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I A new approach: retrocausality in QFT

  1. Jul 5, 2017 #1
    I looked at the other threads that have discussed retrocausality, but a scan of the article
    seems to take a new approach.
    The paper also gives two references.
    Proceedings of The Royal Society A. DOI: 10.1098/rspa.2016.0607 .
    Also at arXiv:1607.07871 [quant-ph].​

    The following paragraph is a quote from the article that seems to explain the meaning of the authors' idea.
    First, to clarify what retrocausality is and isn't: It does not mean that signals can be communicated from the future to the past—such signaling would be forbidden even in a retrocausal theory due to thermodynamic reasons. Instead, retrocausality means that, when an experimenter chooses the measurement setting with which to measure a particle, that decision can influence the properties of that particle (or another particle) in the past, even before the experimenter made their choice. In other words, a decision made in the present can influence something in the past.
    The following is another quote which seems to be the theoretical basis for the idea.
    Can't have both time symmetry and no-retrocausality​

    I have two questions about this idea.
    1. Can any of the forums participants think of a way that this idea can be experimentally tested?
    2. What does anyone guess about the chances that this idea will at some future time become mainstream?
     
  2. jcsd
  3. Jul 6, 2017 #2

    DrChinese

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    Sure, I'll take a shot. :smile:

    1. That is more or less the $64,000 question. There are many experiments which have been run that strongly suggest retrocausality. However, competing interpretations are not excluded by such experiments. (Examples include entangling particles after they have been measured, seemingly impossible.)
    2. No chance it will become mainstream without more experimental evidence to exclude other interpretations.

    I am an advocate of examination of time symmetric class theories/interpretations, including ones featuring retrocausality. From the paper you cited by Leifer/Pusey:

    We conclude that the most plausible response to our result, other than giving up Realism, is to posit that there might be retrocausality in nature. At the very least, this is a concrete and little explored possibility that holds the promise of evading almost all no-go theorems in the foundations of quantum theory, so it should be investigated further.

    Another approach, along similar lines, is Relational Blockworld. This features a type of time symmetry but is considered "acausal" rather than retrocausal. The distinction being that neither the past nor the future can be considered as causing anything independently. So there is no direction of causal flow.

    http://www.ijqf.org/wps/wp-content/uploads/2015/06/IJQF2015v1n3p2.pdf
    This includes related discussion to work by Leifer and Price (related to your cited article).

    But in discussions here, support for any variation away from forward in time causation is very low.
     
  4. Jul 6, 2017 #3
    Currently I am considering the acausal correlations between past and future Dr. Chinese mentioned in combination with non-realism, or non-CFD.
     
  5. Jul 6, 2017 #4
    What is the distinction between
    "...signals can be communicated from the future to the past..."
    and
    "...that decision can influence the properties of that particle (or another particle) in the past..."
    ?

    2. What does anyone guess about the chances that this idea will at some future time become mainstream?
    Depends on the decisions made in the future that would influence properties back here in the present, right?
     
  6. Jul 6, 2017 #5

    DrChinese

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    1. If I entangle pairs of particles after they are detected, they will have properties different than if I do not entangle them. But such process cannot be used to send a signal anywhere, the results are still random outcomes as far as we know.

    2. The future DrC made me say that. :smile:
     
  7. Jul 13, 2017 #6
    I have always viewed retrocausality as an alternate interpretation - not a theory. The problem is not that it doesn't predict anything testable, but that it doesn't predict anything different and testable.

    For example, it predicts that the arrow of time is set entirely through thermodynamics and, as the article describes it, boundary conditions. But is there a theory that says otherwise? One that poses an alternative time-arrow theory that is specific enough to test?

    I see one immediate advantage in using this interpretation in some discussions: It makes the "interpretation" status of the many-world interpretation (MWI), easier to examine. If you want to reverse the time arrow in MWI, it would seem to me that you need those other worlds you formed to cooperate. If MWI worlds interact in one time direction, why not both? Does MWI presume a fundamental direction of time? If it does not, then why not presume that other worlds are rejoining with our world to form our future?
     
  8. Jul 13, 2017 #7

    RUTA

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    My colleagues and I have a book due out in Nov with Oxford UP called "Beyond the Dynamical Universe" in which we argue that many mysteries in physics are due to time-evolved explanation in the dynamical universe and all these mysteries disappear using adynamical explanation in the block universe (our substitute language for retrocausality as Dr Chinese points out). Here is a blurb describing the book http://www.relationalblockworld.com/blog

    So, we're hoping it becomes mainstream, but regardless, it is certainly a very powerful explanatory schema as we show.
     
  9. Jul 13, 2017 #8
    The block universe really is the only way to make sense of relativity. We draw all these pictures of world lines, light cones, and coordinate transformations treating space and time on similar footing. The future exists as much as the past or down or to the side. If we imagine that the future is generated on the fly, then due to relativity of simultaneity, the universe must be specifically generated around me. I think this kind of solipsism is silly.
     
  10. Jul 13, 2017 #9

    PeterDonis

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    This is not a valid inference. It could be that "the universe being generated" is not an absolute thing at all, but relative to an observer.
     
  11. Jul 13, 2017 #10

    RUTA

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    The block universe is a reasonable inference from relativity of simultaneity (see the protracted argument in chapter 2 of the forthcoming book or http://philsci-archive.pitt.edu/3696/ which was published in Space, Time, and Spacetime - Physical and Philosophical Implications of Minkowski's Unification of Space and Time, Petkov (ed.), 2010, Springer). But, we don't have to argue that point here, since we must assume the future already exists in some sense to discuss retrocausality, which is the topic of the thread.
     
  12. Jul 13, 2017 #11

    PeterDonis

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    A possible inference, yes. But many discussions of this make the stronger claim that the block universe is required by relativity of simultaneity; this claim is false, as I argue in my Insights article on the topic:

    https://www.physicsforums.com/insights/block-universe-refuting-common-argument/

    Yes, this I agree with.
     
  13. Jul 13, 2017 #12

    strangerep

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    Exactly. I was just about to complain about that very point.

    IMHO, the paragraph quoted is self-contradictory nonsense. o_O
     
  14. Jul 14, 2017 #13
    Hi RUTA:

    I get from the blurb that the concept is that conceptualizing the universe as a 4D whole without time slices avoids (some?) issues about causality between past and future. What the blurb omitted is any use of the word "causality". I wonder if you might post a brief explanation of how you see this 4D view relating to causality.

    Regards,
    Buzz
     
  15. Jul 14, 2017 #14
    "The following paragraph is a quote from the article that seems to explain the meaning of the authors' idea.
    First, to clarify what retrocausality is and isn't: It does not mean that signals can be communicated from the future to the past—such signaling would be forbidden even in a retrocausal theory due to thermodynamic reasons. Instead, retrocausality means that, when an experimenter chooses the measurement setting with which to measure a particle, that decision can influence the properties of that particle (or another particle) in the past, even before the experimenter made their choice. In other words, a decision made in the present can influence something in the past."


    1) Compare the rates of expansion now with that at the big bang. Retrocausality assumes that the future already exists and influences the past, so the initial rate of expansion will be different as there is less/more force acting on it.
    2) Possibly. Personally I think all aspects of QM make more sense when you consider retrocausality.
     
  16. Jul 14, 2017 #15

    DrChinese

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    That might be true. However, I think that expansion is a free parameter. How would you be able to separate the component of expansion related to the future?

    ----------------------------------

    I should note that there are other possibilities for representing the Big Bang in a time symmetric universe. We assume there was something like this, based on the evidence we can see:

    Big Bang ----> Present day universe we inhabit ----> Future

    There could also have been something like this, in a time symmetric universe, which I acknowledge as purely speculative (but reasonably so if we are considering/assuming time symmetry in the first place):

    Anti-future <--- Anti-sub-universe <--- Big Bang ----> Present day sub-universe we inhabit ----> Future

    Which would presumably explain why the thermodynamic arrow of time runs in one direction, as we see it. In the anti-future, they too see a single thermodynamic arrow of time - although they are unawares it run counter to ours. This model neatly explains the apparent asymmetry of time. RUTA's relational block world extends in both directions of time. :smile:
     
  17. Jul 14, 2017 #16
    Interesting, but which spacetime does the anti-future occupy? Or does it exert an infuence from outside of spacetime? Forgive me, but I am assuming spacetime begins at the big bang? Do you think that our perceived arrow of time is down to entropy and how biological organisms must be required to collect energy from available states to survive disordered states thus giving rise to memory formation time bias?
     
  18. Jul 14, 2017 #17

    PeterDonis

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    The Carroll-Chen model works something like this:

    https://arxiv.org/abs/hep-th/0410270
    In the Carroll-Chen model, the same one. See the paper above.
     
  19. Jul 14, 2017 #18

    DrChinese

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    Nice, thanks! Gives me some good reading (on top of some of RUTA's newest works).

    -DrC
     
  20. Jul 14, 2017 #19

    RUTA

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    In my opinion, causality is a concept relevant only to dynamical explanation. Obviously, the retrocausality camp disagrees, see their "interventionist" account of causality in my Insight https://www.physicsforums.com/insights/retrocausality/. I think it's best to forget that expression when dealing with adynamical explanation in the block universe and merely talk about the 4D patterns. But that's just my preference.
     
  21. Jul 14, 2017 #20
    H RUTA:

    Thank you for your answer.

    Regards,
    Buzz
     
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