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Thread: Retrocausality in action

  1. #41
    Quote Originally Posted by RJBeery View Post
    This makes it clear that Alice and Bob should be able to compare their measurements and decide whether or not their respective photons are entangled or separable with no information from Victor; they should be able to deduce Victor's choice of whether or not to entangle the photon pairs.
    I can see why you might get that impression from the text you quoted, but that text comes from the introduction of the paper. Details given later in the article, not to mention the actual quantum theory the experiment is based on, contradict your interpretation.

    Specifically: if you work in this retrocausal "picture" of the experiment, then an important detail is that when Victor performs an entangling measurement, he effectively retroactively projects the state shared by Alice and Bob randomly onto one of four different entangled states, only one of which Alice and Bob are actually interested in. So to see correlations they need to know which of the photons they receive are in the entangled state they're interested in, and they can only get that information from Victor.

    This kind of detail is clear if you're already familiar with the theory behind this sort of experiment, but it is also stated in a few places in the paper. For instance, near the beginning of page 4:
    When used for a BSM, our BiSA can project onto two of the four Bell states, namely onto (both detectors in b'' firing or both detectors in c'' firing) and (one photon in b'' and one in c'' with the same polarization).
    Here they explain that their implementation of the Bell state (entangling) measurement only distinguishes two of the four Bell states. So their Bell state measurement returns a quarter of the time, a quarter of the time, and an inconclusive result half the time. On the next page, they say:
    For each pair of photons 1&4, we record the chosen measurement configurations and the 4-fold coincidence detection events. All raw data are sorted into four subensembles in real time according to Victor’s choice and measurement results. After all the data had been taken, we calculated the polarization correlation function of photons 1 and 4. It is derived from their coincidence counts of photons 1 and 4 conditional on projecting photons 2 and 3 to when the Bell-state measurement was performed, and to or when the separable state measurement was performed.
    Here they say that their correlation function for the entangling measurement cases is also conditioned on Victor getting the result.

    Let me ask it another way, putting Victor's choice ahead of Alice and Bob's measurements: if Victor produces a stream of entangled photon pairs and sends one each to Alice and Bob, you agree that they should be able to measure them, compare their results, and after sufficient iterations determine whether or not the stream is correlated WITHOUT asking Victor, yes?
    That depends. If Victor always sends the same entangled state, then yes. If Victor sends a random stream of the four different entangled Bell states, then no. They just see completely uncorrelated results in that case, and they need information from Victor to filter out the three entangled states they're not interested in.

    That's the whole point of Bell's experiment!
    This is an aside, but the experiment described in the paper doesn't actually perform a Bell test (though they could have, and there's not much doubt about the result they'd have obtained if they had).
    Last edited by przyk; 05-02-12 at 02:27 PM.

  2. #42
    Valued Senior Member brucep's Avatar
    Quote Originally Posted by przyk View Post
    What "traditional definition" are you working with? MWI is deterministic in the sense that if you know the initial quantum state of the entire universe (or an isolated subsystem), and you know everything there is to know about the interactions and evolution taking place, then the Schrödinger equation predicts a unique future state at any future time. But individual observers generally won't have access to complete information about that state for various reasons.

    Huh? The branching that would occur according to an MWI account of the experiment you cited in the OP is probably quite a bit more complicated than you imagine and it doesn't happen all at once. Assuming "free will" (for simplicity, not by necessity), then the global quantum state generally splits into a number of branches corresponding to the number of measurement outcomes for every measurement performed. So if we just look at one iteration of the experiment, then by the time Alice and Bob have performed their measurements the global state has split into four branches (because Alice and Bob each have two outcomes), and when Victor performs his measurement (which ideally has four outcomes), the global quantum state further splits into a total of sixteen branches. Each time, exactly which branching happens depends on the measurements that Alice, Bob, and Victor choose to perform.

    If you drop the "free will" assumption and you imagine modeling Alice, Bob, and Victor as physical systems, then there's additional branching depending on the number of decisions they could make. For example, as you say, in the experiment Victor was a quantum random number generator with two possible outcomes, and in a more complete MWI description, you'd consider that the measurements Victor performs are correlated with this outcome, which doubles the number of branches. In the experiment, Alice and Bob each chose between 3 different measurements they could perform, so if you also think of Alice and Bob as quantum random number generators, that's a total of 16 x 2 x 3 x 3 = 288 branches. That's 288 branches per iteration (generation of four photons) and assuming Alice, Bob, and Victor are no more complicated than two- or three-outcome quantum random number generators.

    With all that said, if these responses don't make too much sense to you, I wouldn't worry about it too much. I've never really considered determinism a selling point of MWI anyway.
    All your responses were great in my estimation. Thanks for linking the paper in the archive. If a QM interpretation makes the prediction that QM is deterministic, makes a different prediction than QM, or a new prediction, it's not an interpretation of QM. QM interpretations use determinism to create student friendly ways to interpret QM. The interpretations must make the exact same predictions as QM. The most interesting interpretation I've read, though the author doesn't claim it's a formal interpretation, was posted on a Internet physics forum by Mr_Homm.
    Last edited by brucep; 05-02-12 at 04:07 PM.

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