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Double-slit experiment: any videos of the Observer Effect?

  1. Apr 14, 2015 #1
    Hello! I've been reading up about Quantum Mechanics, and I'm trying to understand various aspects of the double slit experiment.

    This is one of among many videos I've watched: Dr Quantum - Double Slit Experiment

    Here's my understanding so far of the basic setup (where the gun is shooting out particles one at a time):

    • The source sends out a waveparticle (photon or electron), which reaches the barrier with the two slits.
    • The waveparticle passes through both slits, and the result that emerges from each slit interferes with the result from the other slit.
    • The waveparticle is in a superposition state with a probability distribution defined by its Schrödinger wave function
    • The wave function apparently collapses at the film (the plane where measurement is being made) into a single spike of 100% probability at a specific coordinate, and we finally know where that electron ended up.
    I understand there is still a lot of discussion about the "collapse" phenomenon, and there is not a consensus yet (Copengahen interpretation vs many worlds, etc).

    But what I'm trying to wrap my head around is the Observer Effect as mentioned in the video at 3:40, where a detector is placed at the barrier to see what is actually happening at the slits. Apparently this causes the wavefunction collapse to happen early.

    So my questions:

    1. Is there any real life video / pictures out there showing the setup with such a detector at the barrier? I've seen live videos/demos of the basic setup that results in the interference pattern, for example this one and this one . But I have not been able to find a similar video that shows the result of the "detector at the barrier" scenario.

    2. What is the nature of the detector, or what is the detection mechanism? How were they trying to determine which slit the particle went through? Was there some kind of electro-magnetic detector that generates a signal when the photo/electron flew by? So the gun fires off one photon, which eventually ended up at the back film. I'm trying to grasp how that could occur if that photon was measured earlier and should either be absorbed or deflected by the measuring mechanism.
     
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  3. Apr 14, 2015 #2

    bhobba

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    I will leave it to those more into experimental stuff than me describe exactly how you observe electrons etc going through a slit.

    Instead I will address some misconceptions.

    First there is no such thing as the observer effect. Collapse is an interpretation thing - its not part of QM - some interpretations have it - others don't. the modern view is when you observe it going through a slit it interacts with what you observe it with which causes decoherence. The result of decoherence is its no longer in a superposition of going through both slits - but has a probability of going through one slit or the other. Technically its converted from a superposition to a mixed state.

    Secondly what's going on in the double slit is not what those beginner videos and other popularisations tell you. Here is a much better explanation:
    http://arxiv.org/ftp/quant-ph/papers/0703/0703126.pdf

    Thanks
    Bill
     
    Last edited: Apr 14, 2015
  4. Apr 14, 2015 #3

    naima

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    You do not need a detector to have a reduced visibility of the fringes. Experomentalists have used excited atoms with the Young slits. These atoms can emit a photon behind the slits. This can give a (partial) which path information. Even if there is no detector for these photons, the visibility of the fringes will decrease. I think that we should stop to think of "observation" (and observers).
     
  5. Apr 15, 2015 #4
    Hi Bill, thanks for your reply.

    Admittedly I'm new to this realm, and the paper you quoted is unfortunately beyond the scope of my comprehension. I am reading up where I can however, and possibly down the line I would be able to grasp more the technical aspects of the phenomenon.

    I couldn't find much on decoherence, but from this wiki article, it appears to be just another interpretation of what's going on in the double-slit experiment, as with Collapse, Many worlds, De Broglie-Bohm, and Qbism. Incidentally, I watched this World Science Festival video where people like Brian Greene, David Albert, Sean Carrol, Sheldon Goldstein, and Ruediger Schack debated those interpretations, but dechorence was not in the discussion.

    But that is getting away from the the main point of my thread. I know there is a long road ahead of me if I want to get an accurate technical understanding of what's going, but for now, the lay popularizations will have to do. The gist of it is that I can physically see real world demonstrations of the double-slit interference pattern, and there are several videos to this effect.

    But many videos I've come across also mention this phenomenon where if you put a detector at the slits, the interference pattern is gone. When you say "there's no such thing as the observer effect", do you mean that "they way these people try to convey this phenomenon is wrong"? Regardless of whether these popularizations are explaining the phenomenon accurately, I am very curious to see a real life video of this phenomenon. I want to see a video (or at least some photographs) where an experimenter has set up the basic double-slit experiment, and then put a detector at the slit (whatever the nature of that "detector" is), and see the interference pattern vanish. If I see this really happen, then I can delve into a technical exploration of this phenomenon.

    Or is this phenomenon purely a thought experiment and it has never been demonstrated?
     
  6. Apr 15, 2015 #5

    bhobba

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    Its a semantic difficulty. Usually when someone talks about observer they mean a rational, concious observer like you and me. The observer effect is taken to mean the presence of such an observer affects things in a way some inanimate observational apparatus wouldn't. That's wrong - and the sense the observer effect is a myth. That observational apparatus can affect systems is almost axiomatic - an observation is an interaction with a system that affects it.

    If you want to understand this a bit better the following may help:
    http://www.ipod.org.uk/reality/reality_quantum_intro.asp [Broken]

    I don't agree 100% with everything in that link such as the block universe but its a good place to start getting to grips with this stuff.

    Thanks
    Bill
     
    Last edited by a moderator: May 7, 2017
  7. Apr 15, 2015 #6
    Are you suggesting we have experimental evidence against:
    http://arxiv.org/abs/1405.0879
    ?
     
  8. Apr 15, 2015 #7

    bhobba

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    I have had bad experiences disentangling papers. How about you give a précis of the argument and we can take it from there?

    But regarding collapse, in interpretations that have it, there is no way of proving it one way or the other because it has been deliberately cooked up to be equivalent to the formalism of QM which doesn't have it.

    Thanks
    Bill
     
  9. Apr 15, 2015 #8
    That's because decoherence per se does not do anything when it comes to establishing a connection between the quantum formalism and the physical world (e.g. by accounting for definite states of physical systems or the appearance of such states to observers)
    http://philsci-archive.pitt.edu/11418/1/Reply.pdf
     
  10. Apr 15, 2015 #9

    bhobba

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    Come again.

    It transforms a superposition to an improper mixed state ∑ pi |bi><bi|. This is indistinguishable from a proper mixed state so explains apparent collapse. It doesn't explain collapse (in interpretations that have it) but it shifted the issue to one of - why do we get any outcomes at all.

    Thanks
    Bill
     
  11. Apr 15, 2015 #10
    Sure. Let me know if you need mathematical details.
    Are you familiar with GRW-style spontaneous collapse theories? Their formalism models the collapse process such that collapse-frequency is a function of the system's size. On the formalism I mentioned, collapse process is modelled such that collapse-frequency is a function of the system's quantum integrated information. Quantum integrated information is the physical correlate of consciousness.

    This is not true. The mentioned theory uniquely predicts that small isolated systems with high integrated information are localised. So the theory makes unique predictions for the behaviour of certain microscopic computers.
     
  12. Apr 15, 2015 #11

    bhobba

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    I know GRW. So we have a theory along those lines.

    What's not true? A collapse theory that was cooked up to be distinguishable from ordinary QM (yes I did gaze at the paper) and states 'We have presented a novel theory which is in conflict with quantum mechanics.' is the same as QM?

    If you like that sort of thing you might like primary state diffusion:
    http://arxiv.org/pdf/quant-ph/9508021.pdf

    Thanks
    Bill
     
  13. Apr 15, 2015 #12
    That site also mentions this phenomenon:

    Which is exactly the scenario I'd like to see videos/pictures of. Are you aware of any photographic evidence of this described phenomenon? Before I try to understand the phenomenon, I'd like to see it.

    The basic slit interference pattern is easy to replicate in the real world with a laser, a box, some razor blades, etc. There are videos of this. But what I can't find videos of, is this mythical "detector at the slits" follow-up scenario.
     
    Last edited by a moderator: May 7, 2017
  14. Apr 15, 2015 #13

    bhobba

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    No.

    It more an experimental thing which isn't my bag.

    Thanks
    Bill
     
  15. Apr 15, 2015 #14

    Nugatory

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    I've been posting this link a lot lately.... http://en.wikipedia.org/wiki/Delayed_choice_quantum_eraser#The_experiment_of_Kim_et_al._.282000.29

    Even without considering the delayed choice aspect of this experiment, it is clear that the mere availability of which-way information affects the interference pattern. It's frustrating that you don't get to see a screen with an interference pattern in it, instead have to accept the word of a hunk of electronics called a "coincidence counter", but that's a consequence of the clever trick used to detect which slot the signal photon went through without disturbing it in any way.
     
    Last edited: Apr 15, 2015
  16. Apr 17, 2015 #15

    naima

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    Hi Nugatory

    How could we define (with math?) the availability of this which way information?
    What is the difference between
    a) a photon coming from the slits can give informarion even if not measured
    b) an electron passing through theses slits gives no WW information if not measured?
     
  17. Apr 17, 2015 #16
    The statement that I quoted.
    All collapse theories are distinguishable from ordinary quantum mechanics.
    That's not hard since ordinary quantum mechanics doesn't make any determinate predictions.
    Excellent! Thanks - I was not aware of this and look forward to reading it.
    The issue has always been whether the physical system (Schrödinger's cat, say) remains in a superposition (as in Everett); collapses at around the time of measurement (collapse theories); or is not correctly described as being in a superposition at all (Bohm). We haven't explained apparent collapse until we've shown which of these theories (if any) is correct. Period. Apart from helping the Everettian respond to certain objections (e.g. preferred basis problem), decoherence is silent on the key issue.
     
  18. Apr 17, 2015 #17

    Nugatory

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    When the photon reaches detector D0 (equivalent to the screen in the text version of the double-slit experiment) its wave function will be one of three possibilities: a superposition of "went through slit one" and "went through slit two"; "went through slit one"; "went through slit two". When we say that which-way information is available, we're saying that it is in one of the latter two states.

    The experiment is designed so that every time that a photon in the superimposed state reaches the screen either D1 or D2 will trigger, every time a photon in the "went through slit one" state reaches the screen detector D3 will trigger, and every time a photon in the "went through slit two" state reaches the screen detector D4 will trigger. And just as predicted, only the photons on the superimposed state produce an interference pattern.

    This experiment (and most such experiments) are usually done with photons instead of electrons because it's easier and less expensive, but there's no reason to doubt that electron's would behave similarly if we could construct an analogous setup for them.
     
  19. Apr 17, 2015 #18

    Nugatory

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    I must confess to being baffled by this statement. "Ordinary quantum mechanics" makes only probabilistic predictions (which is not quite the same thing as not making determinate predictions), but so do many collapse theories, including all collapse theories that are consistent with experimental results.
    So how are they distinguishable?

    For that matter... A significant fraction of the population (maybe smaller than a few decades ago) would consider "ordinary quantum mechanics" to include collapse, so I'm not even clear about what classes of theories are being distinguished here.
     
  20. Apr 17, 2015 #19

    bhobba

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    A few issues.

    The cat in Schroedinger's Cat is never in superposition - its common sense classical - its alive or dead. The observation occurs at the particle detector - its classical from that point on. The issue Schroedinger's Cat addressed was exactly where to place that observation - in any set-up like Schroedinger's Cat its obvious where to place it - but Copenhagen, Ensemble etc doesn't dictate it - and therein lies the problem. Another way to look at it is QM is a theory about observations that appear in a common-sense classical world. But how does a theory that assumes such explain it.

    A lot of progress has been made in resolving it - but a few issues still remain (eg the factorisation problem) although most think its of the crossing t's and dotting i's variety - still one never knows.

    A large part of that progress has been in decoherence which explains how a superposition becomes an improper mixed state. That, at least in interpretations that have collapse, explains apparent collapse, not actual collapse, but apparent collapse. I could point to references supporting that claim but before going down that path I would like to hear in your own words why you believe decoherence does not explain apparent collapse. In particular can you explain how it is possible to tell an improper mixed state from a proper one. If it was a proper one then collapse would have occurred, but it is an improper one then it is only observationally the same - we cant say collapse has occurred - but 'apparently' it has.

    Thanks
    Bill
     
    Last edited: Apr 17, 2015
  21. Apr 17, 2015 #20

    bhobba

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    Collapse theories sometimes introduce small non-linearities that may be detectable.

    Thanks
    Bill
     
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