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I Mind Bending Double-Slit Diffraction Thought Experiment

  1. Jul 13, 2016 #1

    I studied undergrad pure maths now post grad philosophy, not quantum physics, but please consider the following experiment:

    Suppose you set up a double slits experiment with a photon detector attached to each slit, and the photon detector attached to a photon transmitter, so the photon can contine its journey to the screen.

    Now as long as what takes place at each photon detector is not observed, is it not possible for an interference pattern to be observed?

    I ask because under the Copenhagen Interpretation, the act of measurment or observation depends on an act of consciousness, yet the act of consciousness, I can argue, only takes place only at the screen if the re-transmitters are unobserved, therefore the wave can still follow both paths until the screen. Recall, an unobserved event, according to quantum lore, has neither happened nor not happened. So I say as long as these re-transmitters are unobserved, they can be thought not to effect the outcome.

    On the other hand, if the photon detectors are observed, eg by a signal light which the experimenter observes, then the wavefunction has to collapse and the interference pattern will disappear.

    So then do you agree this strage behaviour is indeed possible under the Copenhagen Interpretation?

    Many thanks...
    Last edited: Jul 13, 2016
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  3. Jul 13, 2016 #2


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    If it is possible, in principle, to determine which slit the photon passed through: there will be no interference. Actually looking at the information is not relevant. Of course, there really is no such method of "allowing a photon to continue" - you would have a different photon anyway.

    A better what to get close to what you are looking for (I think) is to place polarizers in front of the slits. If the polarizers are parallel, there IS interference. If they are perpendicular, there is NO interference. Again doesn't matter whether you know which side it went through because in the perpendicular case, you could make such a determination if you so wanted.
  4. Jul 13, 2016 #3
    I realise that is what many would think on the face of it, but it that really true?

    Will nobody reputuble agree with me? Or has the problem been considered before and a certain conclusion reached? That is what I want to see...

    On your polarizer suggestion, it would have to be as follows, the choice of polarizers is made randomly by a mechanical device with a 50-50 chance of a particle going through both or only one, and the choice is not observed, and the polarisers come down and lift up again after some set time, and I suppose one could argue that what could be observed, on my hypothesis, over the set time period, is a mix of the two undetermined possibilities rather than a conventional observation of all interference or all consolidation. Easy to test, but the problem is the observer might choose to see one or the other result at random, that would be his act of consciousness, so the wave would conform to it, and only if he really believed all paths were still possible, would he observe the hypothetical mix I suggest.

    But then again, now I realise my experiment suffers the same flaw, if you believe the re-transmitters have made an observation, then the wave would collapse in conformity with that act of consciousness. So there is no need for my complicated first example, it is equivalent to this much simpler polarizing experiment,
    Last edited: Jul 13, 2016
  5. Jul 13, 2016 #4


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    1. It is safe to say this has been considered before. :smile:

    2. The reason to use polarizers is that light still goes through otherwise unaffected (other than a change in polarization). So no "lifting up" is needed.

    You might notice that by definition, it is not possible to ask what particles do when they are not observed. So to a certain extent, your hypothesis could never be tested.
  6. Jul 13, 2016 #5

    Let me put the experiment to you like this: You prepare two sets of slides, set A has two slides with the same polarisation orientation, set B has two slides with orthogonal polarization. So if you load set A into your experiment you will see interference, but consolidation with set B. On the other hand, if you swiched between the sets every few seconds, and allowed a patern to build up, you would see a mixture of both.

    But now, even on a single trial, if you loaded a set at random and ran the experiment then removed them withot looking, you could make the conscious decision to observe a mixture of both interference and consolidation, saying to yourself I have not observed these slides so the result remains open, and what you observe would conform to that. Most people, not believing in the Copenhagen interpretation would be unable to do that, they would expect the result to be one or the other, and that is what they would see. But the Copenhagen Interpretation by definition allows the possibility for both to exist together if you choose to look at the experiment in the way I suggest- the act of consciousness, or measurement or observation or whatever you want to call it, determining the outcome by the theory.

    So I say it is not true "my hypothesis could never be tested", it could be tested by a true believer. Recall the mysterious stories of the "Pauli effect", how he was famous for ruining people's experiments in some strange sychronistic way. What I am saying makes sense of that, put a person like him in front of the screen and the outcome of the experiment can be completly different because he chooses an extradionary interpretation and the wave conforms to his perspective.

    Do you have a projector and screen? We can do this experiment together, all you have to do is convince yourself of the theory before you try it. Once you have done it, we could demonstrate it to a crowd of non-believers, with sleight of hand. We tell them the slides are half and half, so they will see a mix, then we show them that, then we ask them to examine the slides, and they will think we are magicians. Only if they know the trick in advance will it be impossible to perform. It makes sense for me of something Plato talks about a great deal: his flux theory of conjuction between maker and observer, and his statement the good can never reach the bad, it can only reach those of good bad mixed condition.
    Last edited: Jul 13, 2016
  7. Jul 13, 2016 #6


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    You might want to re-read the above, because being a "true believer" (or not) will not affect the outcome - and you seem to be slipping into the land of non-scientific. No one here can help you with that. And such speculation is not really welcome on a moderated board such as PF.

    And there really is no hypothesis here to test - as I said to begin with, there is no interference if which slit can be determined (regardless of whether it actually is). If you mix the polarizer settings randomly, you will end up with a pattern in between the 2 extremes.

    "Recall, an unobserved event, according to quantum lore, has neither happened nor not happened." - As far as is currently known, there is no sense in which a conscious observer is required to effect a measurement.
  8. Jul 13, 2016 #7
    You are not understanding what I am saying. The manifest observation of the wave depends only on your act of observation of the experiment, it does not depend at all on the objective physical setup in some old fashioned physical way. Since a mixed hypothesis is cognatively open, it is observable in that unorthodox way. Surely you can see what I am saying? Your last statement, "as far as i know there is no sense in which a conscious observer is required to effect a measurement" makes no sense at all.
  9. Jul 13, 2016 #8


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    That is an opinion and you are welcome to it. However, it is really a philosophical point and not science. I have already provided a description of what will occur, and I am not sure what remains to discuss.

    I happen to agree that we live in a subjective (observer-dependent) universe, but consciousness (or lack thereof) does not appear to play a part. My viewpoint is called an interpretation, and it is recognized as being just one of a number perspectives. At this time there is no objective manner to select one over another.
  10. Jul 13, 2016 #9
    For the record I am not talking about relativist morality, I am taking about light falling on a screen through double slits. You say you have provided a desciption of what will occur, I deny that, I have explained at lenght how something you say is impossible is, I am now certain, possible if you choose to look at the experient in the way I suggest. So I claim to be giving a concrete example of how the same scientific experiment can give different results to different observers, which is relativism in the physical not psychological sense.

    You say "I am not sure what remains to discuss". Clearly nothing remains to discuss between us because you are sure I am wrong, but as a result of our discussion, I am now sure I am right, and we are locked in that way. However the thread need not die because perhaps others might stuble across this thread who disagree with you and want to discuss something with me.

    Finally, thank you anway very much for discussing this with me, even though we disagree, I have found our conversation extremely enlightening and I am extremely grateful. The polarizing example is much better than the example I started with, and your feedback drew out my arguments. Please don't think your time was wasted- indeed since you are a believer in humanist relativism if not physical relativism (I am the other way around, I believe in moral absolutism but physical relativism), you can walk away happy to have made someone else happy, even if you think their happiness is delusional!
    Last edited: Jul 13, 2016
  11. Jul 13, 2016 #10


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    So a non-destructive detector. Nothing unusual about that, at least not in a thought experiment.

    The detectors that you put in the slits are the observation.

    You defined a situation where the photons are detected, and then asked us to assume they weren't detected. These are contradictory requirements.

    You're thinking of the VonNewmann-Wigner intepretation. The Copenhagen interpretation doesn't involve consciousness at all. Copenhagen has collapse that happens when superpositions get "too big". "Too big" happens way way before you get up to the scale where a detector clicks or doesn't click.


    Double slit experiments give the same outcome whether or not someone is in the room looking at them. Basically all of the quantum experiments you've ever heard about are done by machines, and their outcomes are predicted just fine by interpretations that make no reference to consciousness.

    So I can confidently predict exactly what you will see if you do this experiment: there will be no interference pattern. I mean... it's already been done. Do you think physicists always sit and watch every experiment they do for hours and hours? That no one has ever set up an experiment and let it run, then gone over the machine-collected data later?
  12. Jul 13, 2016 #11
    If you like... [Obviously Pauli & Bohr would disagree but not worth discussing]
    On the end of the machines is a consciousness...
    For you then, as explained, it will fail, but for someone out there who falls in love with this thread, the answer could be different.
    Last edited: Jul 13, 2016
  13. Jul 13, 2016 #12


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    If you think machines are conscious, and you think consciousness is what's relevant to interference-or-no-interference, why were you expecting your experiment to give different results when the path information was kept in a machine vs told to a human?
  14. Jul 13, 2016 #13
    Yes, good comment, thank you, as explained above, my original idea was wrong, or at least confusing, DrChinese showed me that, and it was a silly mistake that is clearly commonplace and has been well thought about and would seem to make it impossible to test the role of consciousness (like the people who, ignorant of first principles, think they have built a perpetual motion machine by adding complexity, getting lost in thier complexity).

    That is why I reflected on the argument, stripped it of the confusing technology, and brought it down to nothing more that a "true believer" with a projector, screen, slits and two sets of polarizing slides.
    Last edited: Jul 13, 2016
  15. Jul 13, 2016 #14


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    Although there is no universally accepted definition of what the "Copenhagen Interpretation" is, you are right that some forms of it did once consider that consciousness was essential to the collapse of the wave function. However, that notion was abandoned decades ago (but unfortunately not before it made it into the popular imagination, where it has proved harder to eradicate than any invasive weed) and is no longer part of the modern understanding. It was originally introduced (by Wigner and von Neumann among others, but not by the guy from Copenhagen whose interpretation we're talking about) as the only logical way of placing the von Neumann cut between quantum system and classical apparatus. However, that problem disappeared with the discovery of quantum decoherence, and with it any reason for attributing special significance to conscious observers.

    Thus, this entire thread is pretty much based on a false premise and can be closed.
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