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Double split experiment

  1. Nov 21, 2013 #1

    Can someone please explain to me what is actually happening? OK so particles act differently when we observe them. So if I set up the double slit experiment in my room and left the room, came back and I'd see a interference pattern?

    If I stayed in the room, I'd see two lines of light? Do we know why they change when we observe them?
    Last edited by a moderator: Sep 25, 2014
  2. jcsd
  3. Nov 21, 2013 #2


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    The classic double slit experiment involves observations, but not the way you wrote. This experiment demonstrates that photons, for example, can display wave-like properties sometimes and act like a particle at other times. It also demonstrates one of the crazy aspects of quantum mechanics.

    Briefly, whether an interference pattern is produced or not depends on whether the experiment is configured to detect which slit each photon passes through. If no attempt is made to detect (track) the slit that each photon travels through, an interference pattern is produced just as 'waves' would interfere when channeled through two slits. But if you do anything to 'know' the slit that photons pass through, they act like particles and the interference patter disappears.

    The first thought for how the pattern is created is that the photons act like waves and interfere with each other passing through the slits.

    But the pattern is created even if the experiment is set up to deliver single photons, one at a time. So the photons can interfere with themselves, and pass through both slits in their quantum weirdness...? But, once again, if we do anything to detect which slit each photon goes through, the pattern disappears.

    The older phrase to describe this as "wave-particle duality" is out of date, being frowned on in the perpetuation of misunderstanding. Instead, it's replaced with simply the unique nature of objects at the quantum level that, should not be considered waves or particles, they just can sometimes act analogous to waves and other times analogous to particles.

    Although this behavior can be tested and verified in a variety of ways, there's a lot of variation in the way it's interpreted - the what's and the when's behind the behavior. I encourage you to Google double slit and read or view several instances of double slit setups and results to understand better and guide your own interpretations.
  4. Nov 22, 2013 #3
    Thanks for the lengthy explanation. I have tried googling and watching some youtube videos but there seems, for me, to be ambiguity with the word "observe". In the videos I watched they seem to give the impression that simply looking at the experient happen can effect the outcome of the experiment. From what I think I understand the outcome is only changed if we measure the experient?

    In that youtube video I linked he say that they placed a special camera to measure which slit the particle went through and that act of "measuring or watching" effects the outcome.

    At the end he also mentions that if anyone is able to explain this using logic or common sense there is a noble prize waiting for you. I assume this can't be explained logically then? It's just another quantum freakshow that goes against all logic?
  5. Nov 22, 2013 #4


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    This can be explained theoretically and mathematically for each result. When you say 'logically', you're referring to the understandings you've gained from your lifetime experiences in the classical world of physics. One could argue that quantum behavior is as logical as classical once you learn it and understand what (or what not) to expect.

    I won't say it's easy, because it's non-intuitive from what we observe in our macroscopic, classical world. The key, I think, is being willing to keep an open mind and not questioning every detail as you learn, just because you think it doesn't make sense. As you proceed to further things that may not immediately make sense, you'll be surprised at how the earlier things begin to become second nature. You just have to keep riding!
  6. Nov 22, 2013 #5


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    In this context, "observe" means anything done to gain the knowledge (know) which slit each 'particle' passes through. It's all about the knowledge - that's the quantum twist. If we don't know which slit, the wave interference distribution occurs. If we do anything to track the particles, the wave-like interference disappears. We don't have to actively watch - the info could be recorded on computer - doesn't matter. The quantum world knows it's being monitored. <grin>

    I have to stop before others begin adding all kinds of quantum interpretation here. Better to get you off to a good start so you can enjoy what's ahead when you're prepared to.

    I'll try to find a couple of other DS articles if you can't. It would be a good thing for you to get some depth on the base double slit. AFTER that, you'll really enjoy the 'quantum eraser' extensions. These principles apply to all at the quantum level, sometimes double slit is done with electrons.
  7. Nov 22, 2013 #6
    Obviously I won't take that statement literally but it is so strange how the knows that right? lol.

    I'm not actually studying any sort of physics at the moment I simply browse these boards to learn basic stuff, just as an interest really and make a thread every now and then to clear up some of my own questions. Do we know why the outcome changes? Not why mathematically but why what is actually happening to the particles for them to change their behaviour?
  8. Nov 22, 2013 #7


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    Indeed there is in the literature - especially those of the populist bent - the professional literature such as textbooks for serious students (eg Ballentine - QM - A Modern Development) are quite clear though and it is that view I will explain.

    QM is a theory about measurements that appear here in the classical commonsense macro world. That world is assumed to have all the usual things such as existing out there regardless of being observed by us, a tree makes a sound in a forest regardless of if anyone is there to hear it - no wideness there at all. Measurements are also taken in a somewhat general sense, and basically measurements are 'marks' in that commonsense macro-world that numbers can be assigned to if desired. In the double slit experiment that would be when the photon, electron or whatever you are using leaves a mark on the photographic plate, a screen that flashes or whatever.

    You probably have read about Schrodinger's Cat, and some accounts can confuse again for the same reason - they do not take appropriate care explaining exactly what an observation is - you get the impression its when a conscious observer opens the box - it isn't. The observation is at the particle detector - that's where the essential quantum weirdness occurs. Everything is commonsense and classical from that point - the cat is never in some weird superposition of live and dead - it is alive or dead - period - regardless of if the lid is opened or not.

    The real importance of Schrodinger's cat is not what some of the pop-sci press harp on about. Its simply this - how does this classical commonsense world emerge from the quantum world. We have assumed its existence - but really we would like an explanation for it.

    A lot of work has been done on that issue and great progress has been made with the understanding of what is called decoherence:

    'The discontinuous "wave function collapse" postulated in the Copenhagen interpretation to enable the theory to be related to the results of laboratory measurements now can be understood as an aspect of the normal dynamics of quantum mechanics via the decoherence process. Consequently, decoherence is an important part of the modern alternative to the Copenhagen interpretation, based on consistent histories. Decoherence shows how a macroscopic system interacting with a lot of microscopic systems (e.g. collisions with air molecules or photons) moves from being in a pure quantum state—which in general will be a coherent superposition (see Schrödinger's cat)—to being in an incoherent mixture of these states. The weighting of each outcome in the mixture in case of measurement is exactly that which gives the probabilities of the different results of such a measurement.

    However, decoherence by itself may not give a complete solution of the measurement problem, since all components of the wave function still exist in a global superposition, which is explicitly acknowledged in the many-worlds interpretation. All decoherence explains, in this view, is why these coherences are no longer available for inspection by local observers. To present a solution to the measurement problem in most interpretations of quantum mechanics, decoherence must be supplied with some nontrivial interpretational considerations (as for example Wojciech Zurek tends to do in his Existential interpretation). However, according to Everett and DeWitt the many-worlds interpretation can be derived from the formalism alone, in which case no extra interpretational layer is required.'

    Einstein once asked Bohr is the Moon there when you are not looking, and in answer to that and similar questions, Bohr said - stop telling God what to do. The jokes on both of them though - we now know the moon is being observed all the time by its environment, and that in fact is how this classical commonsense world emerges.

    I hasten to add issues still remain such as the so called factorization problem - and you will find them discussed on this forum, but we certainly know a lot more about the real issues with QM than we did - and its most definitely not the mystical nonsense you sometimes read about in popularizations such as What The Bleep Do We Know Anyway.

    If you really want to understand the double slit experiment from the modern view check out Lenny Susskinds lectures on entanglement:

    Decoherence is a form of entanglement and these days it is thought by some, including me, entanglement is really the rock bottom essence of what QM is about:

    Last edited: Nov 22, 2013
  9. Nov 22, 2013 #8


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    When you study physics for a while you realize both are really the same - mathematically why and what is actually happening are really the same thing.

    Physics is really about mathematical models to describe the world out there - the mathematics and 'reality' (whatever that is) are considered the same.

    Also check out:

    What modern physics has revealed is the most startling of insights - at rock bottom symmetry, the most mathematical and abstract of concepts, is at the heart of why the world is as it is.

    But me telling you that is not the same thing as experiencing it by studying the detail.

    I invite you to do that and understand this very very deep insight.

    Last edited: Nov 22, 2013
  10. Nov 22, 2013 #9
    Thanks for the great post Bill, I appreciate the effort. I just about managed to understand most of what you are saying and i'll be sure the read those links. For me though it's actually the laymens explanations that confuse the issue as oppposed to clearing it up haha.

    Thanks again.
  11. Nov 22, 2013 #10
    Not sure if you've come across the Yves Couder material. While this macroscopic system discovered by Couder isn't QM (despite having a number of QM-like properties including single-particle diffraction, interference, tunneling, quantized orbits, Zeeman effect, etc.), a few physicists do feel it might give us important insights into QM:


    Of course the difficulty is still the issue of non-locality/entanglement: to explain the existence of entangled states and long range correlations between the outcomes of distant experiments (e.g. violations of BI in EPR-type experiments). But if one can question the assumptions behind BI (as a minority have) then perhaps "the long range quantum correlations in EPR-type experiments are not more mysterious than the correlations between various random events on the opposite shores of the Ocean caused by Tsunami waves arriving after the earth quake in the middle of the Ocean."
  12. Nov 22, 2013 #11


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    Since I've done the double-slit experiment many times myself, I just want to add some comments:

    There will be an interference pattern regardless of where you are (unless, of course, if you place yourself between the slits and the screen, blocking the light :biggrin: - then the pattern will be projected on you). I've been close to the experiment, far away, and had a camera take pictures of the patterns without me observing - nothing changes.

    No. You will see the interference pattern! If you use a laser pointer, pointed at, let's say a wall, then

    1. Without any slits present in the path of the light, you will see a single laser dot on the wall (with some relatively small position distribution).
    2. With vertical slits present in the path of the light, you will see a wide horizontal interference pattern (many "dots" to the left and right of the central dot).
    3. With horizontal slits present in the path of the light, you will see a wide vertical interference pattern (many "dots" above and below the central dot).
    4. If you rotate the slits between a horizontal and vertical setup, the interference pattern will rotate too.

    If you/they mean human observations of the experiment/interference patterns, then, absolutely not. It does not change the outcome of the experiment.

    If you like to try the experiment yourself, it's easy to do. :smile:
    Last edited: Nov 22, 2013
  13. Nov 23, 2013 #12


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    The interference pattern you see is a result of the photons passing through the slits, reflecting off the surface of the screen, and then entering your eye where they are finally detected. You are observing the photons after they have passed through the slits and formed an interference pattern on the screen. You would need to observe the photons BEFORE all this happens, which would mean putting your eyeball down at one slit and looking at the light coming through. Which I don't recommend.

    When we "observe" a photon, it requires us to interact with it and detect it with some sort of photodetector. This requires that the photon impart its energy into the detector and as such it ceases to exist. This isn't like an electron, which we can bounce light off of to observe. (Yet in both cases if we know which slit the electron or photon went through we don't get an interference pattern)
    Last edited by a moderator: Sep 25, 2014
  14. Nov 23, 2013 #13
    As a theoretical physicist I strongly disagree. In the real world there is a broad range of physical processes going on. As humans we can observe these processes. We can categorize them and analyse them. We can construct mathematical models that describe and predict the outcome of experiments well. But it is still just a description. "Mathematics" is no more the same as "Reality", than a detailed book review is the same as the book itself.

    The fascination with mathematical modelling has led a whole generation of the brightest minds in theoretical physics astray. They embarked on this quest for unification. It has led to fantastic theories of superstrings and membranes in 10 dimensions. What a splendid folly!

    Even here on Physicsforms you can see that the experts have a fascination for semantics and mathematics according to strict rules, which tell them what is acceptable and which type of notation is allowed. But just ask a question that is neither trivial nor profound, say medium-complicated, and suddenly none of the experts has anything meaningful to say. Amazing.

    I recently asked this question: "How can it be understood, that in the hydrogen groundstate the electron has a speed, while at the same time its orbital is time-independent? How can the electron be perceived as moving and non-moving at the same time?" There were a couple of unrelated reactions, but not a single actual response to the question. To me a signal that QM experts know very well how to handle the mathematics of QM, but lack a deep understanding what the physical reality behind their modelling is.
    Last edited: Nov 23, 2013
  15. Nov 23, 2013 #14
    there also is the dimensional focus model, supported by Leonard Susskind's work, where these photons tend to behave this way because they have a greater probability (due to their small size) to reveal the upper dimensions in a strange, hard to comprehend (for us) 3 dimensional model. its like how when you view 3 dimensional objects in an 2 dimensional MRI-esque manor. if you apply this to viewing 4 dimensional models in a 3 dimensional world, they tend to turn out this way.

    this is one explanation but it is a bit more obvious and less sciency (for lack of a better word). lol
  16. Nov 23, 2013 #15


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    Except for the fact that, you know, it works. It works extraordinarily well. This may come as a shock, but at the smallest and largest scales the universe does not appear to behave according to how we see it here at our scale. Trying to explain everything in terms of how we commonly see them is the real folly.
  17. Nov 23, 2013 #16
    I took Mandragonia to be arguing that one shouldn't confuse the map for the territory.
  18. Nov 23, 2013 #17
    There is no evidence at all that we in live in 10 dimensions. Supersymmetry, superstrings, membranes etc are wonderful mathematical concepts, but totally detached from reality. In the mean time there are serious concerns in cosmology about inflation, dark matter, dark energy, vacuum energies etc.

    Some of the greatest physicists (Einstein, Dirac, Feynman) emphasized that one should always keep reality in mind, that one should visualise what the solutions to equations actually mean. A lesson many have forgotten.
  19. Nov 23, 2013 #18

    I must say that your understanding of dimensions (and if i may include the basic framework of the standard model) seems to be a bit flawed. weather or not dimensions exist is not a valid question. you must also consider what Einstein didn't consider.

    also, might I add, each and every one of these theories you just trashed put every single one of the "concerns" you listed in perspective.
  20. Nov 23, 2013 #19
    Right. Consider chess. It is a game played on a discrete 8*8 board, with discrete time element (the move), and with clearly defined pieces. This allows one to record the moves of a grandmaster game with 100 % accuracy. It looks something like:

    Anand-Carlsen, WK-match game 9, 2013. 1.d4 Nf6 2.c4 e6 3.Nc3 Bb4 4.f3 d5 5.a3 Bxc3+ 6.bxc3 d5 7.e3 c5 8.cxd5 exd5 ....... (many moves omitted) ............ 0-1 (white resigns).

    You can print these moves (with or without commentary) in a chess magazine. You can also store it as a little file on your PC or on a USB-stick. Now the question is: is the game notation as printed in the magazine or digitized as a file identical to the actual game as played in Chienna, India by the two grandmasters Anand and Carlsen? Absolutely not! Think about the climate, the food, the atmosphere in the playing hall, the opening preparation of the previous months, the evaluation of the first 8 match games, all the thought processes that were going on during this game......
  21. Nov 25, 2013 #20


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    Veering into multidimensions and superstring are already beyond the scope of the Quantum Physics forum.

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