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Double slit questions

  1. Mar 2, 2015 #1
    1. If the measurement is done anywhere after the slits and before the screen, do I understand correctly that it does not create interference pattern?
    2. Should screen be thought of as a kind of measuring device?
    3. So electron is what? Floating probability? That sometimes can be localized in space (e.g. when measured) and at that time displays particle-like properties?
    4. OK, so when it's measured it behaves like particle. But why does it not go back to wave state after measuring point? Or maybe I should not think of such thing as "before" or "after" measuring point in this experiment, and consider there is "no time" here?
     
    Last edited by a moderator: Mar 2, 2015
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  3. Mar 2, 2015 #2

    jfizzix

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    Quantum mechanics is only equipped to describe what you are likely to see when you measure something.

    If you put a pair of slits in between source and screen, and there exists no way to tell which path the particle went through, you see an interference pattern.

    If you change the experiment by adding some component that allows you to see which slit the particle goes through, you see something different (you see no interference pattern).

    All in all, this is not too unusual. If you change your experiment, you change what you end up seeing.

    What seems unusual is when you try to imagine this experiment going on as though the particles were well-defined blobs of stuff; we would have to imagine that the blobs definitely go through one slit or the other, and yet create an interference patten when we are not able to see what's really going on.

    Quantum mechanics itself doesn't have much to say about this, except to answer the question of what you would likely see, say, if you move the slits and screen around. It describes entire experiments as a whole.

    The metaphysics of what is going on when we're not looking is still under debate.
     
  4. Mar 2, 2015 #3

    jfizzix

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    if you want to describe a completely isolated electron, I would describe it as a quantum wave obeying Schrodinger's equation.

    If that electron interacts with anything else, like a detector, a proper quantum mechanical description would have to include the quantum states of both electron and detector.

    If the detector interacts with a computer screen, then a proper quantum mechanical description would have to include the quantum states of electron, detector, and computer screen.

    But then, it's turtles all the way down.
    Sophisticated descriptions usually just stop at the electron plus detector level.
     
  5. Mar 2, 2015 #4
    Thank you.

    If instead of one electron gun and 2 slits, we use just 2 electron guns close to each other (away from each other the same way slits would be), and shoot 1 electron from each gun at the same time. Would these 2 waves/electrons interfere creating inteference pattern?
    Has anyone tried that?
     
  6. Mar 2, 2015 #5

    jfizzix

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    Ordinarily, I would say no. Two independent electrons emitted from two independent electron guns cannot interfere with each other (at least, not in the same way that one electron can produce an interference pattern with two slits).

    However, if the two electrons were entangled with each other so that their joint state could be described by a single wavefunction, then yes, I expect you could see interference. However, it's an interference pattern you would only be able to see two electrons at a time.

    This has been done in a different form with entangled pairs of photons. I don't know how you would create a source of entangled pairs of electrons, though.
     
  7. Mar 3, 2015 #6

    Simon Bridge

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    1. If the measurement is done anywhere after the slits and before the screen, do I understand correctly that it does not create interference pattern?

    Depends on the measurement.
    If the measurement is such that it determines the path the partical took, then the interference cannot happen.
    A single partical experiment also does not get the pattern... only one particle gets detected in a specific location. Any "pattern" is built up over many particles.

    2. Should screen be thought of as a kind of measuring device?
    It should be thought of as an array of measuring devices.
    The simplest form of the experiment has a source and a detector.
    Over many experiments, the probability of detecting a particle at some location, given that a partical has left the source, can be determined.

    3. So electron is what? Floating probability? That sometimes can be localized in space (e.g. when measured) and at that time displays particle-like properties?

    In the model we are discussing, the electron is a partical.
    The electron itself is never observed to have wave-like properties ... the wave properties are in the statistics.

    4. OK, so when it's measured it behaves like particle. But why does it not go back to wave state after measuring point? Or maybe I should not think of such thing as "before" or "after" measuring point in this experiment, and consider there is "no time" here?

    Time as a special dimension is valid in this model.
    The electron position is no longer uncertain after it has been measured.
    This is much the same way that the number that is rolled on a die is ujcertain before the roll but dertain afterwards.

    Note, you can also do the experiment with two sources instead of one source and slits.
    You can arrange to get an interference pattern just like you can with regular Youngs interference.

    To get a better idea about how this quantum partical thing works, see Feynmans famouse lecture series...
    http://vega.org.uk/video/subseries/8
     
  8. Mar 3, 2015 #7
    The two sources would have to be coherent. What sort of arrangement are you thinking of?
     
  9. Mar 3, 2015 #8


    Thank you Simon for commenting on the questions, espcieally 1 & 4. (I understand pattern is built up by particles over time.) In relation to 1 & 4:

    5. So can I say that: particle is ejected from the gun, goes (as probability wave) through first & second & neither & both slits, is measured on the other side in a way that it would determine its path, e.g. path was slit 1. Does it mean it goes "back in time" and passes through slit 1?
    I'm trying to apply your dice example here, but cannot easily visualize that.​
     
  10. Mar 3, 2015 #9

    Simon Bridge

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    It makes no sense to describe the particle as a probability wave at any stage of it's passage through the apparatus.
    The particle does not turn into a wave and them magically transmute back to a particle in the detector.
    The wave nature that shows up after many runs of the experiment is entirely statistical.

    You are used to this relationship in classical probability: you can throw three dice and add them up - each throw gets you one number.
    After lots of thrown you can build up a pattern which shows the probability distribution of the different possible outcomes.
    The outcome of a dice throw and the probability distribution are different things.

    If we know in advance which die produces which number, then there is no longer the usual probability distribution. The outcome is certain.
    If we don't know what the numbers will be, then the distribution is used to help predict the outcome.

    The outcome of a particular dice throw is roughly analogous to the detection of a particle at a particular position while the probability distribution of possible outcomes is (roughly analogous to) the square amplitude of the position wave-function.

    The difference between classical and quantum probabilities are (a) the probability amplitudes are allowed to be negative, and (b) (loosely) while classical probability describes the experimenter's state of knowledge of the system, the quantum probability describes the Universes state of knowledge of the system.

    The description of the particle's trajectory as having passed through one of the slits or both of them has no meaning where the trajectory has not been measured.
    The only description that means anything is that a particle was emitted, a particle was detected, and there are restricted paths that were available. The detection suggests that the particle must have traversed the apparatus but it says nothing about how it did so.

    Have you watched the Feynman videos yet?
     
  11. Mar 3, 2015 #10

    Simon Bridge

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    Certainly uncorrelated sources just add their intensities.
    I don't want to get sidetracked. You can search the lit for "two source quantum interference" for a host of examples.
    We are basically in agreement - "it is possible to arrange..." i.e. using coherent sources.
     
  12. Mar 4, 2015 #11
    Thank you Simon, I'll need some time to assimilate this subject.

    I have not seen videos yet. Thank you for that too.

    Robert
     
  13. Mar 4, 2015 #12

    Simon Bridge

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    No worries - you've figured out that the others are talking about stuff that normally shows up later in your education I take it?
    I'm concentrating on a "it's real if you can measure it - the rest is just maths" approach. Theoreticians can have other ideas.
     
  14. Mar 4, 2015 #13
    :), yes, re what others talk about here, I'm following that closer in the other universe where I took different path at uni :)
    I hear what you say about statistcs, though in this case stats and theoretical math shows up as real stripes of different kinds on the screen.. I'll think about it.

    Interesting you mentioned education anyway, cause I had a question about it I was going to ask separately, but since we're on it..
    So, I have this friend :), he did his degree in electronics engineering ~20 years ago, did a level of math at that time, but have not touched it since, forgot a great deal. Looking at his theses now, he follows high level process, but knows he could not do that math now without restudying large parts of it.
    Anyway, he finds quantum physics fascinating, and wonders about mathematical side. Wonders which part of math he would need to learn to look at some of the QF equations. He also believes that looking at math side in more details would help him understand some concepts better, though he does not know where to start.
    If there are resources, good books, etc you think would be worth looking at, I'd be glad to pass it on to him :). Or if the comment you'd could offer is of the kind that would bring him down to earth and make him forget about it, I'd like to hear that too.

    Thank you in advance.
     
  15. Mar 4, 2015 #14

    Dale

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    Several off topic posts have been removed.
     
  16. Mar 4, 2015 #15
    That is the shocking thing, the wavelength collapses at the point you observe it. So if you were to measure it closer to the slit, it would still give you the same pattern, the electron is everywhere until you observe it!
     
  17. Mar 4, 2015 #16

    bhobba

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    You are falling into a trap of ascribing properties to a system when its not observed. QM is silent about what's going on until and unless it's observed. All a state tells you is the probabilities of observations if you were to observe it.

    Thanks
    Bill
     
  18. Mar 4, 2015 #17

    Simon Bridge

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    ... no, the wavelength does not collapse. As well as "falling into a trap of ascribing properties to a system when its not observed" (thanks bhobba), it looks to me like you are imagining the "electron wave" as something physical in the sense of a water wave. This is a common but misleading image.

    I think you, also, will benefit from watching the Feynman lectures linked earlier (post #6). Please do so.
    Note: your misconceptions here have been addressed before in this thread. rereading the previous discussions may help.
    There is nothing to be gained from making this experiment more confusing and spooky-seeming than it already is.

    I don't usually comment on these things - learning is so personal: what is good for one person may actually harm another. I just don't know your friend well enough to make a determination and anyway, there is lots of advise out there.
    The maths in QM is mostly linear algebra, with a basic understanding of probability and calculus.
    Most people (in NZ) just pick it up as they go along. I never understood algebra or stats until I did QM ...
     
  19. Mar 4, 2015 #18

    bhobba

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    I just want to add they are very common and particularly difficult to displace.

    Prior to posting here I read a lot of books on QM and I was infected with them. It was only during the first few months of posting here and meeting the 'challengers' of other posters they finally were banished. So don't beat yourself up about it - it takes time for it to 'soak' into your soul so to speak.

    Thanks
    Bill
     
  20. Mar 4, 2015 #19

    Simon Bridge

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    Agreed, it does not help that so many sources are keen to reinforce them.
     
  21. Mar 4, 2015 #20

    bhobba

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    Very true.

    I am discussing this stuff in another thread with a guy who has a copy of the following advanced text which I also have:
    https://www.amazon.com/Decoherence-Classical-Transition-Frontiers-Collection/dp/3540357734

    Presumably he has studied it.

    Schlosshauer is very careful to explain the issues correctly - it really is an excellent book - one of my favourites. To the OP if you want to study the detail of QM then that text is approachable after a first exposure to it in books like the following:
    https://www.amazon.com/Quantum-Mechanics-The-Theoretical-Minimum/dp/0465036678
    https://www.amazon.com/Introduction...d-Griffiths/dp/0131244051/ref=asap_bc?ie=UTF8
    https://www.amazon.com/Quantum-Physics-Dummies-Steve-Holzner/dp/1118460820

    Despite that he also seems to have misconceptions.

    Thanks
    Bill
     
    Last edited by a moderator: May 7, 2017
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