Alan Harrison

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  • #2590

    One nice thing about a smooth transition between the states measured at two different times is that it looks the same for both directions of time–in keeping with the “Time-Symmetric Theories” theme.

    #2589

    Actually, upon reconsidering what I just wrote, it occurs to me that weak measurements may actually give some information about what happens between the two “strong” measurements at t1 and t2, contrary to what I just said. I’m not certain that that’s the case–I’ll have to think more about weak measurements–but if it is, then the following items appear to support my idea of a continuous transition instead of an instantaneous collapse:
    Nature 511:570-573 (31 July 2014) doi:10.1038/nature13559
    Nature 511:538-539 (31 July 2014) doi:10.1038/511538a [This one is a Nature commentary on the first article.]

    #2588

    Well, I don’t think so, but if I’ve misunderstood something I’d appreciate your enlightening me. The point is that if I make a measurement (or prepare a state) at time t1 and again at t2 > t1, with no intervening measurement, then I have no experimental evidence for *how* the system got into the state I observe at t2. The Copenhagen interpretation tells me that it collapsed in an instant, at t2, but if instead I assert (as I do!) that it changed smoothly during the interval (t1,t2), there is no experimental evidence that can prove me wrong. Therefore I conclude that (A) my explanation cannot be refuted by the experimental record and (B) in particular, if there’s any failure of conservation, it can’t be measured.

    #2566

    Hello Ruth et al.,

    Another way to answer Michael’s question, which I believe is equivalent to Ruth’s answer, is that not everything that is real is observable. The obvious example (from a realist perspective) is the wavefunction itself.

    #2561

    Hi Ken,

    I had originally been trying to devise a variational principle (VP) with the desired characteristics essentially as a mathematical exercise, deferring the question of how to justify it physically. The Lagrangian viewpoint tells me that I should use the extremization of the action as my VP, so my current approach is to start with a given Lagrangian and see if the resulting VP does what I want it to. I can find candidate Lagrangians in QFT, where interestingly enough the theory makes it natural to pursue the VP approach, but the conventional approach is to go in another direction altogether (interaction picture, path-integral methods and so on). So I’m trying to see what I can learn from that.
    I agree with your use of the relationship between joint and conditional probabilities to understand what happens in a measurement. But I’ve been hopeful that an improved theory might allow one to calculate more than just a joint probability P(A,B). If the theory doesn’t take us any further than that, we still need a collapse model of some sort to explain how nature picks a particular outcome, and I’d like to see a theory that resolves that issue as well.
    In the work I wrote up previously, I proposed that the phase of the wavefunction (or equivalently, the start time of a measurement) might serve as a hidden variable that controls the final choice among possible outcomes. (Of course, here I use “final” in a logical rather than temporal sense!) [It turns out that Pearle proposed the same idea in 1976 (Phys. Rev. D 13,857-868).] So my idea of prediction would ultimately include something like this, and my comment above alluded to the fact that I don’t understand this mechanism in detail yet. I’d like to find a promising VP first, and then return to this issue.

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