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What are the QM

  1. Jul 31, 2005 #1
    Consequence's for the wavefunction of a Blind Person?. do they (blind-persons) still collapse?..interact with Physical Systems?
    Last edited: Jul 31, 2005
  2. jcsd
  3. Jul 31, 2005 #2
    I'm fairly sure that your question makes no sense whatsoever.
  4. Aug 1, 2005 #3
    Thats the best "Heisenburg-Logic" response I have had in a while, great, and thanks again.
  5. Aug 1, 2005 #4
    I believe this is in the wrong forum, i think it should be under biology because people who study the brain probably know more about blind people then those who just study quantum. Seeing as how photoreceptors are biology, though there maybe biophysicists whose area of expertise may be in this area.

    There are many areas of blindness and different forms/combinations of lesions that could probably take out the visual system.

    But if your strictly talking about photoreceptic region mmm my guess would be that the entire cells would be empty/dead.
  6. Aug 1, 2005 #5


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    Spin_Network is probably referring to the fact that the collapse is usually attributed to observation. However, there is no difference between a blind person and a non-blind wrt QM, since in both cases, a detection apparatus needs to amplify and transform the observed signal in order for an observer to perceive and interpret it. Noone would see a single photon coming out of an interaction. We always need equipment in between.
  7. Aug 2, 2005 #6
    I would add to that that that the observer does not need to be a conscious
    entity. A recording device can perform valid measurements even if no
    conscous entity ever checks the results.
  8. Aug 3, 2005 #7
    I guess there are fundamental interactions in nature, not necessarily related to living things' physical processes, which work like observers and spoil entanglement at at a macroscopically level practically all the time.
  9. Aug 4, 2005 #8
    Biologically of course, "observation" comes in many different forms (e.g., the organs of perception)--thus while I may not collaspe an object (physcial entity) via sense of vision because I am blind, I will surely collaspe it via my other senses. In this way life [L] commutes with objective reality [O] and integrates via electro-chemical wave functions within neurons the totality of that which exists independent of consciousness to form an entangled [O-L] wavefunction. Thus, the answer to the initial question in this thread is "yes".
  10. Aug 5, 2005 #9


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    There is a difficulty in quantum theory here (it is THE difficulty in fact). Consciousness is sometimes called upon as a deus ex machina to get out of the riddle (I'm one of those doing that).
    Schroedinger realized this for the first time, and to illustrate the "ridiculous" idea, he told the story about his poor cat, but the argument still stands.

    If you:
    1) take the state vector ("wave function") as something that describes the objective state of the world out there (and not as an epistemological tool which summarizes your knowledge about it)
    2) you assume that quantum theory is universal and correct (applies to everything, in the way we know it, with a strictly unitary time evolution operator)

    if thing A is a measurement apparatus, measuring system S, in his environment I, then that "measurement" is just an interaction described by a hamiltonian as just any other interaction, and subject to a unitary time evolution operator.
    However, for it to qualify as a measurement, it needs to do certain things, namely, give a result "for sure" if the system is in one of the corresponding eigenstates. Now, consider the initial state:
    |I0> |A0> |S1>
    (the system is in state 1, an eigenstate of the measurement that will be performed, and the measurement apparatus as well as the environment didn't yet couple to it).
    If we let the measurement apparatus interact with the system ("do the measurement"), then the measurement apparatus will have to end up in a clear and stable state corresponding to the outcome, and communicate that to the environment:
    So: U: |I0>|A0>|S1> ---> |I1>|A1>|S1>

    If now |S2> is a different eigenstate of the system, leading to a different result, we can apply the same reasoning and find:

    U: |I0>|A0>|S2> ----> |I2>|A2>|S2>

    So far, so good. The measurement apparatus did what we wanted and "amplified" the microstate of S to the macroscopically distinct states of A1 and A2, and unavoidably its environment I1 and I2.
    Again, A1 and A2 and I1 and I2 are macroscopically DISTINCT states (such as "dead cat, live cat").

    Well, what happens now when the original system is in a superposition ?
    a|S1> + b|S2> ?
    U IS A LINEAR OPERATOR, so there's no choice:

    U: |I0>|A0>(a |S1> + b |S2>) ----> a |I1>|A1>|S1> + b|I2>|A2>|S2>

    The measurement apparatus, and its environment, ENDED UP IN MACROSCOPICALLY DISTINCT SUPERPOSITION (and entangled with each other). Worse, as each term, individually, was supposed to be a stable state, the superposition is stable too, because all we can do is apply a linear operator to it, and by definition, a linear operator acting on a sum is the sum of the action of the operator on each term.

    So the statement that a photodectector "obtained its measurement result" is probably different than what follows according to the strict application of QM: it didn't "see" or "didn't see" a photon: it just ended up in BOTH states.

    This is, however, not what we experience: we only experience ONE TERM, with a certain probability. This cannot be obtained with a unitary time evolution operator.

    So how to reconcile this:

    1) you can claim that the wavefunction is just a description of our knowledge of the system (like a kind of probability distribution), and of course it changes when we learn things about it ; this explains the collapse into one of its terms. But that is a disturbing viewpoint IMHO, because then there's nothing left that describes "the world out there". We don't have then a theory of the world, but a theory of our knowledge! But knowledge of what ? As we don't have a description anymore of what we're knowing things of, we just "know" and describe that, we don't know things "about" something, because that something lacks totally a decription. Strange, for a physical theory.

    2) Unitary quantum theory doesn't apply universally. So that then implies a modification of the theory to tell us what does apply universally, and how it simplifies to unitary QM for microscopic systems. This is very difficult to do !

    3) Unitary quantum theory does apply universally, and these macroscopic superpositions do happen but we consciously observe only ONE term. This is where consciousness tries to save us. This is Many Worlds. Also strange, for a physical theory!

    My point is 3) until we finally find 2)
    My preference of 3) over 1) is that although it is very strange, at least there IS a world out there which is described. I've difficulties letting that go.

    2) is still very remote, if ever. QM is more successful than is confortable. I'm hoping somehow for gravity to settle the issue...
  11. Aug 5, 2005 #10


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    I believe Eugene Wigner was the man who believed human conciousness causes a wavefunction to collapse. It seems like a difficult position to defend.
  12. Aug 5, 2005 #11
    How do you differentiate 3) and 1)? By defining an ontological and non local object "consciousness" (or a god)? Aren't you defining, in fine, a bohmian like interpretation of QM with point 3 (i.e. we may view the bohmian path as the consciousness of the particle in fine :biggrin: )?

    Physics after all is all about the description of the "reality", therefore it should be independent of such concepts, shouldn't it?

  13. Aug 5, 2005 #12


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    vanesch ponders:

    1) you can claim that the wavefunction is just a description of our knowledge of the system (like a kind of probability distribution), and of course it changes when we learn things about it ; this explains the collapse into one of its terms. But that is a disturbing viewpoint IMHO, because then there's nothing left that describes "the world out there". We don't have then a theory of the world, but a theory of our knowledge! But knowledge of what ? As we don't have a description anymore of what we're knowing things of, we just "know" and describe that, we don't know things "about" something, because that something lacks totally a decription. Strange, for a physical theory.

    Life is tough. You pays your money and takes your chances. A hard-nosed analysis easily confirms that we do not have a clue what's out there. it's all surmise, smoke and mirrors, and neural magic.

    Where is it written that we should be able to understand Nature at all, let alone on our terms?

    The plain fact is, a la Hume, what we know comes from two sources: our genetic, structure; and from "whatever it is that triggers our perceptions' -- certainly the two are strongly coupled. Even as far back as Plato, people have accepted the idea that there is an "out there", and an "in here". (Accepted is, of course, the operative word.) It's fair to guess that this distinction is likely hard-wired -- are there alternatives?

    Science is concerned with understanding a posited out-there in terms of our experience in-here, under the assumption that there is enormous stability in our world -- indeed, the sun will rise tomorrow, the Atlantic Ocean will not vanish, my name will still be Reilly. What Hume did was to remind us that our assumptions are just that, there's no way to prove anything about tomorrow. (As financial offerings must admit, "Past performance is no guarantee of similar future performance.)

    vanesch asks, "But, knowledge of what?" That's the real question isn't it. Years ago Aldous Huxley claimed that humans would never be successful studying the human mind, because self-observation is always biased. He got it wrong. My very strong sense is that the best game in town for trying to understand our posited objective external world is brain science, cognitive neuroscience. Let's figure out as much as possible about how and why we know, while gently thumbing our nose at the good Mr. Hume. That is, let's understand the most fundamental measuring device in the world.

    Plato looks better and better to me:his symbolic description of the cave and the dancing shadows more and more, I think, describes what happens, generically not literally, inside our heads. A theory of objective reality? Not yet. We don't even have a good understanding of subjective reality.
    Reilly Atkinson
  14. Aug 5, 2005 #13


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    I do my best. That's probably not good enough :approve:
  15. Aug 5, 2005 #14


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    There is a subtle difference, but you are right that 3) has some points of similarity with Bohm, except, and that's the whole thing: the non-locality! With individual minds travelling each LOCALLY their own path, you can save locality, while Bohm has one and the same "token indicating which branch" for ALL, space-like connected, observers. As such, this unique and universal token had to obey a non-local dynamics in order to coincide with QM and this is with us for ever, thanks to Bell's observations.
    So yes, you are right that what I'm proposing (and what must have been very close to what Wigner proposed if I understood it well), is a kind of "multi-Bohmian mechanics" where there are now potentially MANY tokens (which I called consciousnesses). This allows them, each individually, to travel through the arborescence of the wavefunction. The non-local, deterministic dynamics is now replaced by a random dynamics (the Born rule), determined only locally.
    The problem is of course that different tokens end up in different branches! That's unavoidable if you insist on locally determined walks. But it doesn't really matter, as long as for one token, it is unobservable if the other token is in the same branch or not.
    The reason why locality can be conserved in this case is that the choice between two branches is only imposed when a LOCAL dynamical interaction (unitary evolution) splits the term in two. This splitting determines totally locally the two or more different Hilbert norms on which the token has to decide which way to go (via the Born rule).
    For instance, in EPR like experiments, even though remote Bob "saw" a result, for Alice, "Bob seeing a result" just means that HE chose his branch, but that the TWO results are still there, in two branches, and she's not obliged to choose HIS branch (his body being in the two states, she won't find out). And it is only when the news of the result (that messenger being in the two branches itself) comes to Alice, and she learns LOCALLY of the result, that interference occurs and that she will choose a branch corresponding to probability laws which give us the famous Bell-violating correlations. But that happened LOCALLY when the messenger got back to Alice.

    Well, it should at least contain such a description, and the purely epistemological view denies that from the start ! After that, we can discuss what is allowed in the description, but at first there MUST be a description.
  16. Aug 6, 2005 #15
    This is just a chicken/egg question framed differently, wouldn't you say. Of course the blind man would collapse the system. He has added himself to the system. Probability would have to change when the system changes and when the something is added to the system "an observation" has occurred, whether it be through direct measurement, entanglement ect...
    I think observation is a VERY bad word.

    Umm.. I really don't know what I'm talking about :yuck:
  17. Aug 6, 2005 #16
    We must pay attention to the fact that information gained, emotional responses and all that are ultimately consequences of a physical interaction. Probably, consciousness has nothing to do with QM's wave functions collapses. But these collapses may have to do with the interaction which conduced us to the condition of being conscious of something. By the way, is there anyone here who likes the concept of pointer states ? (W. Zureck)

    Regarding the OP, Schwinger has said once:

    Quantum Mechanics is a symbolic representation of our knowledge about the experimental results on a microscopic scale. Simple, insn't it?
  18. Aug 7, 2005 #17


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    The difficulty with that point of view is then that the system's dynamics is influenced by what we know about it !

    (which is not the case with classical statistical descriptions where it only goes in the other way: our knowledge is influenced by the dynamics of the system (Liouville's equation in phase space, for instance), but no aspect of the probability distribution alters the dynamics of the system.

  19. Aug 7, 2005 #18


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    Not so

    Quite the contrary. We know from lot's of empirical evidence that learning is a function of electrical and chemical processes in the brain and nervous system. We know similarly, that recognition and (possible) memorization are governed by elctrically and chemically induced neural pulses with corresponding physical changes in neurons and brain-structure. That's all you need for collapse.

    As you get from "I don't know" to "I do know" your brain changes correspondingly., and you physically reduce a set of possibilities to the actual one -- seems like a good description of a wavefunction collapse. Probabilities come home to roost whether classical or quantum.

    The primary big-gun behind the state-of-knowledge approach is Sir Rudolf Peierls.
    Reilly Akinson
  20. Aug 7, 2005 #19


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    But a lot of physical processes depend on this collapse of the wavefunction. The point of view you're describing implies these processes do not happen if there is no consiousness to detect it? That's like saying the moon is not there when nobody looks.
    My photodetector behind a beamsplitter does not go off after I fired a photon, but is in a superposition of detection/no-detecion until I check whether it really did or didn't go off? That sounds awfully nonphysical and suspicious to me. The wavefunction clearly does not decribe anything real in this view.
  21. Aug 7, 2005 #20
    By expermentally looking at where the eletron of a trapped Hydrogen atom is, one is very likelly to seei it in the atom instead of out of the atom. What localizes the electron wave function ?
  22. Aug 8, 2005 #21


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    Could you please provide examples of physical processes that depend on wave function collapse?

    Sir Rudolf is a far more eloquent and learned physicist than I am, so I suggest you do a Google on him, and get his take. For many of us, this knowledge-based approach to probability helps, among other things to help remove the mysticism too often associated with QM and other endeavors that use probability.

    With respect to the moon's status, the Peierls' approach deals with your knowledge of the moon, not with the moon itself(Newton would accept that.). and, further note that our knowledge is based on our perceptions, which we posit comes from light reflected from the moon. Perceptions? Knowledge? Very close to the same thing. It's all based on surmise -- who knows what's real?

    Reilly Atkinson
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