Relational Blockworld: Providing a Realist Psi-Epistemic Account of QM

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    Mark Stuckey

    We update our Relational Blockworld (RBW) explanation of quantum physics and argue that it provides a realist psi-epistemic account of quantum mechanics as called for by Leifer. The paper is posted at and will appear in the next issue of IJQF. The paper is 41 pp of text and 8 pp of figures, but you can skip to sections which interest you without too much difficulty (you may have to look up an abbreviation or two):

    Abstract is on p 1.
    An introduction to RBW is on pp 2 – 5.
    RBW as a retrocausal account is on pp 5 – 9.
    A paragraph explaining the adynamical global constraint (AGC) in relation to QFT and QM is on p 9.
    A general outline of the paper in on pp 9 and 10.
    An RBW account of Danan et al (“Asking photons where they have been”) is on pp 11 – 16.
    An RBW account of Dowker’s GHZ set-up is on pp 16 – 18.
    RBW as a realist, psi-epistemic account of QM is on pp 18 and 19.
    RBW as ontic structural realism in a blockworld where “particles” are contextually defined is on pp 19 – 24.
    A mathematical explanation of the AGC is on pp 24 – 28.
    The twin-slit experiment as explained by AGC –> QFT transition amplitude –> generating function –> propagator –> QM probability amplitude is on pp 28 – 36.
    RBW explanations of foundational issues, e.g., the measurement problem, decoherence, commuting vs non-commuting algebra, and the Born rule, are on pp 37 – 41.
    Conclusion is on p 41.

    We will establish a 2-hour live chat time in consultation with the session moderator, Ken Wharton, at a later date.

    Robert Griffiths

    Dear Mark
    Do you think you could prepare a brief summary of what you are doing, say 4 or 5 pages, relating it to issues in quantum foundations and the work that other people are doing, and saying why we should be interested in it? I confess to having gotten essentially nothing from the abstract and introduction of your 54 page article, and lack the motivation to spend hours digging through the rest of it. Nor did I find your hints to look here or look there helpful. There are too many unfamiliar terms and concepts, an enormous number of initialisms, but, worst of all, I do not know where you are going. Which of the big problems of quantum foundations do you think you have solved? Your style may be appropriate for communicating to specialists, but it then leaves someone like me, who has not been in on the discussion, out in the cold. Take a look at some of the other contributions to this workshop and I think you will see the level of writing and explanation needed to communicate to a group of this sort. I hope I am not being offensive by expressing my disappointment.

    Bob Griffiths

    Mark Stuckey

    Hi Bob,

    Thanks for your input, it’s much appreciated and not the least bit “offensive.”
    We were concerned about just this sort of reaction, that’s why we tried the outline. Since that didn’t work, let me attach the slides for a short talk I had planned to give in Vaxjo last month (I instead gave the quantum Cheshire Cat talk posted in the “Other Topics” forum). In these slides, we provide a Relational Blockworld explanation of the standard EPR-Bell experiment and do not include a mathematical articulation of the adynamical global constraint. Instead, we focus on conveying the basic ontological concepts of RBW. Hopefully, this talk is clear enough (conceptually) that you can now understand section 4 of our posted paper where we provide RBW explanations of the standard QM foundational issues. If we still haven’t succeeded in answering your questions, please let us know and we’ll gladly try something else 🙂

    Thanks again for your interest,


    I have a bunch of questions and comments, but let me start with “Is RBW a retrocausal account?” You cite Geroch approvingly saying that nothing changes or moves in the blockworld. But that doesn’t seem right to me. Change/motion only make sense from the perspective of a world-line — the properties of one temporal part of the world-line are different from the properties of another temporal part. Of course the 4D world-line itself doesn’t move or change — that would be a conceptual confusion. But their contents change — in fact, that’s all that change means. But if you buy that, then why resist saying that RBW is retrocausal? Causation is something that appears from the perspective of a world-line. Of course, causation is a lot harder to characterize than change! But the fact that RBW explains a 4D situation “all at once” is no barrier in principle to characterizing the causation implicit in it. There may be other barriers to causal talk though, like the “unmediated” exchange of energy between emission and absorption, or the lack of counterfactual definiteness. But I’d be interested to hear what you think.

    Mark Stuckey

    Thanks for your question, Peter. Causality and change can certainly be represented in 4D spacetime, so I didn’t mean to imply otherwise. The Geroch quote is, as you say, pointing out the fact that the 4D perspective *itself* doesn’t change. We use that “changeless” 4D perspective for two reasons. First, 4D spacetime lacks a Now in the sense that there is no “movie projector for creating preferred moments in M4” per Brian Greene’s analogy [At the 29:43 mark, just after David Albert says, “Physics does radical violence to this everyday experience of time” at the 29:05 mark.] If you want to see how one might account formally (and robustly) for a Now, see Ruth Kastner’s PTI. Second, QM is not compatible with classical causality per Wood and Spekkens (perhaps because QM requires directionless links in its causal diagrams, see Evans’ paper in this forum). So, the notion of retrocausality in a block universe is, as an anonymous referee stated (footnote 3 of our paper in this forum), “superfluous at best, and inconsistent at worst.” If you want an example of non-deflationary retrocausality, again see Ruth’s PTI.

    Since RBW’s fundamental ontological entities for modeling QM phenomena are 4D spacetimesource elements, I don’t see RBW as containing any robust sense of change and causation at the fundamental level. Perhaps Silberstein will have more to say.

    I look forward to your further comments and questions!


    Good. I guess I don’t see why causality in a block world should (in general) be superfluous (or inconsistent). You can ask “Suppose this measuring device setting had been different — would anything earlier have been different?” If yes, then you’re retrocausal. But what I suspect is that it’s hard to answer this question in RBW because of the unmediated nature of the energy exchange. That is, I suspect that it’s not the block world that makes causal talk inappropriate, but something else about your model. Does that sound right?

    Mark Stuckey

    I understand you, Silberstein and Wharton discussed this point and concluded that RBW is, in the sense you state, retrocausal. That’s why we say RBW is retrocausal (however deflationary) in the paper. Frankly, I agree with the referee. As we state in the paper, why bother with retro-time-evolved causal stories in a block universe when the entire 4D pattern is already explained by the adynamical global constraint? The “perspectival view from within the block universe” (Price’s term) is what motivates us to tell time-evolved causal stories. But, GR simply provides a spacetime metric and stress-energy tensor on the spacetime manifold that are “self consistent” per Einstein’s eqns. In most cases, you can read off several consistent time-evolved causal stories from a GR solution. But, in some cases, the time-evolved story is left wanting (or even seemingly inconsistent, e.g., as can arise due to the relativity of simultaneity). For example, someone who wants dynamical explanation might ask, “Where did the big bang come from?” when viewing GR cosmology solutions. It’s a faux mystery that arises simply because they want a time-evolved causal story. So, in my opinion, retro-time-evolved (or even time-evolved) stories are superfluous in the context of any 4D solution, as the referee states. But, I’ll leave it to the community to decide its semantics 🙂

    Ken Wharton


    It’s true that this “interventionist account” of causation kind of disappears when you’re no longer talking about subsystems controlled by agents… Some philosophers might even go so far as to say there *no such thing* as objective causation in a full 4D block universe. This would basically fit with what you’re saying, it sounds like (although it’s unclear to me whether you think there is another non-RBW 4D block picture that *would* have some version of causation).

    But I’m not quite willing to go that far. In a full 4D block universe, it seems to me that the role of the external agent (who might cause something in a 4D subsystem via the interventionist account) is basically replaced by the cosmological boundary conditions. In the same sense that an agent outside a 4D subsystem might “cause” something in that subsystem, it seems basically equivalent to say that events in a 4D block universe are “caused” by the cosmological boundary conditions. So I don’t think it’s a good idea to throw out the connection between RBW and retrocausation: people might read that as you denying any role of future cosmological boundary conditions as a constraint on what’s happening now. (And you’re not doing that, are you?)

    Mark Stuckey

    Glad to see you here, Ken!

    Of course, I believe 4D spacetime can be used to model classical causality. However, the phenomena under consideration here are space-like separated correlations that violate Bell’s inequality and those phenomena violate classical causality (as articulated by Wood and Spekkens, for example). Obviously, I’m not “denying any role of future boundary conditions as a constraint on what’s happening now.” That is germane to the path integral approach which we use in RBW. And, I agree that if I change an equipment setting, I definitely acted “causally” in a 4D situation (experimental process). But, the causation in that case is simply to instantiate a particular trial. I don’t take that as causally related to the correlated outcomes per se. In other words, if someone asked me, “Why did you get agreement in 25% of the EPRB trials in which the SG magnets were 120 degrees apart?” I wouldn’t answer*, “Because I choose to set the SG magnets 120 degrees apart.” My decision to set up that particular configuration is just not relevant to explaining the outcomes. So, I agree with term “deflationary intervention” in that case.

    That being said, I do acknowledge that the classification of RBW as “retrocausal” is not for me to decide. That is decided by the “retrocausal school” where you are a leader. So, if you say RBW is retrocausal, then it’s retrocausal. The bottom line is, we should definitely acknowledge we’re in the same camp and avoid destructive infighting. We’re in a small group who believes that future boundary conditions are explanatory for violations of Bell’s inequality. Exactly how the use of future boundary conditions is viewed should not keep us from supporting each other. Constructive criticism is of course acceptable 🙂

    Hopefully my voicing a difference of opinion is taken as constructive criticism, not destructive infighting.

    *The answer to that question would be the adynamic global constraint for the relevant spacetimesource element. Since RBW’s fundamental ontological entities for modeling QM phenomena are 4D spacetimesource elements, I don’t see RBW as containing any robust sense of causation at the fundamental level.

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