ

    You wrote..


    Following your advice, I pulled this off of wikipedia [http://en.wikipedia.org/wiki/Photon]


    I'm not trying to split hairs here, but it seems clear that the HUP applies to particles with mass. Maybe this requires a higher level of understanding than that of which I dispose!
     
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