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Wave collapse. Fact or fiction?

  1. Feb 21, 2006 #1
    Does the collapse of the wave function really happen when someone observes it or is collpase a fluke of measurement? Thanks for your input and time.
    RAD4921
     
  2. jcsd
  3. Feb 21, 2006 #2

    vanesch

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    You pick your choice. You can have it either way, and arrive at identical experimental predictions (if you're careful).
    Given that experimental predictions are the only final arbiters, there's no way to say. That said, there are *theoretical and philosophical* arguments to prefer one or the other.
     
  4. Feb 21, 2006 #3
    Yes. It really collapses. But this "collapse" is something totally mathmatical, not something physical.

    Pete
     
  5. Feb 23, 2006 #4

    vanesch

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    In the MWI view, it doesn't collapse, for instance.
     
  6. Feb 23, 2006 #5

    Doc Al

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    In the minimalist ensemble interpretation (ala L.E. Ballentine) there is no wave function collapse.
     
  7. Feb 23, 2006 #6

    reilly

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    I agree with pmb_phy, to a certain degree. As I've written many times here: there is indeed a wave function collapse -- in fact there is at least one for any measurement, and it makes no difference whether we are talking quantum or classical. Probability theory is equally applicable to both areas; both can be well described by standard stochastic dynamic equations (see Bartlett's An Intro to Stochastic Processes for a good, sophisticated account of statistical modeling in physics, economics, forecasting, epidemeological studuies,... For QM, see any discussion of density matrix dynamics) /
    /
    Frankly, I've never understood how the extraordinarily brilliant creators of QM could get the probability thing so wrong. Who knows, perhaps they were seduced by the substantial strangeness of the new physics they were creating. But, it is also the case, that probability theory was less developed and cetainly much less widely known in the 1920s than it is today. Also, we have, as they did not, quite a sophisticated understanding of how our brain and perceptual systems work. Then, there's now almost a hundred years of experience with QM.
    As far as I know, the issue of wave fuction or probability function collapse is not much present in discussions of classical statistical physics, nor in the theory of games, nor in sports betting, stock picking, .... Yet, apart from the specifics of actual rules of computation, the classical and quantum probabilities are just that -- probabilities./

    To me it is very obvious that there is a wave function collapse; it's the collapse or change from uncertainty to certainty in the neurons in our brains. I like this idea because: it is simple; and it is based on empirical evidence -- in fact, it could even be tested, and perhaps already has been in some of the many, many brain scan experiments and tests. My view is that the QM founders would, today, embrace the knowledge interpretation, a fancy name for what I just described. Why invoke mysticism, or faster than light, or the non-separable Hilbert Spaces that surely must be required by MWI, or whatever just to get around the idea of probability. KISS.

    In many circumstances, particularly computational ones, pmb_phy is right on: the collapse is mathematical; like a new assignment for initial or boundary conditions.

    Regards,
    Reilly Atkinson

    (My new take: wave function collapse is all about biology, that is, physics in the large, the brain in particular.)
     
    Last edited: Feb 23, 2006
  8. Feb 23, 2006 #7

    Doc Al

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    You think that that's less wacky than MWI! :bugeye:

    I agree that it's all about probablity. The ensemble interpretation holds that all that QM predicts is the statistical properties of an ensemble of similarly prepared systems. Yes, you can--for convenience--reduce the state vector when a successful measurement has taken place; but that measurement takes place in any (properly working) macroscopic measuring device--nothing to do with someone's conscious awareness of a result or anything happening in someone's brain. At least no more than in classical physics. (Just my pragmatic opinion, of course.)

    What's so special about the brain? Sounds like you believe that the Schroedinger cat state vector doesn't "collapse" until someone happens to open the box--no, until someone is aware of having opened the box and seen the cat. (Schroedinger was just kidding, folks!) Or am I missing your point? (And what about Wigner's friend?)
     
  9. Feb 24, 2006 #8

    reilly

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    No brains. No physics, nor Physics Forums. We differ in how we interpret "....cat state vector" In my view, the cat's wave function is strictly a human artifact (that's a no-brainer, but an important one) just as is the name 'cat" is and the Visual perception of the cat, .....) "Cat" and cat are not the same. Thus, the knowledge interpretation makes very good sense -- it is directly connected to reality, as we know it, because it is working in the same part of our brain that stores our knowledge of reality.

    Cats are alive or dead, not both -- why mess with common sense in a case in which it is perfectly applicable. And yes, my assertion about cats is an inference, but based on overwhelming evidence from centuries of human experience, (No falsification as yet, with cats.)


    Probability, wave functions, real numbers, groups, differential equations are part of what might be called an extedend language, English (or Russian, or......) along with mathematics and associated conceptual notions. Always, in normal life, and in the arcane world of physics, we are bound by the limitations of language, brains, and so forth. We talk about language( and sometimes we have the conceit that we are talking about Nature rather than a description of Nature), which, we hope, gives a good account of Nature. All you know; all you experience is in your head -- that's the universe. That's the empirical Occam botom line. The rest is inference, subject to all manner of opinions.

    I was fortunate to hear Wigner talk about his "friend" and QM interpretation (1964, Washington U in St. Louis), and, much to my surprise I still have my notes. He talked at length on the consciousness issue, delayed examination of experiments. chains of communicating observers. At the end of the day, he remained puzzled -- consciousness counts, doesn't count. I've also heard Prof. Weiner and Prof V. Fock(yes, that one) debate the finer points of QM(1959, Cambridge, Mass.); but their accents were so thick that I got very little from the debate itself. But from these experiences, I came away with the strong feeling that the masters themselves were highly perplexed about many aspects of QM. I also have the feeling that the masters did not have a clear picture of the role of the brain in all of this. Modern neuroscience, to some degree, tries to finesse the consciousness issue by concentrating on measuring what the brain does. Ask yourself how things might look if say, Professor John Dowling's masterpeice, The Retina, had been available in the 20s, or the Hodgkin-Huxley explanation of neural dynamics, or ..... (The Retina provides a brilliant account of the vision system in extraordinary detail.) It seems to me that given more hard-nosed info on minds and brains, the mysticism and confusion of the founders would have been substantially lessened.

    Consider: while you or I write, that there is always a probability that the next letter will be p, perhaps z, or may be 2, when we graduate to alphanumeric. Further, there's a probability that the next word will be "the"
    or "collapse", and so on. There's a good bit of complexity in patterns of Ianguage usage. Nonetheless, individual usage patterns can differ lsubstantially, and, possibly, can serve as an identifier of the writer -- compare Einstein and Bohr, for example.

    I claim that the only superposition here is in the brain's representation of alternatives; my brain, conscious and unconscious, knows, sorry about that, what I'm doing, and what i'm about to do. Why would I go to the trouble of inventing a 'superposition of ra persons, each living for one alternative -- do the ra persons who guessed wrong die? Where do they go?

    Suppose that I "plagerize" a style, that tells others I'm Prof. X, rather than ra. Am I thus in a superimposed state: partly ra, partly Prof. X? Does it matter whether anyone reads my plagerized masterpeice?/


    What I know has no influence on Nature unless I do something based on that knowledge. (Of course there's the radiation that comes from neural activity, not to mention heat. But these are tiny effects, which we presume have virtually no impact on anything, except for brain scans.) If a meter reads 10 volts, when it could also have read 5 or 7.8, then until I look, I don't know, so I have that superimposed representation in my brain But common sense says that the meter reads 10 quite independently of my attention or lack thereof. Why? Centuries of human experience confirm in such a case that there is continuity of effect, Newton and all that.

    Sorry, this got a bit out of hand.

    Regards,
    Reilly Atkinson
     
  10. Feb 24, 2006 #9
    So, do you mean that the QM is a clever mathematical trick in order to overcome the lack of knowledge ?

    Is the missing information from our brain processed by QM's clever mathematical model of probability?

    Leandros
     
    Last edited: Feb 24, 2006
  11. Feb 25, 2006 #10
    Wave packet collapse its a theorethical result. When our system is a Hamiltonian stationary state and we measure an observable which eigenvectors are the same as the Hamiltonian.

    E.g. a particle in a harmonic oscillator potential, [tex]\hat H = \frac{\hat p^2}{2m} + \frac{1}{2}m \omega^2 \hat x^2[/tex].

    Eigenvectors are tagged by [tex]\vert \varphi_n \rangle[/tex] and eigenvalues are [tex]E_n = \hbar \omega \left( n + \frac{1}{2} \right)[/tex]

    Before we measure, we don´t know nothing about its energy or its state. We know that we can express the state in terms of the eigenstates of the hamiltonian. But, if we measure the system and it is in an eigenstate of the hamiltonian, then forever will be in this state -of course, if we don´t perturb the system-. We have passed from a discrete infinite spectra of different states to only one state. That is the wave packet reduction.

    This can be proof with quantum theory and has been observed many times. That is like uncertainty principle, it cames from theory and makes the things so easy (or so difficult :D).

    MiGUi
     
  12. Feb 25, 2006 #11

    reilly

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    Why not? It's done. generically if you will, in courts of law (circumstantial evidence, expert testimony) businesses (market segmention studies, portfolio analysis, meetings -- well sometimes) medicine (CAT scans, electron microscope (a quantum device)). The point is that we use all manner of techniques and ideas to expand and enrich our knowledge -- to get ahead, to catch up -- QM is just one of many that we use to help make sense of Nature. You will note that I make no distinction between QM and classical physics as far as probability is concerned -- other than the obvious dynamical differences. So this knowledge stuff and probability, in my view, prety much covers all the bases. And, as I've suggested, this idea, not original with me, most likely can or will be thoroughly explored by neuroscientists. In my view (sorry 'bout that) we will be able to see whether or not there is such a thing a a collapse of probability -- my money says it's a non-brainer; the answer is Yes.

    And, don't forget that the whole point of science, indeed of virtually all academic disciplines, not to mention law enforcement, parents, ..... is to expand and refine our knowledge -- always we want more(cf Pandora). Thank god we do have clever schemes

    How in the world could QM process something missing from a brain? Do you suggest that QM is some independent entity that could do so? I simply do not understand your question.

    Again, QM, sports betting, which is the best shortcut, what's for dinner? all deal with probability, a basic idea we use on a daily basis -- it's natural to our fundamental human properties .in the same sense that language is (cf Noam Chomsky, whose seminal ideas are nicely elucidated in Steve Pinker's book, The Language Instinct. )In fact, I would argue that probability is indeed part of our human language -- that is, the manner and substance of how we use it.

    What is it about QM and or probability that bothers you so much?

    Regards,
    Reilly Atkinson
     
  13. Feb 25, 2006 #12
    So you argue that the only reality is what is inside your head?

    Explain then why nobody agrees with the perception that there is no attraction between opposite magnetic poles? I prove this every day at home: I put north and south together and the do not attract one another, but nobody agrees with me!! If reality was only what was in our heads then everybody would agree with me! But they dont.. so reality cant just be in our heads.

    This stinks of philosophical blah blah blah by the way..
     
  14. Feb 25, 2006 #13

    reilly

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    I greatly appreciate your civil tone; I shall take a shower at once. Sorry 'bout that.

    I'm afraid I'm missing your point about magnets.

    About this reality stuff: One of the many things about practical reality is consensus. That is, virtually all of us agree that Katrina made a mess of the Gulf Coast. Why? Because, even if very hard-nosed analysis says what we know is in our heads, where else?, we agree for practical purposes that indeed there is an objective reality out there. This assumption makes practical and intellectual life possible. But, it is an assumption; one that we cannot prove. So yes, reality is in your head. But our DNA and our neuroscientists indicate we all function in very similar fashions. So, it is indeed reasonable, that we can safely assume the standard objective reality, as we do, and, amazing as it is, we mostly concur with each other -- yes it is raining, there's a red apple,......(If you have not read Hume, I suggest you do. If you have, then we do indeed differ at a very fundamental level.)

    Regards,
    Reilly Atkinson.
     
  15. Feb 25, 2006 #14
    What is this "MRI" view? Its always useful to not assume that whom you're directing a comment to will understand that acronym that you're using.

    Pete
     
  16. Feb 25, 2006 #15
    What is lacking in this conversation is a clear and precise definition of "wave function collapse."

    Question to Reilly - A measurement is automatically taken on a quantum system on Feb 25, 2006 at 7:15pm. The results are stored on a computer's hard drive. At 10:00pm Feb 25, 2006 Reilly prints out the stored data and reads it. At what time does the wave function of the quantum system collapse?

    Pete
     
  17. Feb 25, 2006 #16
    i didnt mean to offend, I´m sorry if i did.

    but i disagree. every time someone agrees that opposite poles attract, we are furnishing proof that there is an objective reality. yes everything is subjective also but the existence of objective reality is certainly not just an unproven assumption.

    And so reality is not just in our heads.

    I look at at this way:

    There is an objective reality which we determine (to a certain degree only, not absolutely of course) by experiment. We model this not purely objective reality using mathematics if we are being precise, or simply using handwavy arguments and pictures/concepts if we are not precise. If we are precise we still attach handwavy arguments and pictures to the mathematics. These handwavy arguments and pictures are what forms our concious version of reality.

    But anyway you cannot argue that objective reality is an unproven assumption. We prove, as well as anything can be proven, that it is a sound assumption.

    Now of course there is the pointless argument that we assume somthing is "proven" but that there is no proof that it will always be so. But that really is a pointless point, because if science has no true value then how do you explain that we develop technology? Its certainly not trial and error. And I think the burden of proof is on YOU to prove that the assumption that the laws of physics will hold (in as far as repeating the same experiment over and over and over) is not a sound one.
     
    Last edited: Feb 25, 2006
  18. Feb 26, 2006 #17

    vanesch

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    MWI = Many World Interpretation.

    I understand reilly's point, and we've been battling over it in the philosophy forum. His viewpoint is purely epistemological, and as such, avoids the issue of ontology. His viewpoint is that quantum theory is a way to calculate probabilities of outcomes of experiment. And of course, once the experiment is done, and the result is known, of course these probabilities change (collapse) into p=1 for the actual result, and p=0 for all the others.

    The problem (for me) with that approach is that it is purely epistemological - no ontological description of the world is attempted, there's nothing postulated to be "out there" and to be "described" (= mapped upon concepts in the formalism). The nice thing with that view is that of course, anything goes. There are never interpretational problems with such a viewpoint, because there's nothing to be interpreted. Physics in its most bare-bones version: calculation of outcomes following a certain formal procedure (guided by some intuition), and verification with experiment. But NO description of nature.

    I think that view is giving up too much, namely that we are not able to describe nature, but just have tricks up our sleaves to calculate outcomes of experiment - but on the other hand, it has to be said that such a viewpoint is the one that gives you least headache ! "As long as our results correspond with experiment, hey, I'm happy".
     
  19. Feb 26, 2006 #18

    Doc Al

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    While I certainly agree that one must distinguish our mental concept "cat" from the real, physical cat, I certainly believe that the physical cat exists independent of whether or not I think of it. Only in the most trivial sense can one say that "everything is in your head". Of course it is! But to think the world is only "in our heads" is arrant solipsism.

    What you call the "knowledge interpretation" seems unnecessarily subjective. A much simpler view is that whether the cat is alive or dead, or whether interference is exhibited in an electron diffraction experiment, is an objective fact of the world, not something that depends on the neural workings of our brains.

    This does sound like a common sense view of the external world, not of the inner world inside your brain. No problem here.

    Of course we are subject to the limitations of our brain function. (Hey, no one is more aware of that than I! :uhh:) And, yes, any assumption of an outside world--or even of a physical brain--is an assumption, albeit a necessary, trivial one. To me, not making that assumption of an outside world is a rampant violation of Occam's razor! It is by far the simplest explanation of what we see.

    I think much of the confusion comes from not accepting that standard QM can only provide probabalistic statements about ensembles of systems, not individual systems. (Perhaps other theories that go beyond standard QM, such as Bohmian mechanics, can get us deeper. Beats me!)


    While a study of how the brain works is fascinating and important, I think it has nothing whatsoever to do with understanding quantum mechanics (or classical mechanics). Pondering how our vision system works will no more clarify electron diffraction through a double slit than it will improve one's appreciation of Shakespeare. (Unless you need glasses, of course.)

    Not sure what you are saying here. A quantum superposition is a description of the properties of an ensemble of indentically prepared systems. The consequent interference is an objective fact, verifiable by anyone trained to do so. Nothing, beyond the trivial, to do with the brain and its workings.

    It seems that you are agreeing (with common sense) that there is an outside world in which the meter reads something definite regardless of your personal knowledge or lack thereof. No argument from me on this point. But the idea that you don't personally know what the meter reads until you look has nothing whatsoever to do with the quantum measurement problem or the supposed "collapse" of the wave function.

    Very soon the philosophical waters will be too deep for me to keep afloat--I can only tread water for so long before swimming to shore! :smile:
     
  20. Feb 26, 2006 #19

    reilly

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    Pete -- Collapse, in my view, is the generic word for a change in a probability due to measurements. I make a right turn when lost. Prior to that turn I was not sure what I would do, so my mind was split, or indecisive, or weighing the alternatives, and, clearly I entertained the idea of either a right or left turn. That state of uncertainty surely vanishes once I make the turn. The probabilities in this case have change substantially.

    So, in my world, there is not a single wave function, but there are many states of knowledge, hence many wave functions. Prior to reading it, I don't know about the data. So no matter what you call it, there's a place in my mind in which the neurons give physical expression to my uncertainty, lack of knowledge, or whatever. The resulting neural pattern changes once I read the data -- and that is an established fact. If you looked at the data before I did, your wave or probability fnction would collapse before mine does --unless you lay it on me before I read. (This is the so-called Wigner's friend problem.)
    /
    Regards,
    Reilly


    More later.
     
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