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Statistical Interpretation

  1. Dec 9, 2006 #1
    I thought Statistical Interpretation of quantum mechanics is a very good alternative besides the orthodox interpretation. But I just can't explain the particle-wave duality from the statistical point.

    How could the Statistical Interpretation explain the interference effects in the Quantum mechanics?
     
  2. jcsd
  3. Dec 10, 2006 #2
    Forgive me my ignorance but the p/w duality is described by statistics per definition. I mean, if you look at the dubble slit experiment, the probability of a particle "passing through both openings" is NOT equal to the sum of the probabilities of "passing through one opening" (while the other one is closed).

    Conclusion : the classical additivity for probabilities does no longer work here !!! That is the entire point of the p/w duality. Statistics are an inherent part of this concept from the beginning on.


    marlon
     
    Last edited: Dec 10, 2006
  4. Dec 10, 2006 #3
    Perhaps I could be of service here. Lets illustrate the idea on the double-slit experiment.

    Single electrons (i.e. one at the time) pass through a screen with two slits and hit a
    detector that tells their positions. One electron leaves one spot on the detector. There
    are no waves.

    After the detector collects many spots a pattern in their positions appears: the interference
    fringes.

    Does this emergent pattern mean that electrons are waves?
    Not at all. It means that there is a correlation between all the electrons.

    How can they be correlated if they have never been in contact with each other, you may ask?
    The preparation procedure of the electrons is common to all of them. They all are
    prepared in the same way. And this preparation procedure is responsible for the correlations.

    If one prepares the electrons in different ways, no interference fringes will show up, in general.

    Cheers!
     
  5. Dec 10, 2006 #4

    I don't think this is a very good conclusion. First of all, one should not be asking the question whether electrons ARE waves. This is NOT what the double slit exp tell us. It tells us that particles can BEHAVE like waves. the interference that is shown is a typical wavelike behaviour in classical physics. Particles do NOT exhibit this type of behaviour in that particular physical regime. However, the double slit exp proves us that on the atomic scale, particles (as defined in classical physics) DO exhibit this wavelike behaviour that we defined in classical physics.

    Also, what do you mean by saying "the electrons are correlated" ? How are they correlated ???


    Are you saying that the outcome of the double slit experiment depends on how the electrons are prepared ? If you look at the formal description of this experiment in any intro QM book, can you tell me what they say about "electron preparation" ? Finally, what exactly do you mean by "electron preparation" ?

    regards

    marlon
     
  6. Dec 10, 2006 #5
    Ohw, do you have experimental evidence to back that up ?! I mean, you have some hidden messengers in mind here I guess which communicate wheter both slits are open or not depending upon outcomes of previous measurements ?
     
    Last edited: Dec 10, 2006
  7. Dec 10, 2006 #6
    What is the very good conclusion, my dear physics guru?

    And which particle behaved in such a way? In my eyes no one. The interference shows up
    on the level of an ensemble of electrons. No single electron has been captured behaving
    like this.

    I don't know what you are writing about. The original question had nothing to do with
    classical physics and as far as interference is concerned one can do without.

    The probability of finding a spot in some regions of the detector is greater than in others.
    By the "correlation" I mean that all the electrons where drawn from the same probability
    density (i.e. a single particle correlation function) which in turn is determined by the
    incident electron preparation procedure.


    Yes. I'm saying precisely that.

    One more example that reading without understanding has no use. Check out
    the electron preparation procedure in the Tonomura experiment (A.Tononmura et al. ,
    Am. J. Phys. 57, 117 (1989)).
    Since you seem to like classics check out what has to be done to observe fringes in
    the Young experiment with a white light source. Or better yet try to do this pretty
    old experiment on your own. Is it possible to see fringes there with the use of only two
    screens (one with the slits and one as a detector), my guru?
    Or, perhaps, I should not be asking this question either?

    Cheers!
     
  8. Dec 10, 2006 #7
    Hi Careful,

    Yes, I do. The electrons have to be selected very carefully. For example, if their momenta
    vary too much the interference fringes go away. This is because incident electrons with different momenta give interference fringes in different places.
    More the colimation of the beam of electrons has to be good. Take a look at the
    figure 3 in A. Tonomura ..., Am. J. Phys. 57, 117 (1989). It shows what it takes
    to prepare the electrons.

    Another example are experiments with light. It is very easy to see fringes if you have
    a laser at your disposal. With a white light source it is not so simple.

    No messengers in mind :).

    Cheers!
     
  9. Dec 10, 2006 #8

    Gokul43201

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    marlon, see Sakurai Ch.1 Section 1.4 and Ch.3 Section 3.4 for preparation of a pure ensemble (unless this is not what zbyszek is refering to).
     
  10. Dec 10, 2006 #9
    The one i gave you.

    I am not talking about one single particle. Remember that in my very first post in this thread, i clearly stated that the "statistics" are inherent to the double slit experiment. So for me to be talking about one single particle, would contradict my own words.

    What i said was that on the atomic scale, EVERY particle exhibits wavelike properties. The socalled duality does not apply to certain particles and not to others. One can replace the electrons by photons etc etc...The outcome of the double slit exp would still reveal the same kind of information.

    But I WAS talking about ensembles...STATISTICS remember...


    Actually, what i wrote there is the ONLY and correct way in which you should interprete the p/w duality. We don't call this atomic scale behaviour a DUALITY between waves and particles for no reason, you know.

    One does NOT need all that to properly describe the particle wave duality and how it can be shown by the double slit experiment.


    Well, my point is that within the context of this thread, that is not the key aspect because you don't need it to describe the duality.

    For that matter, check out how the double slit exp is introduced in Bransden and Joachain's INTRO TO QM.

    Again, i know what the experimental requirements are, but that is NOT what i was talking about. I already told you that i was objecting against the way you look at the actual duality (ie the point i made, to which you so eloquently replied "i don't know what you are talking about").
    LOL...This is irrelevant. I am not talking about "white light" versus laser light etc etc. i know what happens with each light source. You are missing the point : in the case of the double slit exp, the formalism is important and is very clear. I objected against your interpretation of what happens with the "particles" in the double slit exp.


    I dunno...

    Cheers
     
    Last edited: Dec 10, 2006
  11. Dec 10, 2006 #10
    So, what do you have in mind then if I may ask ?

    Careful
     
  12. Dec 10, 2006 #11
    Thank you for your patience, my guru! Let me ask for some more, please.

    Your are not talking about any single particle, right?
    Still, "EVERY' one exhibits wavelike properties. Is "EVERY" somewhat different from 'every'? If it is not, then can one infere from the above that each particle behaves like that? If this is so, could I humbly ask about the source of such wisdom?

    I have heard a heresy that members of the ensemble do not necessarily inherit its properties.


    Next time my weak mind deviates from THE WAY, I promise to punish (i.e. enrich, of course) myself with contemplating the depth of your original post.

    Cheers!
     
  13. Dec 10, 2006 #12
    Well, let me explain. In every intro to the double slit experiment, people talk about a bunch of particles that behave like waves. This is obviously correct. If you take out one particle, and both slits are open, we cannot deduce which slit the particle went through. Nor can we explain what happens with the actual trajectory from source to destination. All we can do is "sum up over all possible trajectories". If a particle would NOT respect the wavelike duality (which you seem to imply) this behaviour would NOT be possible for very obvious reasons. One particle with certain initial conditions will appear somewhere on the detector. Another particle with the exact same initial conditions has a large probability of appearing in another position of the detector. Moreover, the spread in momentum will be INFINTE. Hey, guess what, this story sounds familiar, no ? THAT IS MY POINT.


    Also, you are clearly not famliar with the QM concept of self interaction. This applies TO ONE SINGLE PARTICLE, and guess what this stuff means in the context of the double slit exp. YES : Shooting a single photon (or electron) at a wall that contains two slits will result in an interference pattern on the wall (detector plate) beyond the holes.

    Like you said yourself, you should not only read without understanding. Check out any intro QM book and read between the lines as well !!!

    True yet irrelevant in this case. One single particle does NOT behave "classically" on the atomic distance scale. What do YOU think " NOT behaving classicaly" might mean. Try to find out this apparent mystery, my dear.


    Not necessary, any self respecting intro QM text will do just fine.
    Besides, another think for you : destructive interference does NOT mean NO interference...my dear

    Good Luck

    marlon
     
    Last edited: Dec 10, 2006
  14. Dec 10, 2006 #13
    I am not sure if we connect. I say that in statistical interpretation the fringes in the double
    slit experiment come from the preparation procedure and not from some assumptions
    about wavelike nature of the electrons.
    In other words: from the fact that the ensemble of electrons is prepered in some
    wave function that exhibit fringes under position measurement.
    Let me emphasize that the wave function describes an ensemble and not a single member
    in that interpretation.

    No need for any communication between any two electrons or a to-be-send electron and the detector.

    If this doesn't help, could you formulate a specific question?

    Cheers!
     
  15. Dec 10, 2006 #14
    I presume that in double slit experiments, the electrons can be controlled in such a way that they hit the screen one by one with a considerable time delay. Even then, we are told, the wave pattern builds up. I do not see how you could get such result by preparing the source in some special way unless you assume that there is some physical wave guiding the particle or so. To phrase it correctly, what is the dynamics at the single particle level you have in mind to get the outcome of the double slit experiment.
     
  16. Dec 10, 2006 #15
    WRONG. You keep telling me that in some cases the wavelike behaviour of particles cannot be seen from the double slit experiment. Even in the case on destructive interference, there is wavelike behaviour because interference IS BY DEFINITION a wavelike property. You are using the terminology of waves to say, hey in some cases the duality is not clear. Can't you see the contradicitio in terminis, my dear ?

    WRONG AGAIN !

    Look at the intensity profile of the double slit diffraction. One peak occurs in between the openings etc etc. Such behaviour CANNOT be explained when the particles are "only particle like". This violates the laws of physics. Again, you are forgetting the concept of self interaction here.

    marlon
     
  17. Dec 10, 2006 #16
    What would you say about Bohmian trajectories, my guru. Particles there do not behave like waves at all and still give correct predictions as to the fringes. Does this Bohm's speculation
    have bad karma? What invalidates the interpretation? Why is it impossible?

    Spread of what in momentum?

    You do not surprise me with your knowledge of other branches of physics. But still
    your revelations would be better understood if you had given a reason behind them.
    How do you know the behavior of a single particle? Does it behave classically on 1 meter scale? One kilometer? Is there any theory in your textbooks of single quantum particles?

    Why go after second-hand truths, if the guru is in the mood for explanations?

    Cheers!
     
  18. Dec 10, 2006 #17

    The guy is talking about the initial wavelike properties that need to be respected for destructive interference to occur (no interference pattern on the detector). A necessary condition for acquiring a interference is determined by the difference in pathlength between two paths that light can take to reach a zone of constructive interference on the viewing screen. This difference must be the wavelength of the light that is used, or a multiple of this wavelength. Newton rings learn us that light does not have to be coherent to produce interference patterns but such patterns are very clear when monochromatic or near-monochromatic light is used. Laser light gives the best (most visible) interference pattern.


    He concludes from that that particles do not behave like waves in that case but that is WRONG since all kinds of interference are wave like behaviour, obviously.


    marlon
     
  19. Dec 10, 2006 #18
    Yes, that is why I asked him about the single particle dynamics. You know that I would not say that particles behave like (or are sometimes) waves but that the wavelike property is due to a self interaction effect (actually, you could make a (classical) particle theory of waves, but that's rather difficult to control mathematically). But clearly, something wavelike needs to be present at the single particle level; or you could think about a random walk with complex amplitudes with ``ket's'' and ``bra's'' and define a particle event as the merging of a ket and bra, then you give up the notion of a fundamental particle which exists all the time but this leads to other kinds of undesirata.
     
    Last edited: Dec 10, 2006
  20. Dec 10, 2006 #19
    O.K. Now I get it.

    I have no idea, what is the dynamics of a single particle. It could be bohmian trajectories
    or another non-local hidden variables speculation. The theory of single particles
    is not available so far.

    However, in the statistical interpretation as formulated by Ballentine in Rev.Mod. Phys. 42, 358 (1970), one can read where the followers place the origin of the interference fringes.
    Instead of blaming single objects of behaving like waves it is pointed out that the preparation
    procedure will suffice.

    Cheers!
     
  21. Dec 10, 2006 #20
    Don't bring in other topics. Let's stick to what we are talking about : the double slit experiment and how it proves the wavelike nature of particles. Particles, in QM, ALWAYS exhibit wavelike properties : THAT IS THE ENTIRE POINT OF THE DUALITY. This duality does NOT give us an "either wave or either particle like" picture.

    LOL. Don't you know what momentum is ?

    HINT : momentum of a particle + uncertainty principle, my dear believer

    Self interaction IS an essential part of QM.

    Are you being serious here ? When did i ever claim we knew the exact boundary between classical physics and QM ? There is NO such boundary. We are ONLY talking about what actually happens at the atomic scale where basic QM principles like the p/w duality, superposition or the HUP manifest themselves.

    But to be clear, are you saying that a particle's self interaction does not exist ?

    That's not good policy to be doing since. Take THAT as lesson nr 1 from your guru.

    marlon
     
    Last edited: Dec 10, 2006
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