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I Double slit experiment questions

  1. Jan 23, 2017 #1
    I have a question on a particular version of the double slit experiment (I know there are many). I am not sure I understood it correctly, so I am going to explain it in a very basic manner and in my own words. You tell me if my explanation is correct or not, and then I have another question.

    This particular version of the experiment had many mirrors and half-silvered mirrors with light bouncing this way and that, and also some kind of splitter which split the light into what I believe were entangled particles. I don't want to go into the details of that (indeed my eyes glazed over reading about it), and I don't want you to go into the details of it either. All that is important to me is that I get the basic 'gist' of it, and the outcome.

    1) Split light into entangled photons. 2) We'll call the first photon the 'control.' It simply has a path directly to the 'wall' if you will, where it will leave a mark as either a particle (a 'dot') or a wave pattern. This is also the shortest path in the experiment, so this photon will arrive and strike before any other photon. 3) The second photon goes through the double-slit, and a detector which can detect which slit it went through lies between the slits (after it has passed through) and the 'wall' where it will strike. we either choose to activate the detector after the photon has passed through the slits but before it hits the wall, or we don't.

    If I understood the experiment correctly (and I probably didn't, which is why I am here), the 'marks' left on the 'wall' by the two photons ('control' and 'other') always 'match,' meaning they are both either always a 'dot' or a wave. It is as if (one interpretation) the first 'control' photon to hit has the power to go back in time and alter its decision on whether to strike as a wave or a 'dot' depending on what choice the observer made and therefore what choice the 'other' photon made. Do I have this correct?

    If that is correct, here is my question. Could we not make the path of the 'other' (non-control) photon much longer, to where we would have seconds to minutes to determine what the first photon (control) did? For instance bounce the non-control photon off a mirror on the moon for all I care. The point is to simply see what the first photon does ('dot' or wave) then make the choice to do the opposite for the other photon and see what happens.

    My first prediction is that you will say I have totally (or largely) misunderstood this whole thing. My second prediction is that you will say that both photon will strike as a wave no matter what we do. But would anyone say that our choice for the 2nd photon (bouncing off the moon or whatever) will somehow affect the first photon we saw hitting seconds or minutes ago, i.e. could we witness 'dot' for 1st photon, select 'wave' for 2nd photon, then see the recording for the 1st photon magically change to wave as well? Or would we magically lack the free will to make the 2nd photon do the opposite of the first photon?

    Thank you for your patience, and I await your response.
  2. jcsd
  3. Jan 24, 2017 #2


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    You will get better answers if you tell us which version you're thinking about. Do you have a link?

    However, some of what you say below has to be misunderstanding, no matter which experiment you're thinking of (but it would still be good if we knew which one it is):
    A single photon always leaves a dot, never a wave pattern (which means that the "wall" actually has to be a piece of photographic film, or an array of particle detectors). The wave pattern is built up over time as we send more photons through the apparatus and more dots appear in some areas than others. Thus, there is no "strike as wave"/"strike as a dot" distinction to be made.
    Not correct, because that distinction does not exist. We'd have to know more about the experiment you're thinking of before we can say what is correct.
    Chances are that the experiment you're thinking of (did I mention that you really have to tell us what it is before we tell you how it works?) was intended to do that - these are called "delayed-choice experiments" and google will find many references. We don't need seconds or minutes or mirrors on the moon to see the effect - modern lab instruments are easily able to see the timing difference between one photon taking a straight line across a tabletop while another one zigzags between a few mirrors.
    None of the above. When the dust settles, we will have a whole bunch of dots on the screen. Some of the dots will be made by photons that had to have gone through one slot or the other (but be warned that "went through one slot or the other" is not really right, it's just as close as we can come using imprecise natural language instead of precise mathematical statement) and others by photons that could have gone either way. If we consider just the first group of dots, we will see one clumping pattern, and if we consider just the second group of dots we will see a different clumping pattern. But if we look at the wall where the impact of all the photons is recorded, we won't see any interesting pattern at all.

    I should add that we can't really discuss this sensibly until you provide a link to the specific experiment that you're asking about.
    Last edited: Jan 24, 2017
  4. Jan 24, 2017 #3


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    This spells trouble. Your eyes glaze, probably also when reading the famous Marcella paper, but you do want the understanding. Check out some of the many links at the lower left of the page !
  5. Jan 24, 2017 #4
    Ahhh, but we do need seconds or minutes to see the effect (at least I do). I want to either see one pattern change to another pattern before my eyes (if that is in fact what would occur), or I want to see that I don't have the free will to choose one thing or the other, or I want to see something else occurring. That's what I want to test.

    So let us forget a bunch of details for the moment, or even the exact specific experiment (this particular kind of delayed choice, that particular kind of delayed choice, etc). Again, I just need more of a 'general gist' or 'big picture.' Would it be possible to construct an experiment 'similar' to what I am suggesting, meaning, enough time for me to visually see what is happening on one 'wall' or 'film' (no need for us to be too anal with the terminology - it is unimportant for my purposes here, and I don't care what we call any of these things), then make a 'choice' as to what is happening with another photon, and then see what happens or doesn't happen? You will want to say 'we are already doing that - the dots here represent such-and-such, and the instruments over there record blah-blah.' No, that doesn't convince me of anything. Again, I want to see stuff happen in real-time with my own eyes, then I want make a choice myself and see results or consequences happen in real-time with my own eyes. If this is possible, what results would you expect? If it is not possible, why? Again, particular experiment isn't important, either use whichever one you think is closest to what I described, or invent/modify whatever is close to create it. You don't even need to tell me which experiment, which apparatus, etc.
    Last edited: Jan 24, 2017
  6. Jan 24, 2017 #5


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    You never, in any variant of the experiment, see the quantum mechanical effect come and go with your own eyes. The only way you can see an interference pattern on the wall with your own eyes is to shine a coherent light source on the screen with the slits; in that case, an interference pattern will be visible when both slits are open and will go away if you block either slit. However, this is a purely classical effect that was discovered and explained more than a century before quantum mechanics.

    To see quantum mechanical effects in a double slit, you have to send particles towards the barrier one at a time, and then each one forms a single dot. The interesting quantum-specific behavior is that if you do this long enough the dots will build up into an interference pattern if both slits are open. However, no individual particle switches between wave-like and dot-like behavior - it's always a dot at the point of impact.
  7. Jan 24, 2017 #6


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    As Nugatory already said, it is difficult to discuss the double slit, entanglement, wave/particle, etc. if you don't specify the setup and a focused question. And the details matter. Trying to read between the lines, I would comment as follows:

    Entangled photons do NOT produce interference effects (act "wave-like") when run through a double slit. (They are not coherent, a requirement for interference.) So while you can perform delayed-choice experiments on them (what you call a longer path), you cannot use that to much avail. I can't answer further without more specifics.

    As to convincing you of anything using your own eyes: I question how much help anyone here can help you with that.
  8. Jan 24, 2017 #7
    As in the movie Coolhand Luke, 'what we have here is failure to communicate.' That's fine, I don't blame anyone for that. It's the nature of the beast.

    At any rate, fine, I 'get' that there's a dot at point of impact blah blah. I 'get' that the pattern builds up over time as a series of individual dots, blah blah. But I don't understand why I can't get what I'm after here in an experiment.

    We are getting bogged down in particulars and details - something I tried to avoid at the outset, but something that seems to always happen nevertheless when I ask technical or scientific questions somewhere. So let's try to generalize yet again.

    We have some apparatus for producing a short path and a long path, and some apparatus to somehow 'split' or create 2 'related' or 'entangled' photons or whatever, as previously discussed (don't waste time correcting my terminology here - just mentally correct it on your end and lets move on). We have the double slit, detectors, screens, yadda yadda. Particular setup is unspecified, CREATE ONE in your mind - the 'right' one, based on what I'm trying to test or do here.

    A photon goes through the short path and hits the screen. Since we are doing something unspecified which gives me a lot of time (bouncing things off the moon or whatever - the particulars don't matter, MAKE SOMETHING UP), I am able to watch the dot appear with my own two eyes, record where it hits on the wall, etc. Now I have time to make a choice on whether to detect which slit the 'other' photon is going through, or not detect. So I push one of two buttons which is labeled 'detect' or 'not detect' and wait for the photon. Then I observe what happens, record results, and repeat.

    Question. Would I ever see the 1st photon - the 'control' - change where it struck on the wall, as a response to some choice I made with regards to the 2nd photon? For instance would I ever see the 'strike point' change position - for example move from some highly-clustered and defined area of dots roughly matching the shape of one of the slits, to some other area of the screen, for example to an area that most certainly would not match up with an outline and 'fill' of the shape of one of the slits?

    Question. Would I ever find that I lacked the 'free will' for lack of better words to make a choice with the 2nd photon that is inconsistent with what the 1st photon did? For instance, assume I witnessed the 1st photon strike outside of an area that matches up with an outline and 'fill' of the shape of one of the slits. Could I then try to make a choice that is inconsistent with that? For instance I believe choosing to know which slit the thing went through means you get the non-interference pattern built up over time (if I have something wrong or backwards, just correct it in your mind or substitute the correct thing - no need to hash it out here). But I will make that choice after the first photon hits. Is it possible for the 1st photon to hit outside this non-interference pattern zone? If so, could I not try to make the choice for 'non-interference pattern?'

    Question. If no, why not?
  9. Jan 24, 2017 #8


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    No. That would be mean that a developed spot on the photographic film you're using to build up the pattern would somehow undevelop itself... and photographic film doesn't behave that way. For a more stark example, imagine that we have a particle detector that is wired up so that if it detects a particle it will detonate a bomb... and by coincidence it happens to be at the point where the first photon lands, so the bomb goes off. How could doing something to the second photon somehow unexplode the bomb?
    No, because all choices of what you do to the second photon will lead to a state that is consistent with what the first photon did. Exactly how this works out will depend on those specific details that you don't want to go into.
  10. Jan 24, 2017 #9


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    As mentioned previously, entangled photons are exceptions to the rule. They don't produce an interference pattern ever (while still entangled on momentum basis). So there is no possibility on the very inconsistency you ask about.

    In any quantum experiment, the entire context is part of the setup. Timing and/or ordering is not really a factor, despite what your intuition tells you. Thus, the order of observation of a pair of entangled particles is NEVER discernible.

    And... ditto to everything Nugatory said. :smile:
  11. Jan 24, 2017 #10
    How could any of these things happen? Common sense, intuition, etc. agree with you. Then again, we are often taught that the intuitively obvious isn't always correct, and quantum physicists on pop-science shows like Discovery or Nova or whatever, lead you to believe these things. That's why I asked.

    Thanks for your answers.
  12. Jan 24, 2017 #11
  13. Jan 24, 2017 #12


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    Are not good sources if you want to learn the actual physics. Physicists will say things on these pop science shows that they know they would never get away with in a peer-reviewed paper.
  14. Jan 24, 2017 #13
    A good book you might find easy to read, and which is usually recommended by Nugatory, is "Sneaking a Look at God's Cards" (http://press.princeton.edu/titles/8006.html). From there you could introduce yourself to basic Maths of QM by looking at https://www.amazon.com/Quantum-Mechanics-Theoretical-Leonard-Susskind/dp/0465062903
    Last edited by a moderator: May 8, 2017
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