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Retrocausality solves QM problems?

  1. Jan 31, 2010 #1

    jimgraber

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    Retrocausality solves QM problems??

    Does retrocausality successfully solve the problems of QM?
    This recent paper seems to claim it does.

    http://arxiv.org/abs/1001.5057

    Comments, anyone?

    Also what is the relationship of retrocausality and collapse?

    TIA
    Jim Graber
     
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  3. Jan 31, 2010 #2
  4. Jan 31, 2010 #3

    DrChinese

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    Re: Retrocausality solves QM problems??

    And also good is:

    http://arxiv.org/abs/0908.4348" [Broken]

    As an acausal account, RBW rejects any kind of common-cause principle and holds that detector clicks and the like are not evidence of microscopic dynamical entities propagating through space and impinging on the detector. Rather, detector clicks evidence rarefied subsets of relations comprising the Source, detector, beam splitters, mirrors, etc. in the entire worldtube of the experimental arrangement from initiation to outcomes, i.e., in an ‘‘all at once’’ (blockworld) fashion. Therefore, causality, dynamical entities and dynamical laws are emergent features in our view, not fundamental. In this way, we have been able to provide an account of NRQM that resolves all the foundational issues therein.

    And reference is made therein to Price and Backwards Causation (BCTS) as well:

    As we have argued elsewhere(14), the blockworld (BW) perspective (the reality of
    all events past, present and future including the outcomes of quantum experiments),
    which is necessarily at the heart of most time-symmetric approaches, is implied by RoS.
    Thus, Huw Price(15), who calls this perspective the “Archimedean view from nowhen,”
    was motivated to take seriously the idea of time-symmetric QM (TSQM) and so-called
    backwards causation in quantum mechanics (BCQM). As he says in his book defending
    BCQM(16): “the aim of the book is to explore the consequences of the block universe
    view in physics and philosophy.” Price is attempting to construct a local hidden variables
    interpretation of NRQM that explains EPR-Bell correlations with purely time-like
    dynamics or backwards causation. According to Price(17), BCQM provides an explanation
    of the Bell correlations “which shows that they are not really non-local at all, in that they
    depend on purely local interactions between particles and measuring devices concerned.
    They seem non-local only if we overlook the present relevance of future interactions.”
    The key explanatory move that Price makes is to have information travel backwards
    along the light cones of the EPR particles, converging at the source of the entangled state.
    Presumably, this is the point in spacetime where the entangled state is “prepared.” The
    picture is that the future measurement in separate wings of an EPR apparatus is as much
    the cause of the (earlier) entangled state as the converse, so the entangled state at the
    Source1 is as much the “effect” of a causal chain “originating” with the measurement of
    the entangled state at the detectors as the converse. This is to put the point directly in
    terms of backwards causation. The causal connection is not between the space-like
    separated experimental outcomes, but rather it is time-like between outcomes and
    Source. Essentially, in backwards-causation time-symmetric (BCTS) approaches, one
    eliminates the directional nature of a causal relationship so that there is no distinction
    between “A causes B” and “B causes A,” but rather it is merely the case that “A and B
    are causally related.” In this sense, the outcomes of QNL experiments are “causally
    related” to the state preparation so the demand for a causal relationship (per the violation
    of Bell’s inequality) between the space-like separated, correlated outcomes is achieved by
    allowing for the fact that outcomes “influence” the state preparation (thus, the term
    “backwardly causal” although “bi-causal” might be more appropriate). BCTS provides
    for a local account of entanglement (one without space-like influence) that not only keeps
    RoS, but in some cases relies on it by employing its blockworld consequence. Given the
    future boundary condition in spacetime, one is free to view configuration space (the wave
    function) as a mere calculational device (because we need only take the actual outcomes
    of experiments seriously), thus rendering the quantum and spacetime pictures apparently
    fully harmonious2.

    Despite the potential explanatory power of BCTS, we have shown(18) that it is not
    sufficient to account for all QNL experiments in a local fashion. For example, in the
    quantum liar experiment (QLE) entanglement is generated via the spatiotemporal
    configuration of the experimental set-up in a way not explicable by any kind of commoncause principle and its deepest explanation therefore requires invoking the entire actual history of the experiment in question. QLE, if it is to be interpreted in a way that saves locality, requires a different conception of entanglement than that offered by BCTS.
     
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  5. Jan 31, 2010 #4

    DrChinese

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    Re: Retrocausality solves QM problems??

    A diagram I like from the paper is attached. They point out what they refer to as a "trilemma" related to Bell:

    This leaves us with a choice between three options:

    I. Much more action-at-a-distance than we thought. We can reject
    the project of finding a Locality Model for SEPRB altogether, and conclude
    that action-at-a-distance is much more widespread in QM than is
    usually supposed. We thus reject the intuitive causal model for SEPRB
    (and presumably a vast number of similar cases).

    II. Much less action-at-a-distance than we thought. We can put our
    faith in the project of exploring symmetric Locality Models, which account
    in an action-by-contact (and hence potentially Lorentz-invariant) way for
    both the EPRB and SEPRB cases.

    III. As much action-at-a-distance as we thought, but less symmetry.
    We can retain the “intuitive” view that we should expect a Locality Model
    for SEPRB but not for EPRB, provided we are prepared to reject both the
    symmetries that otherwise take us from Locality in SEPRB to Locality in
    EPRB. If we choose this option, we commit ourselves (a) to the view that
    the identical correlations in these two experiments have a fundamentally
    different explanation; and (b) to fundamental time-asymmetry in the ontology
    of the quantum world, at whatever level provides the beables of a
    Locality Model for SEPRB.


    I think the interesting part is I above. This is an objection I have always had to non-local interpretations, specifically: where is the rest of the non-locality? It seems to only manifest itself in entangled situations, which seems unreasonable.
     

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  6. Jan 31, 2010 #5

    jimgraber

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    Re: Retrocausality solves QM problems??

    "also good is:

    Relational Blockworld Interpretation of Quantum Field Theory"

    Interesting reference. I have just glanced at it. I will print it out and read in detail tomorrow. My first hunch is we need backward in time information transmission, but not time symmetry. Somewhere or other there must be an arrow of time, even if only effective or emergent.
    More later.

    Thanks for the reply and the reference.
    Jim Graber
     
  7. Jan 31, 2010 #6
    Re: Retrocausality solves QM problems??

    I didn't read this paper as making a strong case for backward causality. I did read it as a significant challenge to naive ways of introducing causal models for this paper's "SEPRB", which is stated quite forcefully at the end of Section 4, straight after DrChinese's quote,
    This statement, however, leaves one with a relatively empiricist option, of action-at-a-distance, more-or-less intact. In terms of the quote given by DrChinese, Option III is argued against quite strongly, leaving Option I and Option II. Read in the context of Huw Price's advocacy of backward causality, the paper can be taken to advocate Option II, but I think the paper in itself does not make such a case very strongly.

    There are also other interpretations of QM that are untouched by the argument, but I nonetheless liked the paper a lot, for what it's worth. Section 3.1 was an LOL for me.
     
  8. Jan 31, 2010 #7

    DrChinese

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    Re: Retrocausality solves QM problems??

    Ditto. :)
     
  9. Feb 1, 2010 #8

    zonde

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    Re: Retrocausality solves QM problems??

    Yes, it is tempting to compare cos^2(theta) of Malus law with the same of entanglement. But still there are quite obvious differences in experimental setups i.e. in case of Malus law all required information is "encoded" in photon ensemble but in case of entanglement additional classical information channel is required to make event by event comparison and to reveal interference pattern between two ensembles.
    Another strong point in favor of causal model for "SEPRB" type experiment is it's usefulness for practical purposes. Wire-grid polarizer model is quite simple and intuitive. And you don't need to consider receiver end for causal model i.e. reductionism works here with all it's advantages.
     
  10. Feb 4, 2010 #9

    DrChinese

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    Re: Retrocausality solves QM problems??

    Another paper by Price on the subject:

    Time-symmetry without retrocausality: how the quantum can withhold the solace

    "It has been suggested that some of the puzzles of QM are resolved if we allow that there is retrocausality in the quantum world. In particular, it has been claimed that this approach offers a path to a Lorentz-invariant explanation of Bell correlations, and other manifestations of quantum "nonlocality", without action-at-a-distance. Some writers have suggested that this proposal can be supported by an appeal to time-symmetry, claiming that if QM were made "more time-symmetric", retrocausality would be a natural consequence. Critics object that there is complete time-symmetry in classical physics, and yet no apparent retrocausality. Why should QM be any different? In this note I call attention to a respect in which QM is different, under some assumptions about quantum ontology. Under these assumptions, the option of time-symmetry without retrocausality is unavailable in QM, for reasons intimately connected with the fundamental differences between classical and quantum physics (especially the role of discreteness in the latter)."
     
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