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Quantum Perspectivalism by D.Dieks
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[QUOTE="DrChinese, post: 6837542, member: 323"] 1. The 1 & 4 photons end up entangled if and only if a successful BSM occurs. That is not post selection, and I can explain exactly why that is so: a. There is no question that when the original biphotons (systems consisting of an entangled photon pair) are created (i.e. pairs 1 & 2 and 3 & 4), they are maximally entangled. They will of course exhibit perfect correlations and violate Bell inequalities. They cannot be entangled with any other object due to monogamy of entanglement. b. There is no question that when the final biphoton is created (i.e. pair 1 & 4), it is also maximally entangled. The pair will of course exhibit perfect correlations and violate Bell inequalities. The photon pair cannot be entangled with any other object due to monogamy of entanglement. c. The "cause" of this change is the BSM, and nothing else. There can be no post-selection issue because there is one and only one point of interaction between the initial biphotons, and that is at the BSM. There is no statistical connection from the past of the 1 & 2 and 3 & 4 pairs that can connect to the subsequent statistics of the final 1 & 4 pair - precisely because of the monogamy of entanglement. So there is nothing to see in the way of post selection. The BSM is the crossover point from 2 biphotons down to 1 biphoton. If and only if the 2 & 3 photons click within a small coincidence window - such that they cannot be distinguished as to source - will the BSM succeed. They must be indistinguishable. d. And if there were only post-selection on the 2 & 3 photons - and not some kind of quantum interaction - you should be able to simply post-select on the identical quantum attributes - such as polarization, arrival time, transmit or reflect through the Beam Splitter, wavelength, etc. - without allowing any interaction. The 2 & 3 photons would pass the post-selection tests... but would be distinguishable. Obviously, the BSM fails and the 1 & 4 photons do not pass the Bell test. What kind of post-selection requires interaction? Either you are post-selecting on some list of criteria, or not.2. I never say there is "retrocausality". I have no idea if there is or isn't. What I can say is that there is NO observable change in outcomes whether the BSM occurs before or after Alice (and/or Bob) makes a measurement on the 1 photon (or 4 photon for Bob). Ordering changes nothing in the measurement context. (That is therefore the argument for asserting that QM is acausal.) All I know is that Einsteinian causality fails. *You* might see this as a claim to retrocausality, because you assume there is causality in nature. What is generally agreed upon is that there is NO possibility of explaining the random outcome of a quantum interaction. There is nothing in the past context to explain it, and nothing in the future that explains it either. Again, what I can say is that in a Bell test: only Alice and Bob's relative settings (the measurement context) are relevant to the statistical outcomes.Of course, we have again somehow deviated from the thread topic. :smile: [/QUOTE]
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