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I "The act of measurement"

  1. Nov 18, 2016 #1
    I'm an Engineer by profession with less knowledge of physics, so please be basic with me.
    After recently being intrigued by the double slit experiment, I started to read more and more about the same.
    At most of the places, it says that as soon a sensor is kept near one of the slit to find out which slit the electron passes from, the wave nature of the electron collapses and the interference pattern disappears, i.e. a "conscious mind" collapses the wave function.
    My question is, is it the act of measurement that is causing the change or is it just the sensor that is altering the particle nature?
    I mean, if I shoot a particle and a sensor is mounted at one of the slits, but I am not measuring which slit it is passing through; will I see an interference pattern? Will the electron behave as a wave like it normally would, or will the wave function collapse and particle like properties will be seen?

    It may be a basic question but please bear with me and explain.
    Thank you.
  2. jcsd
  3. Nov 18, 2016 #2
    Dear Abdullah..
    there is always a wave associated with a particle; according to de brogli's equation the associated wave is inversly propotional to mass... as electrons are really tiny fundamental particles so they posses wave nature. there is always a probability of existance of electrons we dont know exactly their positions; we can predict only as we solve by using schrodingers wave equation..
    so in my point of view there is no such a sensor that can detect exactly the pessage of electron.
    regarding detection we know that when light strikes the particle and reflect back to us then we can detect the existance od something but for the case of electron when we try to detect the photons (light packets) we fall will interect with the wave of electron and by interference it behaves like a particle otherwise like a wave.
  4. Nov 18, 2016 #3


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    If you don't measure you get the interference pattern.
  5. Nov 18, 2016 #4

    A. Neumaier

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    The interference pattern solely depends on the experimental setting, not on an act of observation or on the presence of a conscious mind.

    However, a sensor placed at one of the slits changes the experimental setting, and hence the pattern observed. Since the sensor interacts with the electron, the electron is influenced by it (action=reaction) and the response observed can be calculated by taking this interaction into account. It will reduce the quality of the interference pattern or even completely destroy it, depending on the amount of interaction.
    Last edited: Nov 18, 2016
  6. Nov 18, 2016 #5


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    For an electron, if you have the sensor, there will be no interference. The presence of a conscious observer is not required, as A. Neumaier mentions.

    But there is another way to further understand the situation using photons and the double slit. Mind you, the same issues apply to electrons but it is bit harder to set it up appropriately.

    Using photons as the particle source: You place 2 polarizers (which are somewhat analogous to the sensors), one each, behind the 2 slits, and there is no other sensors to otherwise record the path taken: If they are parallel, there WILL be interference. If they are perpendicular, there will be NO interference. The reason is that it would be possible to observe the polarization of photons as they hit the screen and determine the path when the polarizers are perpendicular. You don't actually need to take that step though, to eliminate the interference.

    The moral is: you cannot pin the issue of interference on the fact that you are "disturbing" the particle by measuring it. Clearly, in this case, the only thing that changes is the relative orientation of the polarizers.
  7. Nov 18, 2016 #6
    The short answer is that your sensor is an observer. A conscious being could come along later and check it and get the information. So the logic behind all science says that any later observations by any being or apparatus in the universe must be consistent with the information from your sensor (and anything else that happens).
  8. Dec 10, 2016 #7
  9. Dec 11, 2016 #8
    It's interesting to note that no one knows whether the conscious mind has anything to do with the collapse of the wavefunction. Many physicists think it does. When people tell you otherwise, ask them to prove it. You'll find, they can't.
  10. Dec 11, 2016 #9


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    It is impossible to prove a negative, so of course no one can prove that the conscious mind has nothing to do with the collapse of the wave function. However, that's not especially relevant to the original poster's question: will an interference pattern form if there's an unexamined detector at one of the slits?

    Please try to keep the the thread on topic here; interpretation discussions are hard enough to moderate effectively already.
  11. Dec 11, 2016 #10
    This depends on how one defines 'unexamined', and that's why in the standard approach one always ends up having to define what it means to be 'examined'--an observer-dependent term. This is why I posted the link above concerning how to define the act of measurement. The ambiguity remains in Feynman's comments in his Lectures:
    “Suppose you only want the amplitude that the electron arrives at x, regardless of whether the photon was counted at [detector 1 or detector 2]. Should you add the amplitudes [for those detections]? No! You must never add amplitudes for different and distinct final states. Once the photon is accepted by one of the photon counters, we can always determine which alternative occurred if we want, without any further disturbance to the system…do not add amplitudes for different final conditions, where by ‘final’ we mean at the moment the probability is desired—that is, when the experiment is ‘finished’. You do add the amplitudes for the different indistinguishable alternatives inside the experiment, before the complete process is finished. At the end of the process, you may say that ‘you don’t want to look at the photon’. That’s your business, but you still do not add the amplitudes. Nature does not know what you are looking at, and she behaves the way she is going to behave whether you bother to take down the data or not.” [Feynman 1965 Vol 3, 3-7; original italics and quotations]
    The ambiguity about what constitutes a 'final condition' -- 'at the moment the probability is desired' is removed if absorption is taken into account, which is what is noted in the linked blog post. Then one can define when the experiment is 'finished'--at absorption (detection) of the relevant quantum. Absorption is what makes alternatives distinguishable. It doesn't have to depend on anyone 'desiring' a probability.
    Regarding the original question, without appeal to whether anything is 'examined' or not, or whether anyone 'desires a probability', an interference pattern will form at the final screen if the original momentum superposition of the photon is retained at the point of detection of the photon. In principle one could even have individual detector screen elements that could couple to the momentum of the photon, but as long as there is no detection corresponding to that coupling, the superposition of momentum states is retained and the pattern is preserved. If in contrast you provide for physical detection of a momentum eigenstate at the screen elements, there will be no pattern. It is the mode of absorption/detection that defines the distinguishable alternatives, not anyone having to desire anything or examine anything,
  12. Dec 11, 2016 #11
    Sorry, I certainly don't want to make extra work for moderators, whose efforts on our behalf are, or should be, appreciated by all.

    I've said this myself many times, but actually it's not entirely true. For instance Bell proved "there is no realist, local HV model for QM". Indeed all so-called "no-go" theorems prove a negative! I could give more examples from physics, and elsewhere.

    True; nor can one prove the opposite. That's how one can tell it's an interpretation - not science, really, but philosophy. No interpretation can, at this time, be proven one way or the other. However some posts, for instance @rkastner's link above, assert definitely that consc has nothing to do with it. Since that's impossible to say, I thought it worth correcting.

    Actually, OP did ask specifically about "conscious mind":

    But let's ignore that. Still there is a point worth clarifying here. OP assumes "interference pattern disappears" and "collapse" are the same thing. But they're not. The detector introduces decoherence to destroy the interference pattern. It's irrelevant whether anyone looks at the result. But that's not the same as collapsing the wavefunction, reducing it to a single eigenvector. Instead both eigenvectors (|particle detected in slit> + |particle not detected>) are still present in the wavefunction, but not interfering. The collapse happens sometime between this event and the observer's involvement, no one knows where. Anyway, decoherence answers OP, and "collapse" just confuses the issue.
  13. Dec 11, 2016 #12
    I'd like to correct that I never asserted as a matter of 'impossibility' that 'consciousness has nothing to do with it'.
    Rather, I presented an argument opposing the many extant categorical claims that consciousness is required in order to define measurement. That's a very different thing. For the sake of making progress in understanding what measurement is, I hope that my arguments will not be mischaracterized in the future. Thanks!
  14. Dec 11, 2016 #13
    No, decoherence and unitary-only evolution does not resolve the issue of measurement (this is an ongoing debate in the literature). See, e.g., https://arxiv.org/abs/1406.4126 , which shows how the whole Quantum Darwinism program, relying only on 'decoherence' in a unitary-only dynamics, is circular.
    To assert that 'collapse confuses the issue' is to adhere to a particular interpretation of measurement in QM (unitary-only, Everettian relying on decoherence) that is arguably inadequate.
    There is in fact a consistent theory out there involving well-defined physical collapse, and there is nothing necessarily "confusing" about it, unless perhaps one has not read the relevant peer-reviewed literature with an open mind. See, e.g., https://arxiv.org/abs/1204.5227 , https://arxiv.org/abs/1608.00660
  15. Dec 11, 2016 #14
    I agree.

    No, you misunderstand. My point was it "confuses the issue" of OP's question, which is answered by decoherence, which removes interference pattern. In my judgment he's not really asking about collapse. Of course issue of collapse (apart from OP) is very important. I hope that my arguments will not be mischaracterized in the future. Thanks!

    AFAIK TI is a valid interpretation.
  16. Dec 11, 2016 #15
    We seem to be having a miscommunication. I'll try to clarify.
    You seem to be saying (again) here that decoherence is necessary and sufficient to remove an interference pattern. I know that's a standard position, but that's what I"m taking issue with.
    Interference, reflecting nonzero off-diagonal terms in the density matrix, can be understood without appeal to any decoherence argument--even though I know that is the 'mainstream' approach that everyone is taught now. Unfortunately it is quite inadequate, and many of those who originally championed it (e.g., Max Schlosshauer) have now realized that. Zurek still clings to the idea that decoherence-only can account for phenomena such as whether or not there is interference, but one can see from his response to my letter in Physics Today (https://arxiv.org/abs/1412.7950) that he continues to assume what he claims to demonstrate. (Basically, his response was just to say that of course cats aren't in superpositions. I.e.--to appeal to our macroscopic experience, which is what he claims to be deriving from unitary-only QM via decoherence.)
    Recall that decoherence invokes entanglement of the system with many environmental degrees of freedom which are then 'traced over' to get an (almost) diagonal density matrix for the system. But that is neither necessary nor sufficient (since the off-diagonal terms are never really zero in that approach, and one also has to assume classical distinguishability of the environment as an ad hoc assumption).
    What I tried to point out in my post with the Feynman excerpt is that all you need (rather than 'decoherence') is the response of absorbers, which lead to non-unitary collapse. That is what would destroy interference in the case of a two-slit experiment in which the relevant absorbers could respond only to a particular momentum eigenstate (as opposed to the superposition)--that would collapse the photon to only one of the momentum eigenstates. And that's why collapse is relevant to this issue.
    I do appreciate your saying that TI is a valid interpretation. With TI you don't need decoherence arguments to explain the measurement process, including whether or not there is interference. That's not to say that we don't see decoherence (diminishing of off-diagonal terms due to entanglement of a system with other systems) occurring in certain situations--it does occur under certain conditions. But decoherence alone is neither sufficient nor necessary to explain the measurement process and what we see as a result of measurements.
    Best wishes,
  17. Dec 11, 2016 #15


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    Yeah, I said something about like that, concerning interpretations, almost a month ago, right here.... lol
    I informally, more or less, said the same thing... right here .

    Obviously, it wasn't in this thread, so it might not count.... :oldeyes:
  18. Dec 12, 2016 #16

    Not necessary. There are many other possible explanations, such as predestination, absorber response, "God did it", and so on. To justify "necessary" one would have to deny every one of them. To justify that denial, one would have to study them all, and prove them all wrong - a never-ending, hopeless task. Decoherence is sufficient to remove interference pattern. AFAIK.

    You point out that it only explains how off-diagonal elements become near 0, but not exactly 0. That's right. But if they're near enough to 0 you can't get an interference pattern, FAPP.

    As far as I know, it is. But it makes the crazy (to use Bohr's word) assumption that influence can travel backward in time. Maybe so, who knows?

    That's right. Decoherence doesn't explain, at all, what we see as a result of measurements, nor did I say it did. That's because it doesn't address, at all, the collapse. But collapse is why we see one particular result out of all the possibilities. Or, more precisely, it's the word we use to describe that fact.

    BTW I reviewed your website a bit. The only thing that puzzles me is why you don't like the idea that consc may collapse wavefunctions (of course, it may not). I'd like to discuss it further but this isn't the place for it - especially, not this particular thread.

    Great minds think alike!
  19. Dec 12, 2016 #17


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    I would say even more strongly that there is absolutely no reason to think that there is something special about consciousness in QM. A measurement made by a machine seems to act exactly like a measurement made by a human being. I think that Von Neumann/London/Bauer/Wigner (whoever's idea it was to put the collapse at the conscious observer) did a disservice to physics by giving fuel to those who would connect quantum mechanics to mystical notions of the mind creating reality. QM is weird enough without that.

    Having said that, it does seem to be the case that QM has a macroscopic/microscopic distinction, which is almost (but not quite) as weird as the consciousness distinction. Once a physical property such as a particle's spin has been amplified to make a macroscopic difference, then the value seems to be definite in a way that never happens for microscopic values.
  20. Dec 12, 2016 #18


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    Yeah... that does seem to be the case .

    Oh snap!... I forgot... :headbang:
    In fact, just yesterday, I made a $20.00 donation to Wikipedia®... :oldgrumpy:

    I wonder if I can get my damn money back ?...:oldeyes:
    Last edited: Dec 12, 2016
  21. Dec 12, 2016 #19
    There is still some distortion of my statements--and possibly yours--going on here. What I did was contest your replies to the original question of loss of interference, which repeatedly asserted that decoherence is what answers it" That is an unsatisfactory answer; it's really NOT sufficient--that's my point. There is much in the published literature pointing out why it's problematic, so I'm just asking people to stop categorically asserting it as if it's settled science. It's not. Take a look at the Stanford Encyclopedia entry on 'decoherence' to see all the problems with it as a putative explanation for all cases of loss of interference.
    Re consciousness collapsing the wf: that fails to explain the form of the measurement transition, and it relies on an ill-defined appeal to a 'conscious observer' necessarily outside the scope of the theory. That's why I find it inadequate. The appeal to 'consciousness' as separate from the quantum system under study is a failure to solve the measurement problem, which can be solved if one takes absorption into account.
    Last edited: Dec 12, 2016
  22. Dec 12, 2016 #20
    Yes. TI provides an explanation for that.
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