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Double-slit experiment

  1. Nov 15, 2009 #1
    Hi. I'd like to talk about the double-slit experiment performed with electrons.
    We know that any device, used to tell us which slit the electron passed through, destroy the interference pattern that we would observe if we used no devices.

    Well, but what does it happen if we don't receive the signal from the device?
    In other words, if we are out of the laboratory (or we are blind and deaf) while the experiment is performed using a device?
    Would the interference pattern be still destroyed?

    The main question is what destroy the interference pattern:
    -the interaction device-electrons;
    or
    -the informations WE have about the process

    Thanks.
     
  2. jcsd
  3. Nov 15, 2009 #2

    Cthugha

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    It is the first one - at least if you already consider a block put in front of one of the slits as a device. The double slit is not at all about consciousness and scuh stuff. The interaction causes some which-way information to be present and therefore also changes the state of the electron. Whether someone notices that somehow is not important.
     
  4. Nov 15, 2009 #3
    http://en.wikipedia.org/wiki/Double-slit

    Any modification of the apparatus that can determine which slit a photon passes through destroys the interference pattern, illustrating the complementarity principle; that the light can demonstrate both particle and wave characteristics, but not both at the same time.

    However, an experiment performed in 1987* produced results that demonstrated that which-path information could be obtained without destroying the possibility of interference.



    * P. Mittelstaedt, A. Prieur and R. Schieder, Unsharp particle-wave duality in a photon split-beam experiment, Foundations of Physics 17, 891-903 (1987).
     
  5. Nov 15, 2009 #4

    Cthugha

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    Hmm, this Wiki article does not really use good wording. It says:

    "Any modification of the apparatus that can determine which slit a photon passes through destroys the interference pattern, illustrating the complementarity principle; that the light can demonstrate both particle and wave characteristics, but not both at the same time."

    This is true for the case that the photon path is unambiguosly determined, but there are more possibilities than 100% certain path and 100% uncertain path.
    From a more exact point of view this should be formulated as "light can demonstrate both particle and wave characteristics, but not both at the same time to full extent."

    A more detailed description is given in terms of the Englert-Greenberger duality relation
    http://en.wikipedia.org/wiki/Englert–Greenberger_duality_relation
     
  6. Nov 15, 2009 #5
    You missed the most important part, directly related to OP question.

    However, an experiment performed in 1987* produced results that demonstrated that which-path information could be obtained without destroying the possibility of interference.


    Meaning that 100 year old interpretation about observer and "knowing" is a plain nonsense in the light of many recent experiments. It's amazing how long it takes for text-books to be updated and why people are inclined to accept voodoo as scientific explanation.
     
  7. Nov 15, 2009 #6

    Cthugha

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    Ehm...do you even know the paper and do you know modern textbooks?

    I do not know any serious modern textbook, which is talking about observers in terms of the double slit. Also any serious textbook is not talking about knowing, but about knowable. This is by no means changed by the results by Wooters and Zurek (the Mittelstaedt paper is just an addition to the work of Wooters/Zurek). Incomplete, but finite which-way information gives also rise to incomplete, but finite fringe visibility. This is covered by any modern textbook on modern optics, whether it is the book by Mandel and Wolf, the one by Scully and Zubairy.

    And please do not post that Helix dance crackpot stuff again. Getting banned twice should be enough.^^
     
  8. Nov 15, 2009 #7
    http://resources.metapress.com/pdf-preview.axd?code=x5p103384543pjk6&size=largest


    Good. Then we kind of agree, except that instead of "knowable" one should speak about "measurement", which really means "modification of the apparatus".


    What do you mean by "Helix dance crackpot stuff"?

    It's like you were talking to someone else, I don't understand.
     
  9. Nov 15, 2009 #8

    Cthugha

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    I think this might be misleading. In the early days of QM one of the interpretations of the double slit was that devices (or better measurement arrangements), which are able to extract which-way information are not able to extract information about interference and vice-versa. However, there is some truth to this at the core, it can be very misleading and lead someone to think that complementarity is just a flaw of measurement, so I do not know, whether modification of the apparatus is the best wording.


    Let me put it this way: The paper you just cited does not get cited very often here. Last week someone cited it here. He did some calculations on em-waves and was obviously pretty overwhelmed by finding something which looked like an em-wave in some calculations. However his results were pretty...wrong, he did not notice in his state of euphoria and he got banned twice or so for posting them. Well, he just sounded similar to you, posted in similar threads like the one you now post in and cited the paper you just cited. However, if you are not that guy, I am even happier. ;)
     
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