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Double slit experiment - bias towards normal pattern?

  1. Jul 27, 2015 #1
    WARNING. I'm not a physicist! If there was a universe that only had 10000 photons and we observed all of them then ran the double slit experiment would we get a normal pattern or an interference pattern? If we observe a particle is that it forever in the universe that we happen to be running our tests in? If this is the case then when we run this test in 'our universe' should there not be a bias towards a standard particle distribution as some particles in the test will have been observed in the past?
     
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  3. Jul 27, 2015 #2
    Do you mean observing them as in finding which slit they went through? If so, we get two the pattern that pertains to them going through either slit, not an interference pattern.* It doesn't matter if they have been observed in the past (i.e. a different double slit experiment) -- what matters is the experimental set-up in the present.

    *Technically two interference patterns are present, but hidden. This is because entanglement occurs between the photons, and detector which we treat as quantum mechanical. If we do quantum-eraser, then by matching certain data with points on the back-screen we would see two interference patterns emerge, opposite to each other. (http://www.arturekert.org/miscellaneous/quantum-eraser.pdf)
     
  4. Jul 27, 2015 #3
    Ok...thanks...I think your first paragraph answers my question. If I've interpreted this correctly, if we run a test and detect what slit a particle went through it won't effect a future test of the same particle? As for your second paragraph mentioning entanglement mind blown. I'm not ready for entanglement but thanks for replying....
     
  5. Jul 27, 2015 #4
    Well, the photon will hit the back-screen. Then the photon will be no good for any future tests; it would have been absorbed.
     
  6. Jul 27, 2015 #5

    K41

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    It can't be all absorbed, otherwise you would see no interference pattern. The only reason you see the pattern is because the light from the slit reaches the surface of the back-screen and then scatters the light. Only some of the light is absorbed.

    Quoting Bohren:
    http://photonics.intec.ugent.be/education/IVPV/res_handbook/v1ch06.pdf
     
  7. Jul 27, 2015 #6
    I assume by referencing to light, you actually mean photon? So then "only some of the photon is absorbed"... I don't quite understand. Where does the other part of the photon end up?
     
  8. Jul 27, 2015 #7

    DrChinese

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    Regardless of the reference (I won't even bother to try and explain it), the screen has absolutely nothing to do with the interference. You don't need a screen to observe an interference pattern, for example.

    And in the perfect case, all light going through the double slit will be absorbed. The amount of light absorbed is not dependent on the pattern.
     
  9. Jul 28, 2015 #8

    K41

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    I apologize for the lack of clarity. What I meant to say was that I assumed by "black screen" you just meant a regular black screen used in a classroom experiment, in which case some of the photons would be scattered back instantaneously rather than all absorbed and lost forever (although obviously if it as a black body, then it may emit all the light but I'm not sure on this). I should point out, this wasn't clear in my original post, that indeed all the photon's would be absorbed, what I actually meant to say was that after that, the molecules would scatter the light or emission would occur etc. If it helps, I was thinking about extinction cross section, absorption cross section and scatter cross section etc.

    Without anything to absorb the light and scatter it, how can one expect to see the light? In my post, I explicitly stated "to see" the pattern. I didn't say the pattern was created by the screen. I just said you need a screen to see it. How the pattern is created is explained by other theories. Once the light goes through the slit(s), interference occurs. If there was no screen, how would you see the interferometric pattern? If you didn't need the screen, why bother having the screen in any experiment? Just have the slits, don't bother having anything behind it. Yet every classroom experiment on this has screens, or at least a wall, some object behind the slits for people to actually see the pattern. So in my opinion, you need it, not because it creates it (it doesn't), but to see it after it has been created.

    Finally, the purpose of the forum is to help people and to learn. If you are going to dismiss a textbook, then you should have sufficient material to back up your claims rather than ego.
     
    Last edited: Jul 28, 2015
  10. Jul 28, 2015 #9
    I think DrChinese is referring to other means of observing interference, e.g. Mach Zehnder interferometer.

    The last part of your post is unnecessary, and could lead to disorder.
     
  11. Jul 28, 2015 #10

    K41

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    Ok fair enough. It wasn't my intention but I do feel it unwise to dismiss a textbook without sufficient grounds. If I think there is a mistake in a book, I spend hours, days even just wondering whether its me and looking at other sources. I'm not claiming the source I quoted is 100% correct, I just wanted guidance as if it was incorrect, as to why, rather than just claim it is. This doesn't help me understand the material better otherwise. Anyway, good luck!
     
  12. Jul 28, 2015 #11

    DrChinese

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    Funny, because you said: "It can't be all absorbed, otherwise you would see no interference pattern. The only reason you see the pattern is because the light from the slit reaches the surface of the back-screen and then scatters the light. Only some of the light is absorbed."

    As I said, that is completely incorrect. The light is absorbed by the screen (or film), and an interference pattern results. The pattern in no way results from scattering of different photons or from any of the apparatus itself. In the ideal case, all light is absorbed by the screen.

    As to my comment that a screen is not needed: there are double slit experiments in which there is no screen (other techniques are used to record the pattern). So obviously the screen itself is not the cause of the pattern in any way.

    Going back to your reference: in the normal usage of the language, I would say that self-interference of a photon with itself in a vacuum is in fact the cause of the interference pattern, which is dependent on whether there is one source slit or two. So I would heartily disagree with the reference as written. Somewhere, I'm sure, the author probably explains his reasoning. But most of the rest of us use more common terminology, and that wouldn't be it. So you can either use terminology most of us use, or not. As mentioned in another thread today: you can also call a pig a cow. Most people probably wouldn't understand you though, which is the purpose of posting here.
     
  13. Jul 28, 2015 #12

    K41

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    I clarified what I said later. You've not read my clarified second post. See further point below.

    I don't think you understand what I am saying. I'll repeat it again for further clarity. To observe the pattern, you need something there. Whether it is a screen or a detector, you need something to physically see the pattern. In your post, you said, "You don't need a screen to observe an interference pattern". You've clarified this somewhat by saying you can use a detector. Okay, I can accept that. My point still stands though. You NEED something there to actually see the pattern. I'll repeat what I said in the second post. I'm not suggesting the screen (or detector since you've brought it up), "creates" the pattern. I merely stated that to see the pattern you need those things. Seeing the pattern does not imply that those things are the source of the pattern. I think you believe I implied that, which I stress is not the case. To summarize, the interferance pattern occurs without a screen/detector. I'm not debating that whatsoever. I'm just trying to say that behind the slits, you will need something if you want to "detect" it, for want of a better word.

    Imagine you have two double slits and light shining through it. We know that an interference pattern will occur after the slit. Fine. But when Young did his experiment, he would not have known this. Physically the light has gone through and interfered. This is fine. But we need to detect or observe this. The author is merely stating that to acknowledge that interference has occurred using a screen, then some scatter will have to have occurred and what your seeing on the screen is that scattered light. The light has already undergone interference. Yes, there are theories to describe how, you point one out about "self-interference". Okay. But the author isn't describing or rebutting that. Rather, he is saying, if you were to use a screen, what you actually see on the screen is the scattered light from whatever light has gone through the slit (which includes interference).

    EDIT (please ignore this paragraph, the analogy will only serve to confuse):
    If you don't agree with this, consider that after the slits, interference has occurred. The energy in the light (whether photon or wave), still exists. The energy hasn't been destroyed. So if it reaches a screen, what happens to that energy? If it were totally absorbed, how you you actually SEE the interference pattern? I'll agree perhaps the author wasn't clear, but this is the distinction he is making.

    EDIT 2 (better analogy): Or rather, let us entertain your idea. Light goes through a slit. If the light was completely absorbed by the screen, what then happens to that energy? After absorption, electrons would exist on a higher state. Following this near instantaneous process, what then happens to the energy of those electrons? They must surely be emitted, scattered, or lost through some collision processes. Your using a detector but my original post was in response to someone saying a screen, not a detector.
     
    Last edited: Jul 28, 2015
  14. Jul 28, 2015 #13

    DrChinese

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    As long as we are agreed that your reference is not suitably descriptive and your analogy makes no sense either, I think we are fine.

    Of course any detector absorbs the energy of the light. Any that it doesn't (ie lost as heat) is irrelevant to the entire double slit experiment.
     
  15. Jul 29, 2015 #14

    K41

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    Firstly, please stop referring to a detector. Use a black screen because my reply was to a black screen (or any regular surface at home, see the video), not a detector. If you are using the word detector as all encompassing, fair enough, but make that clear.

    Look at this video 2 minutes in:


    He creates the interference pattern. Fine. I am not disputing how the interference pattern is created, nor the theories, or your theory. I need to make that very clear. Lets now assume that we are discussing what happens AFTER the slit. The interference has already occurred (or is occurring, as the waves interfere etc, lets not get into semantics here). I am not disputing this. Now the light continues to travel through space. When he puts the laser against the "white surface", according to your logic, all the interfered light would be absorbed or lost to heat. But what do you think happens AFTER absorption? Light doesn't just absorb and the process stop. If light is absorbed by a material, if its the white surface in this video or a black screen, it HAS to do one (or some) of the following:

    - Emission:
    --Luminescence
    --Incandescence
    --Other
    Scattering:
    --Rayleigh
    --Raman
    Losses
    --Kinetic Energy
    --"Heat"

    There are probably others but yeah...

    Now part of the light will be lost to heat obviously. Nothing is a perfect process. Part will be lost to kinetic energy. We'll also assume that the background light "noise" is sufficient to observe that the surface has a colour to it. Now if your hypothesis was correct and nothing happened to the interferred light as it continued along in space and reached the surface, then how would you SEE the interference pattern on the white screen? Absorption would not let you see the pattern on the screen. On a detector, I don't know how they work, but perhaps a detector would absorb a photon, but a screen is an everyday object. When the everyday object, the white surface absorbs the light, the electron is excited. You are suggesting that the electron would permanently stay in that excited state until it loses energy to heat/ kinetic which doesn't make sense.

    quantum_double_slit_photon.jpg
    From : http://www.physicsoftheuniverse.com/images/quantum_double_slit_photon.jpg

    I just want to be clear, I'm referring to the screen in this diagram, not the slit partition. I know the light has already interfered after the slit partition and before the screen. I am talking about what happens when the light hits a regular everyday object, like a screen (white surface).
     
    Last edited: Jul 29, 2015
  16. Jul 29, 2015 #15
    I think the confusion is simply the difference between "recorded" data and direct observation. What Dr. Chinese is referring to is that each individual photon traverses the experimental set-up and gets "absorbed" by a detector. What you are referring to is continual observation of light reflected from a "wall". The "wall" absorbs some of the photons, but they are never observed by your eyes and are therefore irrelevant to your observation. The photons that are absorbed by the "detector" are the ones that Dr. Chinese is concerned with, any reflected photons are irrelevant to him. They both display the identical interference pattern over 10,000 or 10,000,000 photons that get "absorbed and detected" or "reflected and observed".
    It is just a continual process. The interference pattern is fixed relative to the slits that "allow" the interaction to keep occurring as you are observing it.
     
  17. Jul 29, 2015 #16

    DrChinese

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    As jerromyjon says, what happens after the screen/detector/film records the photon (or other quantum object) is essentially irrelevant. In the particular example you presented in your most recent post, the photon is reflected to a person's eye. But again, this has nothing to do with a double slit experiment's signature results.

    I guess I am wondering if you have a question related to this thread. Your comment (and related) "You NEED something there to actually see the pattern" is off the topic at hand. As we agree, there are lots of ways to detect particles and we can call any of them "seeing".

    To address the OP's original question directly, as I understand it: photon number is not static in the universe. New photons could be created. Regardless: suppose you ran a photon through a double slit experiment and "somehow" were able to re-use it again later in a second double slit experiment (this is theoretically feasible in some senses). The 2nd time through, the photon would have no "memory" of its earlier history. Therefore there would be no "bias" towards a normal pattern.
     
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