The discussion centers around Tim Maudlin's views on Bell's theorem and quantum mechanics (QM), particularly examining the implications of entanglement and locality. Participants explore interpretations of QM, the nature of randomness, and the philosophical underpinnings of Maudlin's arguments.
Participants do not reach a consensus on Maudlin's views. There are multiple competing interpretations of quantum mechanics and differing opinions on the implications of entanglement and locality.
Some participants highlight the need for caveats in discussions about QM, particularly when addressing complex topics like entanglement and locality. There are references to the limitations of the article being discussed, including its accessibility and the nature of its audience.
Yes, but my question is about non-local realism.Morbert said:Think of the Us and Ds as pre-existing properties, revealed by different measurements on individual particles. Local realism says it should be possible to write down an arrangement of Us and Ds that fully specify the outcomes of all possible measurement choices. So e.g. the dotted line represents a choice of the observable ##\sigma^A_x\sigma^B_x\sigma^C_z## and should yield an eigenvalue +1 (the product of the individual measurement results: e.g. UUU = 1*1*1 = 1, UDD = 1*-1*-1 = 1 etc). Other such observables can be constructed by following the appropriate solid lines.
It is not actually possible to write down an arrangement that satisfies all measurement choices. So we cannot write down
Oh oops misread your original post.martinbn said:Yes, but my question is about non-local realism.
So they have some U or D value along some direction and when a measurement is made the values change to what they should be?Morbert said:With non-local realism, there is no need to specify an arrangement of Us and Ds everywhere because a choice of measurement at one site can immediately influence values at other sites. If it were possible to write down all Us and Ds that satisfy quantum predictions, there would be no need to suppose this immediate influencing.
I'm not sure how useful the diagram is for representing the non-local case. Its primary purpose is in representing the failure of the local case. I would imaging a nonlocal realist interpretation would have these values changing to what they should be, but maybe not directly. You would have to ask one of the Bohmian representatives here.martinbn said:So they have some U or D value along some direction and when a measurement is made the values change to what they should be?
The reason i asked was because i dont see the role of the locality. It seems that this shows that realism without any additional assumptions is impossible. I dont think BM is realistic about this observable, only about the position observables.Morbert said:I'm not sure how useful the diagram is for representing the non-local case. Its primary purpose is in representing the failure of the local case. I would imaging a nonlocal realist interpretation would have these values changing to what they should be, but maybe not directly. You would have to ask one of the Bohmian representatives here.
vanhees71 said:In this interpretation there is no causal explanation, why a specific measurement outcome when an observable is measured that's not determined due to the state preparation occurs, but it's assumed to be "objectively random" with probabilities given by Born's rule. E.g., if you have a polarization-entangled photon pair and you measure the polarization of one of these photons (which is exactly unpolarized in this case) by e.g., using a polarization filter there's no causal explanation whether a specific photon goes through the filter or not. All you can say is that with probability 1/2 it goes through, with probability 1/2 it doesn't. There's no cause for the one or the other outcome for any specific photon prepared in such a state.
vanhees71 said:Nevertheless the correlations are due to the preparation of the initial state. In QT these correlations between the outcomes of measurments of the single-photon polarizations are there despite the fact that these single-photon polarizations are maximally undetermined when the photons are prepared in the said entangled state (in the here discussed example a GHZ three-photon state).