Joao Leao: > The association between non-locality and "retrocausality" > (for lack of a better word) is anything but simple! In any > case it has less to do with the flow of time than with its > negation! [...]
Bell's theorem shows that, given the hidden variable lambda, the result of the experiment at B is dependent on the angle of the measurement at A, *or* the the result of experiment at A is dependent on the angle of the measurement at B, *or* both. Now, because of symmetry, it must be both. Thus, if there are "retrocausations" (or "influences", or "weak signals" as Ian Percival calls them) they are in both directions (and with the same probabilities). So yes, it is difficult to show that the flow of time is involved. Antoine Suarez (and the Geneva Group) speaks of a-temporal quantum effects. Now let us imagine this set-up. I suppose it can be useful also within the MWI, at least as a possible answer to the question "If we live in all of them can we pick the cheapest one?". So I go on trying to describe this gedanken experiment (or perhaps lunacy). There is the usual SPDC source, two correlated photons, mirrors m1 and m2, one human observer, polarization detectors (measuring photon-1) and, very close, 4 boxes to collect photon-2. Of course the path of photon-1 is shorter that the path of photon-2, so there is a time-delay, for photon-2 going into one of those boxes (possible delayed choice here?). m1 /----------<-----------<--source-->------>- detectors | | | \------------------>----------------->----- boxes (1,2,3,4) m2 Now the observer can measure, with his detectors, or the linear polarization of his photon-1, or the circular polarization of his photon-1. Of course the observer, having measured his photon-1, can predict what is the polarization state of photon-2. There are 4 possibilities: linear/x, linear/y, circular/+, circular/-. Being very short the distance between detectors and boxes, the observer has time (due to that time-delay) to move there and pick up the right box (that one with the right label: linear/x, linear/y, circular/+, circular/-) and collect, into the right box, the photon-2 which is arriving. This is possible because he *knows* what was his *choice* while measuring, with detectors, the polarization state (linear *or* circular) of photon-1. And he also *knows* what was the measurement outcome for photon-1: i.e. linear/x, or circular/+, or ... This is also possible because the observer has *time* to move to the other location and pick up the right box, to collect photon-2. But before observer makes his *choice* the photons (and especially photon-2, which is "late") were already flying. So you could ask: what was the polarization state of photon-2, before the observer made his choice measuring, with his detectors, the polarization state of photon-1? The answer seems to be that photon-2 fits equally well in both categories, that is to say: linear polarization and circular polarization. Thus neither of these properties can be ascribed to it as an objective property. Now you can also ask: what if I cut the path lenght of photon-2 and I make it equal to the path lenght of photon-1? It happens that the observer becomes unable to move from the detectors location to the boxes location, because there is no time-delay now. So, in these conmditions, the observer, loses control of the situation. His information remains hidden, or useless, ot impossible. But this, imo, does not mean that photons gain some objectiveness. Or not? Of course you excluded the possibility of (weak or strong) signals traveling FTL, from detectors or from photon-1 to photon-2. In example making the path lenght of photon-2 much much longer than the coherence lenght of the photon(s). But imagine that your procedure (here above) is not enough, and actually there is some FTL effect. The interesting point here is that any FTL effect from detectors or photon-1 makes actual, objective the state of photon-2 *before* its measurement.
