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I QM Assumptions Regarding Entanglement Properties

  1. Nov 10, 2017 #1
    In a nutshell I think that in local realistic theories it is assumed that:

    Each entangled object has definite properties at all times, even when not observed.

    I know the assumption is proved to be incorrect but is that an assumption actually made in such theories?


    But what assumptions about properties, if any, are made in QM? Are either of the following assumptions made?

    When not observed each object has the property of existing in all possible states simultaneously but observations reveal one state only for each object.

    Each object cannot be described as having properties at all, until and unless an observation is made.

    Are there other assumptions and do the assumptions made depend on what interpretation of QM is used?

    Thanks to anyone who replies
     
  2. jcsd
  3. Nov 15, 2017 #2

    Zafa Pi

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    I think it would be of help if you chose to do some reading in a beginners text.
     
  4. Nov 16, 2017 #3
    Thank you Zafa Pi. Your advice is good but I have done so much reading on this subject that my teeth are beginning to itch. I have gone through some texts several times. I think I have the general idea about entanglement, Bell and Bell tests but I'm stuck on what I think are very relevant assumptions made by local realists and by QM adherents. There is something that seems a bit strange and perhaps contradictory to me and I can't even quite pin down what it is. It's just a feeling. Hence my post above which was asking for clarification. Please allow me to comment on each of your five comments above.

    1. The properties I referred to are the entangled properties whatever they may be, for example entangled spins or polarisations.

    2. As I understand it realists believe(d) that the non entangled and entangled properties of each entangled particle has definite values at all times.

    3. 4. 5. I'm fine with those comments.However, I had forgotten that objects can be prepared in a known state.

    Mainly what I want to know is whether or not, what I have written in note two above is correct. The assumption of "definite properties at all times" covers realism and counter factual definiteness. I think.

    Now if what I have written in note two is correct can I further assume that Bell test experiments disprove the assumptions made by realists as in note two? Is it that simple? If so I'm finding it rather odd.
     
  5. Nov 16, 2017 #4

    Zafa Pi

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    I suggest that you pick a particular short article (e.g. wiki) on Bell's theorem, or entanglement and we can go from there.
     
  6. Nov 16, 2017 #5
    thank you again Zafa Pi. I will take your advice and look at some of the literature again and probably find some new stuff to look at. I shall probably have time at the weekend to do that properly.
    I should point out that I am reasonably familiar with entanglement, Bell theory and Bell tests, the concept of local realism etc but I'm just stuck on one thing that goes right back to first principles That one thing is the assumptions made by local realistic theories.

    Do all local realistic theories assume that, along with the principle of locality, each entangled object has real properties even before observations are made.

    In a nutshell that's all I want to know. Everything I've read so far seems to claim the above assumption is made but the assumption seems strange and that's what's niggling me. Hence it would be nice to get the views from an expert to confirm,or otherwise that the assumption is made (and proved to be incorrect by Bell test experiments).
     
  7. Nov 16, 2017 #6

    Zafa Pi

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    I'm not an expert
    You should specify which properties you are referring to. The QM view of an entangled particle is that it has no state.
    In proving Bell's Theorem, besides locality, one of the following is assumed (with my take):
    Realism: Alice's measurement result does not depend on which measurement that Bob makes.
    Hidden variables: The particles measured come endowed with proscribed values for each measurement. (Is this your "has real properties even before observations are made"?)
    Counter factual definiteness: A particle will have some value (unknown) if measured, regardless of whether it's measured.

    I prefer using CFD in proofs of Bell's theorems because it seems the most intuitive, and arises naturally.

    You might like this elementary way to distinguish classical from quantum.
    Let us suppose that:
    1) Alice and Bob are isolated from one another, so that no communication or influence can pass between them and neither knows what the other is doing.
    2) If Alice and Bob both perform experiment X they will get the same result.
    3) Alice performs experiment X and gets value 0, while Bob performs experiment Y and gets 1.
    Then
    4) If Bob had performed X instead of Y would he have necessarily gotten 0?

    Classical physics says yes and quantum physics says no.

    With classical physics we know that the reality facing Alice is unaffected by what Bob does, so she would have had to get 0 if Bob did X instead, and thus yes, Bob must get 0 because of 2).

    The classical argument above is sufficient to derive Bell's inequality which is denied by quantum physics thus yielding no.

    The question posed by 1), 2), 3), and 4) is both short and requires no knowledge of physics.
     
  8. Nov 17, 2017 #7

    zonde

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    Small correction. Bell test experiments do not falsify that particles have properties before measured. Instead what they falsify is that "particles having properties before measured" alone can not explain entanglement. So to explain entanglement you might speculate that particles have properties plus some additional physical mechanism. Or you might speculate that particles don't have properties, but then you would have to give some alternative explanation for phenomena like linearly polarized light.
     
  9. Nov 17, 2017 #8

    Zafa Pi

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    I'm confused. At the beginning of the paragraph you are talking about entangled particles. But in the last sentence are you still talking about entangled particles?
     
  10. Nov 18, 2017 #9
    Mainly what I'm trying to find is a simple yet rigorous description (one that can be understood by an interested amateur) of what exactly it is that Bells theory disproves. I have looked at many papers on this including the original EPR paper but I think the quote below is close to what I'm looking for:

    Below is the quote which is from a Wiki article on "Principle of Locality"

    "Einsteins principle of local realism is the combination of the principle of locality (limiting cause-and-effect to the speed of light) with the assumption that a particle must objectively have pre-existing value (ie a real value) for any possible measurement ie a value existing before that measurement is made".

    I think the description is simple but can it be considered to be rigorous? I have a few points that I would like to be clarified if possible

    1. Does the word value refer to anything and everything that can be measured, including, with the electron as an example, properties (such as electron mass) and non properties ( such as electron location at a particular instant)?
    2. Can the reference to locality be ignored since if particles have pre-existing values the reference to light speed seems irrelevant?
    Thank you
     
  11. Nov 18, 2017 #10
    Thank you zonde is the "additional physical mechanism" you refer to equivalent to the "hidden variables concept " referred to in EPR? If so, if Bell tests falsify the idea that particles have real properties etc do not the tests also falsify the concept of hidden variables?
     
  12. Nov 19, 2017 #11

    zonde

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    No, it can't. You say you have read original EPR paper, can't you spot discrepancy? In EPR paper realism is mentioned right at the end of first page.
     
  13. Nov 19, 2017 #12

    zonde

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    No.
    Experiment can not falsify a concept. Experiment can falsify a model.
     
  14. Nov 19, 2017 #13
    You must be referring to the notes in italics and from how I interpret them they are equivalent to the notes i referred to in the Wiki article For example "if we can predict with certainty the value of a physical quantity etc" (EPR paper) seems to imply that "a particle must have pre-existing values etc (Wiki article). So I can't yet spot a discrepancy other than the use of different words to describe the same thing. However I'm in a rush at present so I will go back and take a closer look at it. Thanks for your input.
     
    Last edited: Nov 19, 2017
  15. Nov 19, 2017 #14
    You answered no to the first question so what is the additional physical mechanism you referred to?

    Models are built on concepts.

    Sorry I'm writing this in a rush but will get back to it. But thank you very much.
     
  16. Nov 19, 2017 #15

    rede96

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    Just out of curiosity, if Alice and Bob *always* get the same results when they perform the same experiment how does QM predict something different?

    If it did then Alice and Bob wouldn’t always get the same result when they performed the same experiment.
     
  17. Nov 19, 2017 #16
    If in note 2 you mean local hidden variables, they are refuted by Bell's ineq. AFAIK.
     
  18. Nov 19, 2017 #17
    The difference lies in the fact that correlations at different angles produce different probabilities. The 100% correlation isn't what changes from classical theories, it is the cos(θ) which describes quantum probability in the intermediate angles. (please forgive me and correct me, mentors, if that isn't the most explicit way to describe it)
     
  19. Nov 19, 2017 #18

    Zafa Pi

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    I only assumed they would get the same result if they both performed experiment X. It doesn't hold for other experiments.
     
  20. Nov 19, 2017 #19
    I can't begin to imagine what you mean by that "assumption". I have read many papers on various experiments where "Alice & Bob" got confirmation of quantum entanglement in their results...
     
  21. Nov 19, 2017 #20

    Zafa Pi

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    In post #16 I proposed a situation governed by 1), 2), and 3). Then I asked a question in 4). After that I gave an answer to that question. Can you specify more clearly where your problem lies?
     
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