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Is the future of quantum mechanics retrocausal?

  1. Jan 15, 2015 #1
    In the past decade there has been a blossoming of all kinds of interpretations and variations of interpretations of quantum theory. Recently an outlier has entered the picture more and more, namely the retrocausal view which aims to tackle the measurement problem and no go theorems by taking the blocktime / B-theory of time we got from special relativity very seriously. In the retrocausal interpretation the future and the past together has an influence over the present in a deterministic fashion.

    Retrocausality gives us locality and causality without violating Bell inequalities in exchange for accepting that the future can affect the present. Now as with pretty much everything that is even remotely related to quantum mechanics, this makes your head hurt. But I would argue that it is no more weird than the idea of indeterministic random manifestations of reality, consciousness collapsing a mythical wavefunction, the universe splitting into infinite worlds, spooky action at a distance, anti-realism or any of the other ways of thinking of the quantum world.

    A version of this view is called two state vector formalism (http://en.wikipedia.org/wiki/Two-state_vector_formalism) that has been fleshed out by Yakir Aharanov, which eventually lead to the theory of weak measurements. However this idea of retrocausality had already been considered by Feynman, Wheeler, Watanbe and others before. Other models have been constructed, specifically Huw Price and Ken Wharton (as well as others) has contributed greatly to this view, which they've developed in numerous papers over the years.The latest of which was put up on arxiv just a few weeks ago and can be read here: http://www.ijqf.org/

    While the retrocausal approach has been known for over half a century and been at least considered by most of the "titans of QM", it's first in the recent years it has gained coverage. In 2014 there were quite a few good articles written for laymen about this. This is a good one: http://nautil.us/issue/9/time/the-quantum-mechanics-of-fate.

    Another big event for retrocausality took place in 2014; A conference stretching over 4 days were held where most of the quantum foundations researchers and thinkers gathered with different presentations and held stimulating discussions on this topic. Some arguing for, some being agnostic, some raising critical questions. One thing is for sure, there are quite a lot of the core of quantum foundations that are starting to take this view very seriously. It parallels what happened to the Many Worlds interpretation in the mid-00s with the Everett@50 conference, which spawned a lot of popular media coverage as well as interesting debates, which made people take it seriously. But this time around, it's literally times time to be picked apart and studied intensively.
    All the talks from this conference can be viewed here: https://newagendasstudyoftime.wordpress.com/events/retrocausality-conference/

    As with all interpretations, there is no single experiment that can prove/disprove any retrocausal model, at least not yet, but there are those who make the argument that the delayed-eraser experiment and partial/weak measurements give creedence to and is most naturally interpreted via the retrocausal view. Such argumentation can be found here: http://plato.stanford.edu/entries/causation-backwards/ and here http://scitation.aip.org/content/aip/proceeding/aipcp/10.1063/1.3663720

    I personally think it's time to take a deeper look at time...
     
    Last edited: Jan 15, 2015
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  3. Jan 15, 2015 #2
    Could we tell the difference between a spacetime that is changeless from one that is changing?
    Seems like retro-causality if consistent would not distinguish between them, but the latter would not be quite the same kind, more like a continuous adjustment for consistency.
     
  4. Jan 15, 2015 #3

    bhobba

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    And rest assured there are scientists doing just that. But since it is science it wont be like one guy went on about in another thread (thankfully now locked) 'If time exists, where is it? It seems logical to say that if time is only a belief and nothing more, based on the fact of change. Change occurs, but I don't believe time is there as a part of the physical universe, because if that were the case, then how could anything move?' Science isn't semantic philosophical waffle - its based on simple fundamental concepts such as time is what a clock measures.

    These retro-casual interpretations are just like any other - they are a dime a dozen and is more of a reflection on the psychology of its proponents than actual science - interesting to think about - but until it's subject to experimental verification - just that..

    Thanks
    Bill
     
    Last edited: Jan 15, 2015
  5. Jan 15, 2015 #4
    Surely they are no worse than infinite worlds interpretations ?

    And about the time thing, I know of another crazy guy that said "could the reason why one cant travel in time be that time doesnt exist ?"
     
  6. Jan 15, 2015 #5

    bhobba

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    I think you are forgetting MW is a precise mathematical conjecture rather than semantic waffle. Where is time? First of all you need to show where is a concept applicable to time. Time is what a clock measures bypasses all that sort of rubbish.

    Time doesn't exist - yea clocks don't measure anything. And we can easily travel into the future - its the past that is the issue due to logical paradoxes.

    Thanks
    Bill
     
  7. Jan 15, 2015 #6

    DrChinese

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    To be fair, there are 2 very strong experimental supports for the retrocausal class of interpretations. I never gave them much thought until I was fully aware of these. I would not call either of them "absolute" proof as obviously at this point, all interpretations claim a degree of equality. But certainly, only the retrocausal strike at the heart of them.

    1. Zeilinger et al have performed experiments in which photons are entangled AFTER they are detected. How is that NOT a case of the future affecting the past? See page 5.

    http://arxiv.org/abs/quant-ph/0201134

    2. The ONLY non-locality that has ever been seen in entanglement experiments involves pairs connected either by a) relativistic world lines from source to detectors; or b) relativistic world lines from source to detectors with zig-zag from past-to-future and future-to-past. In other words, there has never been entanglement of 2 particles that could NOT be explained by connections respecting c, when causality is dropped. I would expect that a true non-local explanation would not have this limitation.

    Again, no need to point out that the above are not "bullet-proof"... anyone is free to disregard them. But I would NOT say there is NO experimental support for them. If there were no support for them, neither of the above would be true.
     
  8. Jan 15, 2015 #7

    bhobba

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    No its not bulletproof.

    Interesting nonetheless.

    Thanks
    Bill
     
  9. Jan 15, 2015 #8

    stevendaryl

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    Something that bugs me a little about retrocausal interpretations (or time-symmetric quantum mechanics) is that they claim to make things symmetric between past and future. At a certain level of abstraction, it does seem symmetric: You have a measurement in the past (preparation) and you have a measurement in the future (observation of results), and the probabilities are computed using both measurements symmetrically.

    However, it seems to me that there is a big distinction, as follows: A measurement involves amplifying a microscopic signal to make it macroscopically observable. In other words, it's an irreversible process. The time-reverse of a measurement would be an entropy-decreasing "unmeasurement".

    I don't completely understand the relationship between entropy increase and measurement, but I think it's an essential part. Since measurement is what changes things from lots of possibilities with different amplitudes to a definite result, it seems that the irreversibility might be an important part of the interpretation of quantum mechanics. It seems that a time-symmetric formulation obscures this, rather than clarifies it. Just my uninformed opinion, though.
     
  10. Jan 15, 2015 #9

    DrChinese

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    Not that I do either... but a comment. Again, part of why the time symmetric/retrocausal/blockworld class of interpretations seem to have at least *some* merit.

    I know entropy increases to the future. But I think it reasonable (though certainly not the norm) to say it increases to the past as well. Let me explain, and please, this is a heuristic argument and not intended to be rigidly correct.

    1. Any measurement and/or prepared setup effectively gives you a local maximum of knowledge. Essentially, you (relatively) minimize the number of possible states the system could be in at time t=0.

    2. At time t=-1 (the past) and time t=+1 (the future), there are more possible states the system could occupy. So there is more entropy - in both time directions.

    In other words: at t=0, there are a number of past "worlds" (or paths) that could have led to this one. And this one can evolve to a number of future "worlds" (or paths). And in both cases, t=0 represents something of a minimum in whatever terms you could measure states.

    If you accept this view (which as far as I know is contradicted by nothing in the literature), you don't have the same asymmetry of causal direction as would normally be a roadblock.
     
  11. Jan 15, 2015 #10
    A measurement gives maximal knowledge of what you are measuring at the cost of some knowledge of everything else in the universe.
     
  12. Jan 15, 2015 #11
    You might find this passage from Ruth Kastner worth reading. She favours the transactional interpretation but sees retrocausation in that model somewhat differently:
    http://transactionalinterpretation.org/2014/07/06/my-talk-at-cambridge-conference-on-free-will-and-retrocausality-in-the-quantum-world/ [Broken]
     
    Last edited by a moderator: May 7, 2017
  13. Jan 15, 2015 #12
    Do we need causality? When two events are tied to each other, we label one cause and the other effect, based on their order in time. But in the block universe picture, you just have correlations between events. The laws of physics describe how the block universe was initially populated with events (in a seemingly probabilistic fashion). Instead of asking what is the probability of A causing B, we can ask, what is the (marginal) probability of B given A.
     
  14. Jan 15, 2015 #13

    RUTA

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    If the future is "there," i.e., reality is a blockworld (BW), then in what sense does the future "cause" the present (retrocausation) any more than the converse? My interpretation of QM relies on a BW, so I'm not arguing against BW. I just don't understand why anyone would talk about an experiment as an "evolving story" in the context of a BW. Nothing is "evolving" or "happening" backwards in time, unless you're employing a "meta-time," and that is superfluous from the standpoint of physics. If you need to use the past, present and future (detector settings, for example) to explain an experiment to include its outcomes, then your explanation is spatiotemporally holistic, so you should be saying that the past, present and future are "co-causal." The implication is that you should be considering adynamic laws, as Wharton has pointed out.

    P.S. Dr. Chinese, I tried to start a Conversation with you on 16 Dec.
     
  15. Jan 15, 2015 #14

    bhobba

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    If I remember correctly, while discussion of retrocausal interpretations is fine, discussion of the BW is not allowed on this forum

    Best to to avoid it if the thread is to remain alive.

    Thanks
    Bill
     
  16. Jan 15, 2015 #15

    DrChinese

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    Bill,

    I don't think that Relational Block World (RBW) fits in the same category as some of the other BW concepts that are frowned upon. We have discussed RBW and cited papers around that a number of times in the past without issue. For example:

    http://arxiv.org/abs/0903.2642

    "We propose a discrete path integral formalism over graphs fundamental to quantum mechanics (QM) based on our interpretation of QM called Relational Blockworld (RBW). In our approach, the transition amplitude is not viewed as a sum over all field configurations, but is a mathematical machine for measuring the symmetry of the discrete differential operator and source vector of the discrete action. Therefore, we restrict the path integral to the row space of the discrete differential operator, which also contains the discrete source vector, in order to avoid singularities. In this fashion we obtain the two-source transition amplitude over a "ladder" graph with N vertices. We interpret this solution in the context of the twin-slit experiment."

    Or this one, which I believe is still in revision:
    http://arxiv.org/abs/0908.4348

    Nearly any timeline you draw involving an entangled pair is best and most naturally described by saying both a past context AND a future context BOTH contribute to observed correlations. There are a number of serious interpretations seeking to express this. RBW is one such. As best I am aware, it is the most fully developed of those. Although admittedly, I think the field is woefully overlooked by the community.

    Hopefully no one will object to discussion around this. I would urge any Mentor to allow continued discussion around RBW. It is far more developed than Absorber Theory and probably more developed than TSQM too. Anyway, this thread is not about RBW and its merits: it is about contributions from the future which affect the now.
     
  17. Jan 15, 2015 #16

    DrChinese

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    Ah missed that and will read the reference.

    I personally use "retrocausation" and "time symmetry" and even your RBW somewhat interchangeably because there are so few people that are familiar with the concepts. The important common element is that the arrow of time - past to future - is rejected in some manner. The mechanisms/descriptions are quite different, as you point out.

    But the big hurdle for most people to get over is the rejection of causality. I don't really get that, because all quantum processes seem largely random anyway. No root cause has ever been found for any probabilistic behavior, why wouldn't you reject causality out of hand? Especially if it makes all the pieces fit together.
     
  18. Jan 16, 2015 #17
    To me the reason I cannot reject causality is mainly philosophical conviction of the Born Rule. The Born Rule indicates that something is determining these outcomes, if things were TRULY random, we should expect absolutely no statistical rule at all.
     
  19. Jan 16, 2015 #18

    bhobba

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    The Born rule follows from basis independence via Gleason.

    If you choose any other it depends on the basis - which is very much against the vector space formalism which is basis independent. Why would you think such is the case? Of course nature may be like that - but you are speaking philosophically and think simplicity indicates all outcomes would be equally probable - Gleason speaks against that.

    Thanks
    Bill
     
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