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Does different observer affect double slit particle experiment

  1. Mar 23, 2010 #1
    if the double slit particle experiment is observed by an animal and not by any human for eg. chimpanzee, through which slit the particle has passed would it collapse the wave function and form particle pattern.

    The particle are only observed by the chimapanzee, so it can know which slit the particles passes
    but the information is unfetchable, similar to an detection with data being erased(delayed choice).

    To the animal it will see an particle pattern, as it has the information of the particle.

    Is it that an human who is not looking at the slit and the particle, would see an interference pattern since it has no information of the particle?

    The animal can also replaced with an human, if it is possible the information he observed is not known.

    Then would both the observer narrate a different pattern on the screen?
  2. jcsd
  3. Mar 23, 2010 #2


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    You do not need an intelligent "observer" to destroy the interference pattern, intelligence has absolutely nothing to do with it.
    What matters is if a "measurement" is performed, whether it is by the environment or a scientist does not matter.

    See e.g. Hornberger et al "Collisional Decoherence Observed in MatterWave Interferometry" PRL 90 160401 for a very nice experimental demonstration of this.
  4. Feb 3, 2012 #3
    i don't believe that is accurate... the delayed choice quantum eraser experiment (search for it on youtube) shows that even after measurement, once the "information" is erased the interference pattern re-emerges... so the awareness of the information is in fact what collapses wave functions. the animal question is quite significant then... i wonder if there has been an experiment to find out.

    also shrodinger's cat is certainly "measured" inside the box in that countless particles are interacting with cat's body, but the question is still posed if its both dead and alive.
  5. Feb 3, 2012 #4


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    This is completely wrong. You can do postselection and filter the signal to find an interference pattern in the filtered signal, but the signal itself does not change. Consciousness or awareness of the signal does not change the signal.

    No, it is not.
  6. Feb 3, 2012 #5
    Well, the delayed choice quantum eraser causes an unmeasured photon to have its wavefunction collapse because of its entangled partner being collapsed even though the choice of whether to collapse that partner (by the 50/50 random prism) is made after the first one hits the measuring device.

    So the eraser shows that its not disturbing the particle that causes the collapse but rather the information being preserved or destroyed. Now what exactly counts as measurement is a difficult problem. You can say the measuring device is also part of the system since its made up of particles. So what in particular causes collapse? That's why some take it all the way to the conscious mind and some say its the first interaction with other physical particles. This is discussed here:

    http://en.wikipedia.org/wiki/Quantum_mind/body_problem [Broken]

    "Von Neumann, in his analysis of measurements, interpreted the demarcation line as the point where wave-function collapse occurs, and he showed that within quantum mechanics, the point of collapse is largely arbitrary, and may be placed anywhere from the first incoherent interaction with a complex enough object, to the interface of the brain with consciousness.[21]"
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  7. Feb 4, 2012 #6


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    None of this is really correct. DCQE uses a two-photon interference pattern to work which means that the interference pattern is visible ONLY in coincidence counts. The sequence of doing measurements does not change anything in the outcome. The dataset does not change afterwards. You only get to pick a partial dataset afterwards which shows an interference pattern or you pick a partial dataset which does not show it. There is no inversion of time ordering or other metaphysical mumbo jumbo involved although popular explanations of this experiment often claim that.
  8. Feb 4, 2012 #7
    Since neither the particle nor the measuring apparatus care about who is in the room, the result will be the same.
  9. Feb 4, 2012 #8
    I don't think that your example is appropriate. Because human beings or intellectual aliens are not seeing the interfering particles. Instead, they are using apparatus at the slit to detect which slit the particle is going. And whatever the observer is the wave function of the ket vector has already collapsed to a well defined state, whereby exerting an operator would produce a well defined eigen-value.

    I think you are trying to talk about the collapse of wave function in the case of Schrodinger's Cat experiment, where Copenhagen interpretation states that the consciousness intervenes the system which is quite controversial. You can learn more about it relating to Schrodinger's Cat.
  10. Feb 4, 2012 #9
    The Copenhagen interpretation do not say so, only some people (mostly philosophers) claims such nonsense about consciousness collapsing wavefunctions. Ordinary textbooks on QM do not make such claims but only describe the physics.
  11. Feb 4, 2012 #10
    Really? I haven't learnt that far into QM. But in some popular science books, it is said that Copenhagen interpretation claims that the Cat in the box cannot collapse wave function, while humans who open the box can. Is it implying that only intellectual livings can collapse wave function in that interpretation which is exactly what the OP is asking.
  12. Feb 4, 2012 #11
    The cat in the box is just a thought experiment. In really that's not going to happen because the cat can measure what's happening and at large distances away from the nucleus electrons virtually don't exist, so there's not a lot of room for superposition with atoms in the macroscopic realm/
  13. Feb 5, 2012 #12
    Lots and lots of nonsense are said in popular science books (about chaos, QM, relativity, black holes...) and movies are still poor :rolleyes:
  14. Feb 5, 2012 #13
    It seems there is some confusion about the definition of a measurement. If it is clear that only after a measurement the wave function collapses, then only the observer of that measurement can see a particle behavior. It sounds like two observers looking at the same screen and having different resolution on the scenario, so one sees the particle behavior and one sees the wave behavior. I am not very sure and correct me if thats wrong.
  15. Feb 5, 2012 #14
    Measurement is a physical process, both observers detect it. There is not such thing as the «particle behavior» and the «wave behavior» a particle always behaves as a particle. A particle as the electron is always an electron in quantum mechanics or in particle physics
  16. Feb 13, 2012 #15
    I thought the coincidence count is there to distinguish multiple photons. What if you just do it in a dark room with only 1 photon... in the DCQE, the photon hits the closer measurement before its entangled partner goes through the prism which will determine its measurability, yet it has an interference pattern only when its entangled partner does. So what is the state of that photon that is "waiting" for its entangled partner to "decide" what's going on? does it have an interference pattern and then change if needed or is it in some sort of pending state? You could move the 50/50 prism that determines measurability one a light year away so what's going on during that year?
  17. Feb 13, 2012 #16


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    No, it is not. Single-photon interference and two-photon interference are complementary, see e.g. Phys. Rev. A 63, 063803 (2001) by Abouraddy et al.

    The interference pattern is in any case visible only in coincidence counting under DCQE conditions, even at arbitrarily low photon count rates.
  18. Feb 14, 2012 #17
    You lost me there. What does it mean that they are complementary and how does that require coincidence counts? Why is coincidence counts required even with only 1 photon? Thanks.
  19. Feb 14, 2012 #18


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    I suppose you mean two photons (one signal, one idler) and repeatedly sending these two photons down the experiment such that there are never more than these photons present in the setup. Even at arbitrarily low count rates you will never see an interference pattern looking at the results of one detector alone under DCQE conditions.

    You can have an interference pattern in the single photon count rates at one detector under spatially coherent illumination. That, however, breaks entanglement.

    Or you can have a coincidence pattern in the coincidence counts only without seeing one in the single count rates. That requires entangled particles which means incoherent conditions.
    You cannot have both, so these requirements are complementary.

    Your earlier post sounds as if there were interference patterns spontaneously appearing or disappearing. That is not the case. There are no direct interference patterns visible at any of the detectors used. You really need to have a look at the pattern that arises when detector A is placed at some position, detector B is spatially scanned and the coincidence counts are evaluated. If you now place detector A somewhere else and move detector B around again, you will see another interference pattern in the coincidence counts which is different from the first one. You can continue like that for any position of detector A and will get many different coincidence count patterns. The sum over all of these patterns gives no interference patterns at all - these are only visible in conditioned coincidence detection.
  20. Feb 14, 2012 #19
    thanks... i meant one photon that is split into two... so what would you see at detector A where the signal photon hits in a dark room and you place detector b a light year away?

    This physicist john cramer doesn't seem to understand this as an inherent limitation to coincidence counting... i can't find any results of his experiment online, but seems from this that avoiding coincidence counts is a practical obstacle to be overcome:


    here it also described just as a way to distinguish entangled from non-entangled:
    http://en.wikipedia.org/wiki/Coincidence_counting_(physics [Broken])
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  21. Feb 14, 2012 #20


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    It would just be detected somewhere at detector A. Nothing special happens.

    The reason why there are no published results is most likely that avoiding coincidence counts is not just a practical obstacle to be overcome. Cramer provides a strange interpretation which is not backed up by any facts, but sounds spectacular. Show me published results supporting his speculation and there is something to discuss.

    Of course you can also use it also as a means to distinguish entangled photons from noise. However, doing so is not sufficient. If you want to read the whole story in a thorough manner, have a look at http://arxiv.org/abs/1010.1236, which is the ArXiv version of Physics Reports Volume 495, Issues 4–5, October 2010, Pages 87–139. This explains the whole topic better than I could. Especially the chapter on spatial correlations and conditional interference is a good starting point. Also, most of the good references are cited within that paper.

    The situation in DCQE is basically the same and also a situation where the exact shape of the interference pattern seen in coincidence counting depends on the position of BOTH detectors. Having just one detector present is equivalent to averaging over all possible positions of the other detector which cancels any interference pattern. Wikipedia is also not really a good source for scientific debates, by the way.
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  22. Feb 14, 2012 #21
    This is the latest news i can find of cramer's experiments on his website from 2010... toward the end of the ppt he explains that practical issues (e.g. noisy detectors) have not allowed the retro-causality experiments to work yet, but he doesn't believe these are inherent show-stoppers.

  23. Feb 14, 2012 #22


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    He is free to believe what he wants to. That does not make his view which is based on Wheeler's point of view any more correct.

    Even more recent publications explicitly oppose that point of view.
    From Phys. Rev. Lett. 107, 230406 (2011):

    "Discussing the delayed-choice experiment, Wheeler concludes: ‘‘In this sense, we have a strange inversion of the normal order of time.We, now, by moving the mirror in
    or out have an unavoidable effect on what we have a right to say about the already past history of that photon’’ [5]. We disagree with this interpretation. There is no inversion
    of the normal order of time [...]"

    Together with the really good reference I have given you before, it should be clear where he goes astray - it of course takes some time to read and grasp the topic, I must admit. However, it does not make much sense here to discuss bare claims which are not backed up by any experimental evidence. By the way Cramer's claim that coincidence counting is necessary only to overcome noise comes from the time when he noticed an experiment performed by Birgit Dopfer in the group of Zeilinger. These results also stressed the importance of complementarity and noted that single-photon and two-photon interference patterns are complementary. Just do the theory and eliminate the noise and integrate over all possible detector positions and you will notice that there cannot be an interference pattern under DCQE conditions without coincidence counting.
  24. Jan 10, 2013 #23
    I have my best answer to the original question, plus 3 questions of my own.
    I personally don't think the intended "animal" question posed at the beginning of this thread has been quite answered yet. Please allow me to restate my interpretation of the question:

    In the double-slit experiment, assuming a generic non-human animal can accurately relay which-path information to a human, but no human is permitted to see the which-path info, will there be an interference pattern?
    For example, the machine detecting which path each photon passes through commands a perfectly obedient dog to "sit" or "shake" based upon which slit the photon passes through, and humans observe the dog's actions.
    (I am not asking about Schroedinger's cat per se, or about the background light "noise" in the experiment- those topics would be somewhat irrelevant to this question.)

    Correct me if I am wrong, but from every delayed choice/double slit/quantum eraser experiment I've read about, the results generally seem to agree on the following conclusion:
    It's not the act of observing per se, NOR is it the act of taking a measurement (detecting), that disrupts the interference pattern. Though these conditions likely must be met to accomplish disruption, they alone are not sufficient. The disruption occurs when an observer understands WHICH slit the photon travels through.

    So getting back to the question- based on that last statement, I believe the experiment I described would result in the photons behaving like matter, rather than creating the interference pattern. I say this not because the animal itself is disrupting the interference, but because humans are gaining the which-path knowledge by observing which command the animal follows. In other words, the perfectly-trained animal is essentially just an extension of the which-path detectors.

    So, building on that, I ask 3 questions about the double-slit experiment:
    1. Am I correct with my prediction? If not then, in layman terms, why not?
    2. Assume we have a chimp that can look at the which-path results and understand that something is passing through 2 possible paths. Does this disrupt the interference pattern? Does the chimp need to generally understand what is happening in the experiment in order for the disruption to occur?
    3. How much about the experiment does one need to understand in order to disrupt the interference and make the photons behave as matter? For example, say we detect our which-path information, and before anyone else can see it we send the which-path info (whatever info the experiment conductors normally draw conclusions from) to Grandma, who is unaware such an experiment even exists. Assuming that Grandma continues to observe the data without knowing what it means, would the waves make an interference pattern?

    I appreciate anyone's input!
  25. Jan 10, 2013 #24


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    Welcome to PhysicsForums, Spence! This is an old thread, kinda dead and buried. Next time it would probably make sense to start a new one instead...

    I will pass on a few thoughts regarding interference vs no interference. The rules are:

    a) If you know, or could have known, in principle, which slit was traversed, then there will be no interference. This has been demonstrated in every experiment, and is the master rule.
    b) As far as anyone knows, the presence or absence of an intelligent observer makes no difference. Certainly you can record the event and look at it a week later, and rule a) above will still apply.
    c) Notwithstanding b), you could always assert that the pattern changed precisely because an intelligent observer looked at the pattern. You could never prove otherwise, but I would call this an assertion which is not scientific in nature.
    d) In DCQE experiments, the ordering of the events is immaterial to the outcome.
  26. Jan 10, 2013 #25


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    What about this explanation for the disappearance of the interference pattern ?
    https://www.physicsforums.com/showthread.php?t=662920 post#5 by Dali
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