# Retrocausality in action

Discussion in 'Physics & Math' started by RJBeery, Apr 24, 2012.

1. ### przyksquishyValued Senior Member

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3,017
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:
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 $\Phi^{+}$ a quarter of the time, $\Phi^{-}$ a quarter of the time, and an inconclusive result half the time. On the next page, they say:
Here they say that their correlation function for the entangling measurement cases is also conditioned on Victor getting the $\Phi^{-}$ result.

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.

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: May 2, 2012

3. ### brucepValued Senior Member

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3,349
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: May 2, 2012