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Non-Physicist looking for an answer

  1. Mar 5, 2008 #1
    Hey everyone, I don't pretend to understand QM (although i am trying!), and the terminology i use will no doubt be incorrect, but hopefully you will understand my problem and somebody can explain the answer to me.

    In the classic photon through a double slit experiment, I think I'm right in saying that if no one observes which slit it goes through, it acts like a wave (we see interference), but if one does observe which slit it goes through, it stops acting like a wave. If the reason for this is as i understand; that things can only be in superposition when isolated from the environment, then why can we observe the photon in superposition (acting like a wave) at all?

    Well that didn't come out very well, but with luck someone will understand my question...
  2. jcsd
  3. Mar 5, 2008 #2


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    So basically you're asking: "what's the difference when we observe them when they hit something (and notice an interference pattern) and when we observe them as they pass through the slit" ?
  4. Mar 5, 2008 #3
    It's funny you call it the "classic" double slit experiment. The "classic" experiment isn't about photons at all; it was the original proof that light is a wave. In quantum mechanics there is a thought experiment with electrons where they behave like a wave until you look at them, and then then behave like particles. People want to use this same experiment to prove that light is a particle, so they just substitute the word "photon" for "electron" and say that light is a particle. In fact, there is no such experiment with "photons". You can't look at a photon when it passes through the slit. Light always behaves like a wave in the double-slit experiment.

    It's true people say things like what you've said here, but that doesn't make it so.

  5. Mar 6, 2008 #4


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    We can't observe the photon in a superposition. All we can do is to wait until we've sent a large number of photons through, and then examine the spots on the screen that have been hit by photons. If the pattern is such that it can only be explained by interference between the two paths, then we conclude that the particles have "behaved like a wave".

    Note that the interference pattern will appear even when the intensity of the light source is so low that photons are emitted one at a time. So it can't be interference between two or more photons.
  6. Mar 6, 2008 #5


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    You can, and when you do, it doesn't.
  7. Mar 6, 2008 #6
    Thanks for the replies.

    SF, you had got to the root of my question.

    Oh. If that's true, it's a shame there is so much rubbish out there on this. I have read several sources which claim that measurement of which slit the partical goes through changes it's behaviour, is this completely untrue?

    Does it behave like a wave when you use a single slit? My understanding was that if you used a single slit, it acts like a partical, but if you use two it acts like a wave, despite the fact that just one photon is being fired. I'm probably mistaken about this too!


    This is very clear, thankyou Fredik. It seems you and monish have different views on the possibility and effect of measurement of which slit it goes through. Who's word should i take?
  8. Mar 6, 2008 #7


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    What you have quoted from Fredrik is the accepted standard explanation in physics. There has been no empricial observation to contradict such explanation. So you decide.

    Last edited: Mar 6, 2008
  9. Mar 6, 2008 #8
    That sounds more like the Feynman two slit experiment rather than something like Young's two slit.

    There is a really good description here, that I tend to point out when these threads come up, it certainly helped me to grasp the concept when I was learning it.


    To be frank I don't know why you claim that its behaviour is always wave like in the double slit experiment, at least without linking to some relevant evidence. To all intents and purposes, we cannot measure the wave without decohering it. When we do the photon appears to travel through either the top or the bottom slit with a 50/50 probability, causing the photon to strike the screen as if it were a particle. Notice the term as if, this means it is exhibiting particle like behaviour, not necessarily that it is a particle, in any way that it is solely wavelike before we measure it, it is both wavelike and particle like in it's behaviour. This appears to be due to the way in which we measure or don't measure it. Ie the measurement itself effects the "photon" and this in turn changes the behaviour of the "particle". I like the term warticle personally. This is a perfect demonstration of wave particle duality and of the measurement problem, in one fairly simple experiment.
    Last edited: Mar 6, 2008
  10. Mar 6, 2008 #9


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    You can make it possible to retroactively determine which slit each photon went through by putting different polarizing filters over each slit, and in this case the interference pattern at the screen can be made to disappear--see the first article here.
  11. Mar 6, 2008 #10
    thanks for the great link SD, very informative. I think I'm somewhat closer to undertanding the concept now : )
  12. Mar 6, 2008 #11
    Whether you see interference pattern or not, you can still say that the detection pattern is wavelike, in the sense that a wave, any wave, is a frequency distribution of sorts. The problem arises for the individual detection phenomenon when, eg. photon, detections are accumulated one by one on some photosensitive surface or other. I don't know, but I don't think that anyone has a good explanation (in wave terms) of why/how the individual results are dotlike manifestations. In any case, the calculation for single or double slit results can involve the Schroedinger representation, the 'wave mechanics' of quantum theory.
    One way to think of it is that in order to observe which slit things are going through it's necessary to, in effect, close one of the slits thus precluding the interference pattern that would have been seen if both slits had been left open.

    Regarding the observation of superpositions in the world at large -- they're easy to see (eg. interacting water surface waves). In fact, it might be argued that superpositions within and interfaces between the various media which comprise our environment is all that we do see.

    Anyway, quantum superpositions might be said to happen in the imaginary space in which the Shroedinger wave equation and appropriate probability (wave) functions derived therefrom evolve.

    The quantum double-slit experiment is a great example of the need for a nonclassical theory involving superposition of states. And of course we can't observe instruments in superpositions involving mutually exclusive physical manifestations. That is, we never see the pointer at 1 and at 2 at the same time, we never hear the geiger counter both click and not click at the same time, and we never see a single photon going through both slits.

    If you're looking for a deep explanation regarding the mystery of the quantum double-slit experiments, there isn't one. But talking about things correctly (including using operational definitions) can reduce the confusion.
  13. Mar 6, 2008 #12
    All this is exactly what the wave theory of light predicts.
  14. Mar 6, 2008 #13


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    OK, but you can also determine which slit the photon went through in ways that don't correspond to anything in classical electromagnetism, like using entangled pairs of photons which allow you to determine which slit one photon went through based on the position that the other photon is detected. This will also result in the loss of the interference pattern (see this thread).
  15. Mar 6, 2008 #14
    I objected to the claim that the "classic double slit experiment" shows the particle nature of light. The experiment you refer to doesn't fit into that category.
  16. Mar 6, 2008 #15


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    Well, I was responding to your statement "You can't look at a photon when it passes through the slit." And anyway, what do you mean by the "classic" double slit experiment? Any method used to determine which slit the photon went through would be an modification to the double-slit experiment as it was originally performed in classical physics, this would be true even if it were just as straightforward to measure which slit the photon goes through as it is to measure which slit an electron goes through (and even measuring the position of individual photons on the screen is a modification to the classical double-slit experiment, regardless of whether they collectively form an interference pattern or not).
  17. Mar 6, 2008 #16
    If you want to argue that entangled photons show evidence of particle behavior, you may or may not have a point. I'm not prepared to comment on that question either way.
  18. Mar 7, 2008 #17
    I think he's referring to Young's experiment, which first revealed the wavelike nature of light rather than the more recent double slit experiments.
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