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Slit experiment follow up experimentation

  1. Dec 30, 2013 #1
    Apologies in advance. I have only the most cursory exposure to qm physics.

    The famous slit experiment.

    Clearly, the slit experiment has been performed countless times...

    What I have not heard:

    - What happened when detectors were set to observe both slits?

    - As detectors are moved *away* from a slit, at what distance/event/circumstance, did the electrons begin to behave like waves again?

    Thank you!
  2. jcsd
  3. Dec 31, 2013 #2
    See Wikipedia. It is stated that the interference pattern disappears. With photons this is obvious, because during detection photons are absorbed, so disappears, including their wave.
  4. Dec 31, 2013 #3
    Thanks for the pointer... that's interesting...

    But I was speaking about electrons specifically.

    Wikipedia states that "It was stated that any modification of the apparatus that can determine which slit a particle passes through will reduce the visibility of interference at the screen, thereby illustrating the complementarity principle: that light (and electrons, etc.) can behave as either particles or waves, but not both at the same time. However there is no publication where this experiment has been actually done."

    What? Nobody has ever published this experiment?

    And if "absorption" of a single particle was required to measure it, then it seems to me that the experimental apparatus would show no result as all particles are absorbed... but I keep seeing reference to a change of behavior ONLY, and not absorption.

    But that leads to another question... what if the instrument is not one that detects electrons? Do electrons behave differently around instrumentation that can't detect them? Can you put a rubber duck to watch the electron passing by a slit... and have it cause the electron to behave as a particle?

    If not, then how does an electron know the "purpose" of the instrument?

    Interestingly, there is reference to something similar... the eraser variation of the test... where the instrument is used to detect the particle, but then that detection information is discarded... and the particle goes back to behaving like a wave.

    Then again... I see mention of how instrumentation must necessarily alter the particle... so this is not really an issue of *detection*, it seems to me, but rather an issue of *interference* with a particle.

    It's much less mystifying to say "when we interfere with a particle, it behaves differently".

    What seems more interesting is the entangled pair behaviors. Entangled by what?
  5. Dec 31, 2013 #4


    Staff: Mentor

    Position localization occurs when the wavefunction (ie state) interacts with an object. Most of the time, for technical reasons that can be found in textbooks on decoherence, and associated with the 'radial' like interaction most objects have, position is what is usually 'measured'. But not always - apparatus can be designed to measure all sorts of things like momentum and spin. The way it knows the purpose of the instrument is it interacts in different ways with different instruments.

    Entanglement occurs either by interaction or when particles are created. It's rather obvious if you know what entanglement is though:

  6. Jan 1, 2014 #5
    To be honest, I added that to Wikipedia recently. But I see now I must make clear that this concerns only photons. And yes, I have also asked on several forums, and nobody knew a publication of an actual measurement. That is indeed remarkable.
    Electrons are more complicated then photons and are much less discussed and published, so I have not been looking to that yet.
  7. Jan 1, 2014 #6
    Hi Bill...

    Ok... so just to be totally clear, if an instrument is made such that it only affects the wave such that spin (and not position) is the only characteristic that interacts with the instrument, then the effects which rule position are not affected, and the wave behavior is maintained?

    And further, that an apparatus that would otherwise be able to determine position, but for a simple decision to NOT finish the apparatus such that position information is never used/displayed/recorded (like a microphone that is powered, but not plugged into anything), the wave function collapses due to the interaction with the apparatus, irrespective of any actual measurement.

    And lastly, it is the perceived instantaneous responses in entangled pairs or groups that cause great interest in quantum computing.... in other words, the potential of fantastic computational speed (limited only by the "overhead" of the system).

    Thanks ever so much!
    Last edited: Jan 1, 2014
  8. Jan 1, 2014 #7
    Ok, with all this entanglement business... might all of this fit into some notion of the simulation hypothesis... where when we think we are looking at a discreet particle, we're really just looking at a "pointer" to a sort of "register", that also have other pointers pointing to it... so when we think we are changing a discreet thing, we are really changing what is being pointed to... so all other pointers appear to have changed, but really, we're just looking at the same thing?

    In other words... we are bumping into the limits of the simulation...?
  9. Jan 1, 2014 #8


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    You might try this: http://physics.wm.edu/~evmik/classe...ics_251/manual/single-photon-interference.pdf

    Also google around for "Quantum Eraser" to see some publishable experiments with photons and which-way information.
  10. Jan 1, 2014 #9


    Staff: Mentor

    It depends purely on the apparatus. Most actually destroy whats being measured. Only some measurements, known as filtering type measurements, change the state of whats being measured, although some texts give the impression all are like that.

    By not finish I presume you mean not display the outcome. Displaying or not displaying it has nothing to do with if an observation is made. In modern times an observation occurs when decoherence occurs and that has nothing to do with displaying the outcome.

    Not into quantum computing - it might be best to start a separate thread about it.

    Last edited: Jan 1, 2014
  11. Jan 1, 2014 #10
    Thanks, Bill...

    So... if there is no display or record of a reading... then really, an observation has not been made.

    I think it's probably a critical distinction in a search for best terms.

    I think that what is happening has nothing to do with observation, but rather interaction.

    If you place a device near a slit that interacts with the particle/wave in certain ways, you can collapse the wave function while letting the particle pass...

    I think that talking about all this in terms of "if you observe" is rather unfortunate phrasing that only adds layers of irrelevant mystery. (I'm speaking about what I am reading and seeing generally).

    It sounds like the proper technical term is decoherence... so I'll look into that a bit.

    All very enlightening.

    Thank you again.
  12. Jan 2, 2014 #11


    Staff: Mentor

    No - I specifically stated the opposite. An observation is generally considered to have occurred if decoherence happens. Specifically, at a technical level, its when a superposition is converted to an improper mixed state. But if you don't understand the technicalities, just understand that entanglement is a very basic quantum phenomena, and a particular type of entanglement, called decoherence, leads to an effective observation.

    Exactly what all these terms I through at you mean, eg effective observation, decoherence etc etc requires studying of the detail of QM. If you know that then the following will explain it:

    But if you don't know these details then explaining exactly whats going on I cant do. You simply have to accept the hand-wavy soundbites I said above. It will become a bit clearer though if you do a Google search on entanglement and dechorence.

  13. Jan 2, 2014 #12
    Thanks again, Bill..

    I just have learned (from one source) that entanglement and observation is actually the same effect (according to the maths). I still gather that it relates to what is necessary to make a change in the "instrument" (I prefer object)... The exact amount and type of change is important (apparently entanglement is fragile). But a physical system has to change to an extent that it one could conceivably (if not actually) observe the change.

    But no... I don't know the details... so I am having to take a lot of things on faith :)

    And I understand that the Copenhagen view is simply wrong... the wave function does not "collapse"...

    It is all rather confusing, and the issue is of course I am trying to understand it on a classical basis, which I believe Einstein was also enamored with.

    If I am wrong in the same way as he, at least I am, in that general way, in good company :)
  14. Jan 2, 2014 #13


    Staff: Mentor

    You have found a good source - that is correct.

    However some pop sci stuff like 'What The Bleep Do We Ko Anyway' tries to promulgate mystical nonsence that it MUST be human consciousness that causes observation.

    That was a view some of the early pioneers like Von-Neumann and Wigner put forward. It never really caught on because such a drastic view is simply not required - its sort of like Solipsism - sure you cant prove it wrong but virtually everyone exposed to it rejects it.

    Unfortunately Von-Neumann died young and was never exposed to the modern view based on decoherence. Wigner did, and when he found out about some early work on it by Zurek realized such a drastic view was definitely not required and did a 180% turn and embraced decoherence - in fact he embraced a variant where some sub quantum process is involved eg:

    But no need to go into that now.

    These days that consciousness is involved is very much a minority view - but new age touchy feely, well to be blunt rubbish, try to make out its an accepted fact - which it most certainly is not. Its then used to justify what can only be described as total nonsense like for example The Secret.

    Last edited: Jan 2, 2014
  15. Jan 2, 2014 #14


    Staff: Mentor

    Wrong would be too strong a view.

    Check out:
    'Is that true? Is that a sign of a problem of the Copenhagen interpretation? It is surely true. It's how the world works. However, one may also say that this was a point in which the Copenhagen interpretation was incomplete. They didn't quite understand decoherence - or at least, Bohr who probably "morally" understood what was going on failed in his attempts to comprehensibly and quantitatively describe what he "knew".'

    Yes its 'wrong' (incomplete would be a better term) - but the error isn't really all that bad. It's better viewing decoherence as an update fixing a bit of a blemish.

    BTW with decoherence collapse is still there - its just hidden and rendered innocuous. However the exact way it does that, and if its a valid view, is vigorously debated. At your level however don't worry about it it - if you are keen you can learn the details and we can have a debate about it - but until then I wouldn't get too caught up in it.

    Last edited: Jan 2, 2014
  16. Jan 2, 2014 #15
  17. Jan 2, 2014 #16


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    How is a "slit blocker" not a "detector"? That's not a frivolous question - they both generate which-way information.
  18. Jan 2, 2014 #17


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  19. Jan 2, 2014 #18
    The pattern is caused by interference of two waves, through the two slits. If you block one slit there is only one slit open, so one wave, so obvious no interference.
  20. Jan 2, 2014 #19
    Thanks, didn't know that publication. I will look into it. Normally measurements using polarazition thereby disturb the coherence, but perhaps it is different here.
  21. Jan 2, 2014 #20
  22. Jan 2, 2014 #21


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    Of course not. Detection will destroy the photons and will not allow them to reach the screen, so you place reversible markers instead.

    Sorry, I thought you were discussing that wiki quote
    "It was stated that any modification of the apparatus that can determine which slit a particle passes through will reduce the visibility of interference at the screen, thereby illustrating the complementarity principle: that light (and electrons, etc.) can behave as either particles or waves, but not both at the same time. However there is no publication where this experiment has been actually done."

    The quote is not about placing detectors at the slits and the way it is formulated is wrong. Putting polarizers at the slits is a modification that can determine which slit a particle passes through and has been done several times. If you really want detectors at the slits, it is way easier to do that with Rydberg atoms or other particles.
  23. Jan 2, 2014 #22
    You are right, but the present Wiki is better. Probably it is and older state during my editing which was not right yet.
  24. Jan 2, 2014 #23
    Right, which is why I have problems with the term "observation", which I think must be behind all of that nonsense. Observation strongly implies an conscious attempt to discover...

    It has to be just a simple and specific type of physical interaction, completely irrespective of intent of the device. A backhoe interacts with soil in a specific way, but we never say that a backhoe is a "dirt detector" or a dirt observation tool.

    It's just that as smart monkeys, we're pretty good at making things that interact in ways that are useful. I kind of wish "observe" was never used in all of this... I feel it's misleading.
  25. Jan 2, 2014 #24


    Staff: Mentor


    Combined with the fact most of these new age types don't learn QM from proper textbooks like Ballentine - QM - A Modern Development - that make it clear observation has nothing to do with an observer - this leads to much nonsense and confusion. Even some otherwise good textbooks don't do it - but generally they are at the beginner level so is excusable - Rome wasn't built in a day - and you cant delve into every nuance in a complicated subject like QM from the start. I also fell into this trap and it wasn't until I read books like Ballentine it was much clearer.

    But stuff written for the lay public - generally - with a few exceptions - forget it - its often a mish mash of half backed outmoded ideas like an unclear distinction between observer and observation and that damnable wave particle duality:

    Last edited: Jan 2, 2014
  26. Jan 2, 2014 #25
    Wonderful link, Bill. Thank you.

    I've spent about a total of 3 hours looking into QM, and it's been about the most fruitful 3 hours I've spent in a long time, thanks to your most patient and insightful comments.

    All the best to you.

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