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The crazy world of quantum mechanics.

  1. Sep 21, 2012 #1
    I'm not going to be silly enough to as ask why the double slit experiment is the way it is, i am however going to ask for some clarification on it.

    My understanding

    So, i understand the concept of the wave particle duality. Now if we try to measure which slit the photon goes through photon behaves likes a particle and if we do not measure it the double slit experiment shows that light behaves like a wave. This is quite mind boggling to me, and most others i would imagine hahaha!!

    Now my question is this, if we are pointing the photon gun at a particular slit then should the photon not travel through this slit and so there is no need to measure which slit it went through?

    Are things ultimately in all states at the one time before we measure them to find out which state they are in?

    Also what is even more confusing to me is that this behavior is not present in the non subatomic world. I,e. schrodinger's cat must be alive or dead when it is in the box, as it simply cannot be alive and dead at the same time in the non sub atomic world

    Any chat or insight would be very helpful as im completely out of my depth but generally very interested in such things!!

  2. jcsd
  3. Sep 21, 2012 #2


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    Welcome to PhysicsForums!

    1. When particles are "aimed" in these experiments: they either come from a point source (which means either slit is equally likely to be a destination or there is sufficient coherence that either slit is equally likely. If you do something which gives preference to one slit over the other, the results change accordingly. You might be interested to learn that experiments can be performed that clearly show there is nothing about the physics of the slits themselves that is responsible for the double slit interference pattern (or lack thereof).

    2. In some ways this is a fair assessment. Part of the issue is the definitions of the words one uses. Some call the states "undefined". Also, sometimes things are in what is called an eigenstate (a particular value) but that value is simply unknown. So a lot is also dependent on how the experiment is set up.
  4. Sep 21, 2012 #3
    Ah that makes sense, one would not know which slit the photon is going through if we did not measure it to find out!

    The term measuring is also highly confusing as the resulting interference pattern on the screen when no measurement has taken place is itself a type of measurement, no?

    The cat in the box part, i mean on a non sub atomic level then surely the cat must be alive or dead. It cannot surely be both. That would mean people in coffins are both alive and dead at the same time? Do you think that there will ever be a unification of the sub atomic and the non sub atomic world?

    Thanks for your previous reply, it helped clear some things up for me.
  5. Sep 22, 2012 #4
    I just posted in about the cat in the box maybe it is worth a read for you guys.
  6. Sep 23, 2012 #5
    Aiming an electron directly at one of the slits would have the precise same results as blocking the other slit. There would be no interference pattern.
    In any two slit experiment, the wave in the region before the barrier with the slits is prepared in a precise way. This is sometimes referred to as an "experimental protocol." The experimental protocol is derived from the theory, although it never contains the full theory.
    If one wants a "perfect" interference pattern, one prepares the wave in the region so that each slit is illuminated by a wave with an equal amplitude and a fixed phase difference relative to the other slit.
    If one wanted to aim the electron the say you say it can be aimed, one would have to send it through a "tube" or other facility that absorbs all electrons going the wrong way. The associated wave would then start out from the tube with a maximum flux in the direction that it was aimed. Effectively, the "tube" is just a way to aim the electron. Note that there would be a single slit interference pattern associated with the tube, but it would show much narrower peaks then the interference pattern associated with the two slits.
    The experimental protocols in the two slit experiment were developed long before quantum mechanics. The theory of classical optics describes how "pure" waves would behave.
    The experimental protocol for the two slit electron experiment is just about the same as in the two slit light experiment in classical optics. The classical results are valid when the light beam or the electron beam is very intense. Very intense means lots of particles. "Lots of particles" acts like a wave. However, the results chance when the beam is very faint. "Faint" means a small number of particles.
    Part of the reason that quantum mechanics developed later than wave mechanics is that it took time to develop instruments that could detect very faint light sources. When Young first did the light wave experiments, one had basically "see" the light with the unaided eye. Eyes can detect small numbers of photons, but eyes have a large signal to noise ratio that is mostly thermal.
    Instruments were developed that could detect and record the presence of very dim light. The camera is a good example. Once you had film, you could record the profile of energy on the film in an interference pattern even for dim light. It was only then that it was possible to determine that there were "localized" exposures of dim light on film. These "localized exposures" can be characterized by photons. If light were always a wave, then the interference pattern would be continuous across the film but for the grains.
  7. Sep 24, 2012 #6
    Nice summary; however the last sentence is disputed. The interference pattern would be continuous if both the transmission process and the detection process were continuous. It's not a question of grains but of atomic interaction processes. I think that A. Neumaier here elaborated on that point.
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