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Information RE The Double-Slit Experiment?

  1. Feb 26, 2008 #1
    Hi there,

    Let me start by introducing myself. I am a student who considers himself to be a very scientific thinker, that is to say, not one who believes in mystical thinking, but I am not studying physics. I am trying to learn more about this for my own edification (and I will try to be a respectful and nice guy ;)).

    I have read carefully about the double-slit experiment, but one question has bothered me to no end about it. What I read is that time and time again photons fired through two slits behave as a wave with an interference pattern even when fired one-at-a-time, but when observed behave as particles that land in two lines. Somehow, the act of observation interferes with the experiment.

    The question that I can't get out of my head -- and I imagine physicists have talked about this -- is the question of what, exactly, qualifies as an observer for purposes of interfering with the experiment. Apparently our methods of observation cause the wave function to collapse. But what about (and forgive my ignorance) mirrors? Flashlights? Strange materials? Other strange events? Dead cats? ;) What else collapses the wave function; that is, what else is an "observer" to satisfy the purposes of the strange way in which we have chosen to use that word?

    Thank you for your time!
  2. jcsd
  3. Feb 26, 2008 #2
    Nothing says that observation interfere with experiment. That's just ONE of many interpretation:)
    Bohm interpretation says interference pattern is caused by the fact that light is made up of both wave and particle.
    Particle thru one pinhole: wave thru both(therefore causing interference pattern).
    MWI says something as absurd as the whole universe somehow SPLITS and in one universe it goes thru one pinhole and in another the other pinhole(therefore the interference)
    There is several others, all with pro's and con's.
    I ally with Occam and chose Bohm because it explains perfectly our everyday experience, no splitting, no observer dependant reality, no both dead and alive cats. Objective reality. This is personal preference, talk to a MWI'er(amazingly there is many people who actually believe the universe splitting every atto second to cause interference pattern on a screen happens) and you'll get more info on that one, I hate it, not only on personal ground, but it's so farfetched, no axiom for "SPLITTING" universes exist.
  4. Feb 26, 2008 #3

    Thank you for the reply :) Let me make sure I understand this correctly:

    Case 1:
    Photons are fired through a slit sans "observer". The result of later examining the surface they strike is that the photons were distributed in a wave-like interference pattern.

    Case 2:
    Photons are fired through a slit with an "observer". The result of later examining the surface they strike in finality is that the photons were distributed in straight lines.

    If as a very real pragmatic result we get different patterns on the surface after we are finished, I wonder what sorts of things are actually interfering with the experiment and being given the title of "observer". Or am I mistaken about something?
    Last edited: Feb 26, 2008
  5. Feb 27, 2008 #4


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    I consider my interpretation to be close to the copenhagen interpretation, but I take the physical basis of information seriously, however I think the QM formalism is not yet in a final state. So in this way my personal interpretation is probably not dead on to any of the mainsteams, although there are common elements between many of the interpretations.

    In my view an observer of X, is ANYTHING that communicates/interacts with X and somehow retains a part of this information. The internal structure, and the information capacity of an observer varies. One might speculate wether this relates to the observers mass or energy, but this belongs to speculations of possible extensions and revisions of QM. But they nevertheless become unavoidable for me at least when you are working on interpretations. For me, in the "interpretations", there is a hidden intent of extending and improving. That's my motivation for making up the interpretation.

    So if you think communication: We have transceivers that exchange information. The information exchange is the physical interactions and the transceivers are the observers. But that still leaves the question about the origin of the transceivers?

    In the plain copenhagen interpretation, the "transceivers" (observer or measuring apparatous) are classical devices, and simply assumed to be there. Their "dynamics" and origin is not elaborated. This is a major simplification, and is one point where I differ from plain copenhagen.

    IMO, the transceivers can be thought ot as dynamical objects and are under contious selection and evolution. Only successful as in selfpreserving "transceivers" are viable, and are common in nature. Exactly how these transceivers self-assemble and evolve are the part of QM that at least I don't understand yet, but I have expectations on it.

    Of course, starting from the most chaotic state, the question of the origin of transceivers is the same as to try to understand the unification of matter and forces. Why does are certain structures (we can call it "particles" for convenience, but it's nevertheless just a word) more likely to be observered, than others? Note that in this hypothetical scenario, "to be observed" is relative to one of the other newly appeared structures. I would not make much sense to imagine a macroscopic classical observers in this chaos. The observer himself would necessarily be at least a semi-stable fluctuation in the chaos.

    (Note that the mictrostructure of the observers, as the first emerge, puts constraints on the "possible" interactions and communications, this could be a plausible way of grasping the unification of forces going hand in hand with the origin of structures. The simplicity of the transceiver, bounds the complexity of possible reliable communictions)

    To understand the origin of observer also IMO trips over the origin of mass.

    If you have this information view, the collapse of the wavefunction is not an issue. The question for me becomes to understand exactly how the statevector is encoded in the observers microstructure, and how this microstructure evolves and corresponds to the wavefunction evolution in between measurements. And this also leads to the concept that the microstructure is under selection or learning.

    Taking all this into consideration, it's not surprising that collapses occur, because it's just a manifestation of that the observer fails to predict EVERYTHING. But there is no good a prior reason to expect thta in the first place.

    Many may disagree with me on this, so take it as a single view input and form your own opinon.

    Last edited: Feb 27, 2008
  6. Feb 27, 2008 #5
    Since me myself is no specialist in this area, I'll let someone else elaborate on it.
    Just take everything as PERSONAL PREFERENCE and not "truth". Cause when it comes to interpretations most likely NO ONE got it right.
    Every interpretation works with the math. pick one OR, don't:P

    QM tend to cause a headache and confusion, sooo if ur just a guy with too mcuh time on his hands like I was, I'd stop thinking worrying about it:)
  7. Feb 27, 2008 #6
    If the final results on the screen the protons strike are in fact concretely different because of measuring devices, then we can in fact quantify what kinds of effects cause the different results through experimentation. I am not asking about philosophy, which I understand does have a place in understanding the wave function; rather, I am trying to find out if something is wrong with my understanding that, no, "observing" the experiment with some kind of measuring device really did change its results; if my understanding is not incorrect, and the results did change, we should be able to quantify what kinds of things change the results through experimentation, which is what I am asking about.

    I am not familiar with all the philosophies associated with QM, but I am familiar with the interpretation of QM that says, "We describe these photons as a wave function of possibilities because their behavior is truly random and we don't yet know where they are. When we observe them and find out where they are, the wave function collapses to a point where P=1 because we now know where they are". No magic or mystification there, it makes sense, and so does the role of the observer. But my mind is blown away if we get concretely different final results that depend on whether we take some kind of measurement; since at that point we get concretely different results that depend on concretely different input, we could gather scientific data about what interferes with the test. Since everyone's answers place the observer squarely in the realm of philosophy, it would sound like this is not the case. Is my understanding of what happened in the experiment flawed?
    Last edited: Feb 27, 2008
  8. Feb 27, 2008 #7
    I have never heard of what you are saying here.
  9. Feb 27, 2008 #8


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    I'm not sure I got the key here, but how can it make sense that the result would NOT depend on the measuring device? Without it, you get no result at all.

    You're right that we can certainly experiment and find out how different "experimental setups" give different results, and how different questions gives different answers. That's what experimental physics does, and the results from those experiments we can use to learn and evolve our models, to make even better predictions and ask better questions in the future.

    "getting the result" is exactly what measurement means. Without actually doing a measurement, all you can do is to "imagine" what the possible results you can get, if you DID make a measurement. This is where the quantum stuff and the waveequations comes in. It's a predicts the rated spectrum of possible results of a hypotetical measurements at a future time.

    That certainly isn't what I mean to say in my comment. I tried to put forward the idea that yes, observing is about information, BUT the information has a physical basis. It's real. But subjective. Not all observers possess the same information. And the only way to compare information is by communication, and to do one has to find a communication channel. The communication channels are IMO not given, they are emergent.

    Maybe I didn't quite get the core of your issue, but if you think the observer is more than just philosophy, you are certainly right IMO! The observer is at least as real as anything gets (without discussing the meaning of "real") but the tricky part with that is to bridge the subjective view with an emergent objective view we know from classical world.

  10. Feb 27, 2008 #9


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    I *personally* loosely speaking, imagine there to be a physical process behind this "imagine". One can imagine that the environment, that in it's entity is unknowable for the observers, lives as a projected image encoded in the observers microstate. So that the dynamics of the expectations in between measurements - as modelled by QM - corresponds to internal dynamics defined on the observers microstructure.

    This is the respect where I can feel a remote connection to the Bohmian view, however I consider these processes to be generally unknown and not necessarily in line with classical models, like the bohmians seem to suggest. Also my interpretation takes it a further step to suggest that the current QM formalism is not exact, it's rather just an expectation itself, in a larger theory. That suggestion is IMO a result from consistency.

    Edit: Another distinction from the bohmian view, is that my idea of internal dynamics does NOT render the predictions deterministic. Because the internal dynamics correspond to "probabiltiy distributions", not individual events.

    But the mathematical realisation of this, isn't yet on the table. So no matter how much confidence I have in this, it's formally highly speculative.

    Last edited: Feb 27, 2008
  11. Feb 27, 2008 #10
    Hi, Frederik,

    Thanks. As I understood it the results were determined by examining the material the photons struck, which somehow reacted to light. This was the result by which we could quantify that the pattern of the photons against the wall was different when we were not using the measuring device.

    The measuring device looked at the slit itself, and its addition -- to watch what the photons were doing en route -- caused them to behave differently. So the question I am trying to get to here, is, can anyone tell me about what kinds of measuring devices, events, effects and so on cause this strange behavior?
  12. Feb 27, 2008 #11


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    more philosophical reflections

    I percepted and underlying general concern in your OP regarding the concept of measurements in general. That's why I tried to make generic comments, because the specific details on the special cause only add details that doesn't matter to the generic insight IMO. We're all of different personalities. I personally hate special cases. It find it much easier to try to understand the general case, then see why the original problem is a special case.

    If you are actually asking for the special case of double slit experiments of light, or the double slit experiment with electrons, then a simple answer is that any part of the experimental setup and thus construction of the measurement itself, includes in principle everything around it. The construction of the light or electron source, slitses, lensing, walls, housings and the electron of photon detector and of course any ambient fields or disturbances of any kind would could in principle affect the registrations on the screen. All this together defines the "measurement". The human scientist is not technically a quantum observer of the system in this case. We could have a group of humans making the same reading off the apparatous with no problem. This is because the relative stability and information of the measurement device is available in excess, and consuming part of it does not significantly distort it. But that is in principle an approximation.

    One way of seeing it(*), is that the "observer" is the entire classical experimental setup that possibly interferes with the system of study, not just the photographic plate.

    In QM you have the initial conditions, constraints, and the rules of evolution.

    Given that, you can predict the result at a later stage, as given by the "expected self evolution" of your information. This is what comes out of say the schrödinger equation. But if you suddently insert stuff in the setup, tweak it, or try to make anything of tracking of the progress, then you are changing the initial and/or the boundary conditions or constraints. If you account for these adjustments, in the application of QM, then you again get the correct predictions.

    Conceptually, one can say that predictions are made up from applying the "rules of QM", to the initial conditions under any possible additional constraints. Then it shouldn't be surprising that if you change any of the "premises, without updating your expectations, you aren't likely to actually see what you originally expected.

    (*) One can be struck by the feeling that the boundary between the observer and the observer, is arbitrary. And that is true. But my personal position is that you should be able to draw the boundary anywhere! and still have consistency. Consistency doesn't mean that all observers register the same information, it rather means that whatever "system" we suggest to describe this quantum world, must be consistent and as universal as possible. Or as I like to think of it, even if it ISN'T then one can argue that it's probably going to evolve in that direction. This all connects in a evolutionary reasoning. We have no guarantees that it leads to success, but OTOH, the opposite is far more unlikely - and that is all we need - a direction for the future.

    On that point I think we still have missing bits, so if you are bothered by that your at least not alone. It remains for QM to bring the observer itself into the full dynamical picture. This isn't done yet.

    So this argumentation does not serve to make you see that QM is perfect. It's not. As I see it however, one can still see why QM makes sense, and one can see the direction to make the improvement, that is consistent with what we know. This alone is a satisfaction. I don't find satisfaction in that I have all the answers, but I find great satisfaction in at least a somewhat decent idea of in what direction to look for the answers.

  13. Feb 27, 2008 #12


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    OTOH, since you didn't study physics I maybe what I said makes little or no sense, unless you have the basic familiarity with QM principles, probability concepts and so on.

    I'm not sure if you're reflected over the human brain and it's decisions processes. Then there might be a backdoor to insight to conceptual issues possible not too unlike QM. Although I'm sure not everynoe agrees with me on this and I admit it sounds silly! Try to imagine, what would you do if you were an electron? You have first of all severe memory constraints, you are short of memory and processing time. And your life is at stake. One would have to make the best out of the situation or prepare to die. Now add ontop of that you are living in an environment, where everybody else are in a simialar situation as you. You have no fancy rulers or sticks. How could you keep track of anything?? What prevents you from falling apart?

  14. Feb 27, 2008 #13
    Yes, people say this kind of thing. The root cause of most of these paradoxes is the attempt to identify light as consisting of discrete particles called "photons". There is an old, established wave theory of light which explains the double-slit experiment without the need for the photon hypothesis. Yes, the photographic plate shows discrete spots: but we can now understand this in terms of the structure of atoms and molecules. They are not able to receive and store energy in anything other than specific quantities related to the frequency of the light. But it is hard to find anything in the detection process which definitley shows the light to be other than a continuous wave of energy.
    The "thought experiment" of watching the "photons" go through individual slits is not something people are actually able to do.

    In other words, I consider the particle theory of light to be the source of all evil. But my views are not widely held.

  15. Feb 27, 2008 #14
    Yes you have a fundamental mistake here that uncorrected will make it almost impossible for you to understand what is going on. Your description of Case 2 is incorrect.

    To build the correct description you need to be clear that with one slit only open the result on the screen IS NOT a “straight line”, but a classical dispersion pattern spreading the result over a fairly wide area. Likewise leaving only the other slit open creates a near identical wide pattern of dispersion, just slightly shifted. Your Case 2 is simply a sum of these two patterns, that is an equally wide approximately identical to the other two, i.e. bright in the center dim towards the edges. That is a simple sum of the other two.

    Now your Case 1 is identical to Case 2 spead accross the same wide area, with one exception; instead of an even gradient from bright to dim, embedded in the pattern is an interference pattern that duplicates the classical form of wave interference defined by Young long ago (1805 or so).

    Yours is not an unusual mistake, what I find a bit surprising how quantum solutions and explanations can be given by many without being sure this fundamental point is understood.

    Using an appropriate “flashlight” as you suggested in your OP (opening post) is acceptable.
    Just place the flashlight above the slits shining down onto the floor such that any photons getting though either slit will produce a shadow you can observe.
    Even better arrange it so that the beam only crosses in front of one of the slits. That way you can coordinate & remove your screen photon counts to only consider those that do not go though that slit and thus remain undisturbed by your testing flashlight beam.
    You understand of course we are all predicting you will produce Case 2 results when the floor observations are possible (i.e. you don’t need to actually look at them for it to eliminate the interference pattern)

    Let us know when you do the experiment - - - Well it might be easier to find where someone has already done it, which brings us to the “appropriate flashlight”.
    I can understand you may complain that it not easy to use photons to detect the passing by of another photon.
    Best option is to use electrons to create the interference pattern which is shown to work identical to the use of photons. Then your “appropriate flashlight” beam can be made of photons able to detect not destroy the electrons (just disturb then a bit so I still recommend testing on one slit to measure the electrons going through the other slit).
    This would be the experiment that actually has been done. Personally I consider it more historically significant than the fancy modern Delayed Choice Quantum Erasure experiments.

    The issue is also referred to as “The Observer Effect” also discussed in these forums. Sorry, I don’t know by whom or when the electron experiment was actually done. Nor have I found anyone that does, if you come across a reference for it let us know.
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