Peter Evans

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  • #2910
    AvatarPeter Evans
    Member

    Hi Mark,

    Yes – if by SI you mean that any hidden variables are independent of the future measurement settings, then this is a good gloss of the position.

    Pete

    #2794
    AvatarPeter Evans
    Member

    Hi Mark,

    Let me start by saying that I don’t disagree with anything you’ve said there. But I also don’t think you’ve quite understood my point. I’m not trying to advocate a position wherein we need to add some dynamical element to our 4D description in order to satisfy our temporally biased perspective; in fact, quite the opposite. Let me try to explain…

    Let us take it as given, for the sake of this discussion, that reality is best understood as a 4D block that obeys a global constraint of the sort you advocate (and a few others on this forum also advocate). If this is indeed the case, then since it is also the case that we occupy this reality and we have been able to provide rather successful causal and dynamical models representing the phenomena around us, then there must be some sort of story explaining how we can do this given that reality is actually a 4D block obeying said constraints.

    But we don’t do this by giving some competing dynamical story in terms of time evolving laws. What we’re after is a dynamical story that arises as a result of a combination of the global constraints and our spatiotemporally embedded perspective.

    If the global constraints were of a sort that gave us a kind of Newtonian reality, then the task of telling that dynamical story would be trivial: the 4D picture would be one that was consistent with the sort of time evolving laws that we usually take to describe/explain ordinary phenomena. But we know that there are significant problems created for this sort of dynamical view by the EPRB correlations; time evolving laws just won’t “save the appearances”.

    When the global constraints contain final boundary conditions, the appropriate dynamical picture that we tell about our local experience of the phenomena will involve some sort of retrocausality. But we shouldn’t need to introduce any fancy new dynamical mechanism (like the pseudotemporal processes you mention), we just take note of the global constraints, and our particular perspective, and take these into account when enumerating our theory describing the phenomena. In particular, our spatiotemporally embedded perspective is going to mean that we have epistemic access only to initial boundary conditions and so we’re going to be providing some sort of probabilistic story based on these and in the absence of any final boundary conditions.

    That this story is dynamical is a feature of how we tell it, not as a result of some objective “dynamicism” in reality. And I think this is the most fruitful way to understand retrocausality.

    And just a last point on fine-tuning. Faithfulness is an assumption that statistical independences shouldn’t hide causal dependences—and if they did, then they would have to be fine-tuned to do so. Thus the fine-tuning under consideration is of the causal mechanisms relating variables such that they hide the statistical dependence that we would expect to see from such mechanisms. I think this might be slightly different from some claim that rules out ad hoc causal mechanisms.

    #2679
    AvatarPeter Evans
    Member

    Hi David,

    Cheers for having a look at the paper!

    The intermediate settings in Section 4.2 are supposed to be the alternate setting from whatever the final setting is set at, in that run of the experiment (and so other runs are not relevant to the calculation). This then gives probabilities for the actual measurement pairs matching EPRB probabilities. (Since the intermediate measurements are summed over, it’s not like they’re actually measured.)

    Theoretically, they’re chosen as those settings simply because, as a result, the amplitudes cancel to give the correct joint probabilities. More practically (this is the original idea that led Gerard to think of the model), under the right circumstances, one could tune the intermediate measurement to be (i) no measurement at all (normal EPRB correlations), (ii) a projective measurement (destroying the correlations) or (iii) a partial measurement with correlations somewhere between these extremes. This would then allow an experimenter to tune between quantum and classical phenomena.

    On your more general point, yes—this is exactly the sort of thing I’m thinking following on from this. I think there’s scope to point out some simple SEPRB-type cases that involve fine-tuning (one very simple system features in a pre-print of Huw and Ken’s that Huw kindly showed me) and then argue that they are, rather counter-intuitively, subject to Wood and Spekkens’ analysis too.

    Cheers,

    Pete

    #2677
    AvatarPeter Evans
    Member

    Hi Ken,

    I’ll try to respond to your comments in the paragraph order you list them.

    1. Agreed. In the source of the internal cancelling paths idea, a paper by Paul Näger, it is a quirk of the causal relationships that provides the cancelling. The model that I present, originating from Gerard Milburn, employs no such quirk, and the internal cancelling paths arises from restricting the possible amplitudes that contribute to the final joint probability. Insofar as you say, “imposing symmetries is itself a sort of fine tuning, in that a large parameter space is restricted to a special (symmetrical) subset”, I think that these are catching on to the very same idea (though obviously with different detail).

    (In fact, putting it this way could provide a more general statement of the conditions under which an internal cancelling paths mechanism could work retrocausally.)

    2. I think “grab-bag” is the right sort of phrase to describe the mess I left at the end of the paper! The main thrust of that section is that the analyses of Hausman and Woodward are essentially the same, and the view therein is amenable to being interpreted along perspectival lines. I’m hoping that this can be an illuminating exercise for understanding how to tell some sort of dynamical story about the dependence relations (a different story for each perspective)—this is related to the point I was making in reply to Mark above—and to illustrate that the right dynamical story may not be representable in an orthodox causal model.

    3/4/5. Yes, sketchy it is. This issue also relates a little to the appropriateness of the instrument of causal modelling. Causal modelling requires distinct variables with distinct causal mechanisms relating them. Of course, it’s partly up to us as modellers to choose the most suitable variables, etc, and explicate the relations between them. I take your question to be, why can’t we describe the relation as mutual causation rather than some more “holistic” story? Well, I would like to think that that is the way I’m describing it. But my “grab-bag” obviously isn’t doing that job very well! Point taken.

    I think your laser example is great—in fact, as I re-read it a couple of times it really does capture a lot of what I want to say about the EPRB case. This might provide a robust analogy… let me consider it some more.

    6. In what sense do you mean spacetime-local? Am I right in assuming you mean Lorentz invariant or, equivalently, action-by-contact? Hmmm… perhaps you could say a bit more please about this problem?

    Cheers,

    Pete

    #2675
    AvatarPeter Evans
    Member

    Thanks, Mark – appreciate the input! A couple of comments…

    On the footnote comment, I don’t think that dynamical and adynamical explanation should be exclusive. There are plenty of cases in science where two different explanations can be provided for the one phenomenon. For example (I can’t remember where I first heard this; maybe Harvey Brown’s book, or Jim Woodward’s), a helium balloon held by a passenger in a plane that is taking off floats towards the front of the plane. One explanation is that the air in the cabin moves inertially to the rear of the plane, making the front less dense—balloon moves forward. Another explanation is that the plane is an accelerating reference frame, which can be modelled as a gravitational field with the source toward the rear—balloon moves forward. We have a formal relationship between the explanations embodied by Einstein’s equivalence principle.

    While the relationship between the 3D vs 4D cases of explanation the comment refers to are slightly different in form to this example, we have no reason to forgo our 3D explanations in favour of a 4D one (despite the beneficial elements of the 4D one). Keeping in mind that we humans are without exception dynamical beings (we experience the world dynamically in time), it seems disingenuous to deny the possibility of a causal (dynamical) explanation.

    Keep in mind, also, that anything one says about global constraints and retrocausation must also be said about ordinary causation. Seeing as we’ve been able to produce very successful (albeit incomplete, according to this forum) dynamical theories modelling the world, and we regularly employ causal/interventionist concepts as we explore the world, if the world turned out to obey global constraints as per RBW, we should be able to provide a story joining the two explanations together. If we could do that for ordinary causation, there seems to be no reason it wouldn’t work for retrocausation.

    Thus there’s no reason why retrocausality, dynamics or interventions should be “superfluous”. Yes, a properly retrocausal picture, like Ken’s Lagrangian framework and your RBW, is best represented as adynamical, but one would hope a dynamical picture can be extracted therefrom (of the sort that can give us causal and retrocausal explanations). Dynamics, causation, interventions, and so on look a bit different to what we would usually think (and it is an interesting, and largely incomplete, task to spell this out), but they are not, by my lights, inconsistent with the adynamical picture.

    I’m just missing the connection, though, between this footnote comment and the issue of faithfulness. Could you please give a bit more detail for how these relate?

    Cheers,

    Pete

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