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The Observer Effect

  1. Nov 24, 2014 #1
    According to the observer effect, a particle changes when observed. Is it possible that how it changes is predetermined at its creation? Are there any papers that have explored this?
     
  2. jcsd
  3. Nov 24, 2014 #2

    atyy

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    Yes, it is in principle possible, but the theory of the underlying hidden variables cannot be a local theory.

    A theory that reproduces the predictions of non-relativistic quantum mechanics that has deterministic time evolution is Bohmian Mechanics. There the randomness lies in the initial condition.

    At present, it is unclear if Bohmian Mechanics can be extended to describe relativistic quantum mechanics.

    John Bell showed (with some loopholes) that no local deterministic theory can reproduce the predictions of quantum mechanics. Among the loopholes are the assumption that we can set up independent random processes at great spatial separation whenever we want to. Another loophole is the assumption that there is no retrocausation.

    http://arxiv.org/abs/quant--ph/0611032
    What you always wanted to know about Bohmian mechanics but were afraid to ask
    Oliver Passon


    http://arxiv.org/abs/1303.3081
    The device-independent outlook on quantum physics (lecture notes on the power of Bell's theorem)
    Valerio Scarani


    http://arxiv.org/abs/1206.1084
    Overview of Bohmian Mechanics
    Xavier Oriols, Jordi Mompart
     
    Last edited: Nov 24, 2014
  4. Nov 24, 2014 #3

    DrChinese

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    As atyy mentioned, Bell wrote on this subject. So before steering you further, it would be helpful if you are familiar with these important papers on the subject:

    EPR (1935):
    http://www.drchinese.com/David/EPR.pdf

    Bell (1964):
    http://www.drchinese.com/David/Bell.pdf

    If you don't know the material in these, more current papers on the state of the art in this area will likely not be of much help.
     
  5. Nov 24, 2014 #4

    atyy

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    Great pun! :D
     
  6. Nov 24, 2014 #5
    It seems that the amount of hidden information in Bohmian mechanics must have been vast at the early stages of the universe. Since the entropy of the universe is increasing over time, the amount of information in the hidden variables in a deterministic theory must account for the entropy of the far future. This seems like an absurdity to me. You basically need infinite information in hidden variables.
     
  7. Nov 24, 2014 #6

    Nugatory

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    Is it any more absurd than the classical notion that if we knew the position and momentum of every particle in the universe we could predict the entire future evolution of the universe? The amount of information required to specify the initial state is enormous, but not infinite.
     
  8. Nov 24, 2014 #7
    Yes. In a 4 dimensional spacetime - Block universe, as advocated by Minkowski (Einsteins math teacher) this is not only a possibility, but better yet, a neccesity. This regardless of which quantum interpretation one adheres to. There is no "time" for it to be any uncertainty. Time is static in Minkowskis 4-dimensional block-time, a theory based on special relativity.
     
  9. Nov 24, 2014 #8

    bhobba

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    I liked the post, but I am wondering if you could elaborate on the amount of information specifying the initial state being enormous - but not infinite.

    Being a Hilbert space the basis is countably infinite.

    Thanks
    Bill
     
  10. Nov 24, 2014 #9

    bhobba

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    My bible on general relativity is Wald, and it is considered a very good reference, especially on the geometrical view.

    But no-where in it can I recall the concept of block universe.

    I did a Google search and it seems to be associated with a certain philosophical view of time:
    http://en.wikipedia.org/wiki/Eternalism_(philosophy_of_time)

    I don't think it was advocated by Miknkowski who simply noted that SR can be viewed as a pseudo-Riemannian geometry (ie the requirement of the metric to be positive definite is removed) and such turned out to be the the natural generalisation of Riemanian geometry to give GR - although it took Einstein a long tortured route to fully understand it.

    Time is NOT static in Minkowski 4 space.

    Thanks
    Bill
     
    Last edited: Nov 24, 2014
  11. Nov 24, 2014 #10

    bhobba

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    It also must be emphasised that is true only in some interpretations of QM - or rather is very trivial. In the ensemble interpretation for example an observation that doesn't destroy what's being observed simply subjects it to a different preparation procedure (these are called filtering type observations). Of course if you prepare it differently its state will change - in fact in that interpretation state and preparation procedure are synonymous - that's what I mean by trivial.

    Thanks
    Bill
     
  12. Nov 24, 2014 #11

    Nugatory

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    Oops. I think you're right.
     
  13. Nov 24, 2014 #12

    bhobba

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    Actually I think you are right - I just wanted your take.

    My view is the Rigged Hilbert space view where I think the physically realizable states are of finite - but in most cases large dimension - and we introduce its dual which is countably infinite purely for mathematical convenience. But really that is a whole new thread :D:D:D:D:D:D:D.

    Thanks
    Bill
     
  14. Nov 24, 2014 #13

    atyy

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    I think in quantum mechanics one can have a finite dimensional Hilbert space, but in Bohmian Mechanics I think it isn't possible. I'm not sure how to prove it, but the heuristic is that if a precise position measurement is possible and the state collapses to a delta function, then clearly Bohmian Mechanics will fail, since we will have localized the particle exactly for the subsequent measurement, which would contradict the uncertain initial condition that Bohmian Mechanics needs. So Bohmian Mechanics here relies on the delta function being unphysical, and really insisting that the state be square integrable. While I don't know how to turn that into hard mathematics, there are some "excess baggage" theorems in this spirit, for example, Hardy's "Ontological Excess Baggage Theorem" and Montina's http://arxiv.org/abs/0711.4770.

    It would actually make a lot of sense if in general hidden variables could exist, and they required much more information to specify than quantum mechanics, to achieve the same amount of predictability. Hidden variables should exist 'in principle' so that we can have naive reality (which is totally justified from the FAPP point of view, since naive reality is just a tool to help us make predictions, and if we are going to be wrong, we might as well pick the simplest viewpoint*). However, the quantum formalism would make sense if the hidden variable theory is in general unwieldy (and not unqiue for our given experimental resolution) - the quantum formalism would be like a simple "fixed point of the renormalization group" effective theory. Of course that's just heuristic, and I have no renormalization flow for which quantum mechanics is a "fixed point". But Hardy's, Chiribella et al's work about reasonable axioms are in this spirit, I think.

    *Here's the justification for naive reality in the Copenhagen spirit. In Copenhagen we make a classical/quantum cut, so we believe in naive reality on one side of the cut. The cut can be shifted, and so anything can be part of some naive reality, just like everything can be part of something quantum. The usual Copenhagen spirit is to be agnostic whether everything can be quantum, because of the problem of definite outcomes. Noting that definite outcomes are on the naive reality side of the cut, agnosticism about whether everything can be quantum is equivalent to privileging naive reality.
     
    Last edited: Nov 24, 2014
  15. Nov 24, 2014 #14

    morrobay

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    http://www.arxiv.org/pdf/quant-ph/0611259.pdf This is another paper on the subject. 4.1 page 9 :
    Apriori there is no reason to assume that the range of values of an ontic physical variable ( ontic spin) should coincide with the range of values of the corresponding observable ( measured spin of S)
     
  16. Nov 25, 2014 #15

    Nugatory

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    The PhysicsForums policy on discussions of the Block Universe can be found here: https://www.physicsforums.com/threads/what-is-the-pfs-policy-on-lorentz-ether-theory-and-block-universe.772224/ [Broken]

    I'm letting this post stand, because it is responsive to the the original question (and OP may also want to google for "t'hooft superdeterminism") but further discusion here is off-limits.
     
    Last edited by a moderator: May 7, 2017
  17. Nov 25, 2014 #16

    atyy

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    I just realised that your two favourite "derivations" of quantum mechanics point in different directions. If one takes Hardy's, then probably finite dimensional Hilbert spaces are more fundamental, and it's just kinda nice that we can use infinite dimensional Hilbert spaces and rigged whatever to make out lives easy. On the other hand, if we take Ballentine and spacetime symmetries as the basis of quantum mechanics, then it seems infinite dimensional Hilbert spaces are more fundamental, since Galilean symmetry should produce the Schroedinger equation, and Lorentz symmetry should produce quantum field theory.

    So do you have an aesthetic preference: Hardy or Ballentine - finite or infinite dimensional Hilbert spaces?
     
  18. Nov 26, 2014 #17

    bhobba

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    Hardy for sure - it gets to the essence better IMHO.

    Thanks
    Bill
     
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