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Filtering type observation

  1. Jan 25, 2016 #1
    I read that state preparation is simply measuring and confirming the system. Filtering type observation is when the observation doesn't destroy the system. But googling doesn't produce the extra words "Filtering type observation".. why is these words only in physicsforums.. what are the other synonyms of it.. can you give example of a filtering type observation?

    Also someone (bill) said after I know the meaning of state preparation and filtering type observation.. then can chat whether the universe is in pure state (is it.. can someone explain categorically)... How are the three ideas related at all? I'm confused as hell. Thanks.
     
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  3. Jan 25, 2016 #2

    bhobba

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    The answer to your query, and others you have made elsewhere, requires the study of actual textbooks. Filtering type observations (also sometimes called Filtering type preparations) are discussed on page 208 of Ballentine in the chapter on state preparation and determination. You need to slog though the detail to understand it - it can't be done at the lay level.

    Its relation to the state of the universe lies in what a state is defined as. In Ballentine a state is defined as the ensemble of similarly prepared systems - see page 47. Some go even further and have it synonymous with preparation procedure. Either way if you take such a view applying it to the entire universe is problematical - exactly how do you prepare the universe. What preparation procedure did you use? A very common preparation procedure is a filtering type procedure where some apparatus is used to prepare a system. That makes zero sense as far as the universe is concerned.

    None of this can be discussed at the lay level - it requires knowledge of the technicalities. If you want to do it you must get that technical knowledge.

    Thanks
    Bill
     
    Last edited: Jan 25, 2016
  4. Jan 25, 2016 #3
    I'm interested specifically in Many Worlds view of filtering type preparations.. just want a bird eye view.. so in Many worlds.. you don't need to prepare the state because the state is ontologically there already.. I guess this is the distinction to make.. my interests of the state vector is if its ontological... in this context.. do you mean the universe can be pure state if Many worlds with ontological state vector is assumed? Just tell me what interpretations or formalism is the universe is in pure state? again just want short verbal summaries.. thanks.
     
  5. Jan 25, 2016 #4

    bhobba

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    It has filtering type preparations/observations as it must have because its part of the formalism.

    In MW the state is very real, not the equivalence class of preparation procedures, and one can discuss the state of the universe.

    But I again want to stress with the kind of questions you are asking it can't be done at the lay level. For MW that would mean studying something like:
    https://www.amazon.com/The-Emergent-Multiverse-according-Interpretation/dp/0198707541

    Please make the time and effort to get that knowledge. You wont really make any progress until you do. I was going to leave answering this to someone else, but none seemed interested so I relented. Eventually though that wont happen and your questions wont get replies.

    Thanks
    Bill
     
    Last edited by a moderator: May 7, 2017
  6. Jan 26, 2016 #5
    Ok I will read the above book. Later I'll search in old posts using search keywords "many world schroedinger cat pure state". Specifically i'd like to know if the Schroedinger Cat with oxygen inside a theoretical Hilbert box (a hypothetical box that can shield against everything including gravity which Bruce Collins has described in Schroedinger Rabbits: Many Worlds of Quantum) can be a pure state using Many worlds. You said it can't.. but that's in Ensemble viewpoint. In many worlds.. with all diverging histories of the cat, if there is interference maybe it's in pure state. Others who may know this besides Bill can hopefully maybe just give a quick answer of yes or no to the question whether Schrodinger Cat with oxygen tank inside a Bruce Collins Hilbert isolation box can be in pure state using Many worlds interpretation. Thank you.
     
    Last edited by a moderator: May 7, 2017
  7. Jan 26, 2016 #6

    bhobba

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    As explained previously, electrons are entangled with the quantum vacuum that permeates all space and leads to the phenomena of spontaneous emission. It can not be shielded against because everything is excitations of the fields the quantum vacuum is the ground state of. You either have to accept it or learn the details so you can discuss the facts. You will not learn those details from 'google' - only from close study of a good textbook. It will not come from lay treatments with things like a Hilbert Box - only a good textbook will do.

    I have given you your answer, you probably wont like it because you have this preconceived notion that if you are a pure state you become unreal. But it's the answer and will be my last word on the issue.

    Thanks
    Bill
     
  8. Jan 26, 2016 #7

    atyy

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    A measurement produces various results according to a probability distribution.

    Let's say the first measurement has two possible outcomes, A and B.

    If one takes the system after a first measurement and takes only the times when outcome A is obtained, then does a second measurement, the distribution of results for the second measurement conditioned on having obtained outcome A is a "filtering measurement".

    Ballentine's discussion of filtering measurements is among the worst and I do not recommend it.

    Instead one can look up the section about "repeated measurements" in Holevo's text http://www.springer.com/fr/book/9783540420828.

    It is dealt with by the "reduction rule" in http://arxiv.org/abs/1110.6815 (rule II.4, p9).
     
    Last edited: Jan 26, 2016
  9. Jan 26, 2016 #8
    Wow, those are graduate level papers. Can you just please give an actual example of experiments illustrating the concept of filtering type observation.. I read somewhere you said when it is done, a quantum outcome is needed in addition to classical outcome. I need actual examples, thanks.
     
  10. Jan 26, 2016 #9

    A. Neumaier

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    The best basis to clear up confusions is the (free) book ''Quantum Theory: Concepts and methods'' by Asher Peres.

    It makes a lot of sense to first read and understand a large part of this book before trying to understand more esoteric interpretations such as MWI. For unless you have a sensible understanding of what everyone agrees upon how quantum mechanics works you'll never get any clarity about more controversial issues.
     
    Last edited: Jan 27, 2016
  11. Jan 26, 2016 #10

    bhobba

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    Excellent idea.

    Normally if lay people come here and ask questions such as what it's like to be in a pure state they accept the answer - pure states don't really exist except as approximations for things like electrons etc, and certainly not for macro objects like a cat. But you want to argue the point. That's fine - that's what science is about. But you can only argue it if you know the detail of what you are arguing.

    Thanks
    Bill
     
  12. Jan 26, 2016 #11
    Just want emphasis because I want my book to be 100% accurate in the physics and words. I need to write the book because no one else would write it.

    In https://www.physicsforums.com/threads/can-we-prepare-mixed-states.743460/#post-4690273, atyy mentioned in #6:

    "In the sense that decoherence gives mixed states, preparing any pure state of a system consisting of subsystems will give a mixed state, provided you make a measurement on the subsystem. This is an improper mixed state."

    Bill Hobba said atyy mentioned "pure state" above as only for purpose of trackable model.

    So the accurate thing is to say instead that "In the sense that decoherence gives mixed states, preparing any state of a system consisting of subsystems will give a mixed state, provided you make a measurement on the subsystem. This is an improper mixed state"..

    We must remove the word pure above right?

    Because I don't want to write in my book the mentioning of pure state and explaining it is for trackable model. And for accuracy.. we must not use the word pure state describing the systems except for double slit and buckeyball only or other real pure state per Hobba emphasis.
     
  13. Jan 26, 2016 #12

    bhobba

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    Think back to good old Euclidean geometry you learnt about at school. A point has position and no size. A line has length and no width. Such don't exist but are fundamental to the theory. The same with pure states - they don't really exist either except as approximations - but are fundamental to the theory.

    The only reason we got into this is you wanted to know what it felt like to be a pure state. The only answer is the truth - they don't exist. Its like asking what its like being a point - the answer is they dont exist.

    There is no need to worry about it in a lay level book - just mention what I said above - if you mention it at all. I wouldn't, but its up to you.

    Thanks
    Bill
     
    Last edited: Jan 26, 2016
  14. Jan 26, 2016 #13

    A. Neumaier

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    Then you should talk only about the mathematics of quantum mechanics, not of its interpretation.

    Nothing in an interpretation of quantum mechanics is 100% accurate in the physics and words. The notion of probability, prominently used in interpretation of quantum mechanics, is an intrinsically imprecise concept on the level of words only. The notion of a measurement is never precise by its very nature; and the measurements discussed are again highly idealized. The examples discussed in interpretations are highly idealized situations applying to a single degree of freedom of a physical system only (typically a spin or a binary decision), or (like for Schroedinger cats) completely unreal caricatures to illustrate a point by exaggeration.
    No one else writes it because it is impossible to write it.
     
  15. Jan 27, 2016 #14

    atyy

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    Peres's book is very good, but it is not for beginners because it presents the fundamentals of quantum mechanics incorrectly. In particular, it lacks a clear statement of the need for collapse.
     
  16. Jan 27, 2016 #15

    A. Neumaier

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    I very much disagree!!!

    The book by Peres presents the fundamentals very clearly and completely, without the need for ever invoking collapse. This makes it particularly suitable for beginners, since they are spared some of the weirdness introduced by the collapse.

    Collapse is dispensable since it happens on the subjective side of quantum mechanics only - it tells what happens to a system when the observer changes his/her point of view.

    Collapse is an intrinsically classical phenomenon. Suppose someone has cast a die. It has a definite value but if you haven't seen it, your probability for every value is 1/6 although its objective value is already determined. Once you know the value, you change your point of view and update the probabilities to 1 for the observed value and 0 for the others. This is the collapse - except that on the classical level, people refer to it by a different name: conditional probability. Exactly the same happens on the quantum level, though the dynamical system is different, hence the formulas look different.
     
  17. Jan 27, 2016 #16

    atyy

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    This is almost certainly wrong. If collapse could be shown to be derived from conditional probability, then it would not be weird. Many have tried to derive it from those considerations. But there are no derivations that are consensus at the moment. Nielsen and Chuang make that comment. I myself have studied a number of those derivations, and I agree with Nielsen and Chuang.

    In fact collapse is not dispensable because without it, one cannot recover the classical conditional probability.
     
  18. Jan 28, 2016 #17

    A. Neumaier

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    I opened a new thread on this, to keep the present thread clean of this side issue.
     
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