Blue Scallop said:
vanhees71.. may I know what is your answer to the above question by name123?
I'm kinda confused about your position.. do you believe that in entanglement experiments like Clauser and Aspect experiments.. it's due to common causes like red and blue socks being determined from the origin? But in the experiments that violate Bell's Inequality, there were correlations. Or maybe you believed there were correlations but it was not "non-local" because there is no reality.. hence there is nothing to be local about (so spooky at distance is false)? (if so, then this is reasonable). Or do you indeed believe Bell's Inequality is violated due to some loophole that still proves it's like red and blue socks from the initial preparation (this was actually believed by Einstein (the EPR arguments) but disproven by Bell's Experiments that showed there were really correlations even if the entangled particles were light years apart).
This is a very good example for why I thik that quantum states are better interpreted in an epistemic sense and why the collapse hypothesis leads to problems with locality and causality.
My view is that the correlations, as all probabilistic relations of quantum systems, are described by the state of the system, and the state of the system is determined by preparation. The preparation in that case is when the entangled photon pair is created (e.g., by parametric downconversion by shooting a laser beam into a crystal). When A measures her photon's polarization state (her photon is defined by that it is registered by the detector at A's place), she immediately also knows the polarization state of B's photon (his photon is defined by that it is registered by the detector at B's place, which can be very far distant from A's detector). For B nothing has changed. He simply expects an unpolarized photon and gets with 50% probability the one or the other polarization when he measures it.
Let's now assume that A's detector is very close to the photon source, and B's very far, such that A measures her photon earlier than B. In other words, the measurement processes are assumed to happen as time-like separated events. Then "collapse" happens definitely at different times for A than for B: A changes the state of the photon pair due to her measurement result much earlier than B. Still, there is no contradiction by what's known to A and B concerning the outcome of their mesaurements. Both A's and B's photons are exactly unpolarized, i.e., the polarization state if maximally indetermined.
There's, of course, also no problem when the measurement processes are realized at space-like distances. Then you can always find a reference frame, where A and B register their result simultaneously or another reference frame, where A registers her result before B or again another frame, where B registers his result before A. Still there's no contradiction, because both, A and B always just find that their photons sent from the source of entangled photon pairs are precisely unpolarized.
When A and B compare their measurement protocols (always keeping detailed track about the time, when they registered their measurement outcome to be sure to relate always the pairs which where created together at the source), they find in any case the 100% correlations due to the preparation of the photon pairs in the polarization-entangled state.
Of course, here I made two assumptions: (a) the polarization measurements are local events as described by standard QED and thus the linked-cluster principle is valid, i.e., A's measurement cannot instantaneously affect B's photon and/or measurement apparatus (implied by microcausality) and (b) that all there is possible to be known about photons is what is described by quantum states, and since this is probabilistic knowledge (some may think only) it refers to ensembles of equally prepared quantum systems, i.e., the probabilistic information described by the prepared state can only be tested by collecting "enough statistics", i.e., using a sufficiently large ensemble.
The problems start, whenever you try to give more meaning to the quantum state then is implied by this minimal interpretation. Some think (in the past Einstein and Schrödinger were the most prominent physicists to do so) that this is not a complete description of nature since "in reality" (whatever "reality" means to them) all possible observables should have determined values always. It's not completely ruled out that maybe somebody one day finds some satisfactory theory, where this is the case, but Bell's work and the empirical precise findings with respect to it, imply that such a deterministic hidden-variable theory must be non-local, and so far there seems not to be a satisfactory such kind of theory in the relativistic realm.