On Tue, Oct 1, 2019 at 3:37 PM 'Brent Meeker' via Everything List < [email protected]> wrote:
> On 9/30/2019 9:52 PM, Bruce Kellett wrote: > > > > I haven't read Carroll's book, it isn't released in Australia until > > November. I would be interested to see if he has a better account of > > Bell non-locality than Wallace. About the spread of "splitting": > > decoherence is a local physical interaction -- photons interacting > > with walls and the like. This clearly spreads at the speed of light > > (or less). But splitting is not really just decoherence. The trouble > > with Bell non-locality is that the splitting of worlds is not a > > physical interaction like decoherence. Bruno and others speak about a > > "spread of entanglement" as being associated with the splitting. But > > again, entanglement is the result of physical interaction, and the > > interaction of looking at a pointer to see a result is not really an > > entanglement interaction. I think that there is a lot of loose > > thinking about this "splitting" process. > > > > The absence of disallowed branches (Alice and Bob both seeing spin-up > > with aligned polarisers) is not a matter of worlds (branches) > > cancelling by destructive interference, because there is no > > interaction -- the light carrying information from the space like > > separated measurements is not coherent, so it can't interfere. If it > > were coherent, allowing interference, then that coherence itself would > > indicate non-locality. > > But the copying of information as to the measurement result, quantum > Darwinism, is a physical interaction that writes the information into > the environment. So that we can imagine that both UP and DOWN > information spreads from Alice and also separately from Bob. Where they > overlap in the future they must correlate per QM. Why can't we suppose > that the inconsistent worlds cancel out. You can suppose it. But how does it happen? You can't actually write down the wave function of all the particles involved in the spreading of the information. > You say the light carrying the > information isn't coherent, but it's not just the light that carries the > information; it's information encoded in the wave function of the > environment. That information encoded in the wave function of the environment has to be coherent if the impossible branches are to cancel out. And that coherence must originate in the measurement interactions that Alice and Bob perform. And that is non-local, since Bob and Alice are space like separated. Telling a story like you are telling does not remove the non-locality -- it just makes it more mysterious. I need an actual local physical account in order to be satisfied that there is no non-locality. In other words, what happens when the future light cones from Alice and Bob overlap? The information of the separate results may be present only in the form of light emanating from the original pointers. So no small part of the environment (like the light) is > going to appear coherent, but it's still going to be inconsistent with > the opposite result and zero out cross terms in the density matrix. > That's essentially what the mathematical process of taking the reduced > trace does. > I don't see any connection with reduced traces. The reduced trace simply blots out the coherence information disseminated into the environment. It converts the pure state to a mixture. As will happen whenever you consider only part of the system and ignore (trace out) everything else. Bruce -- You received this message because you are subscribed to the Google Groups "Everything List" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To view this discussion on the web visit https://groups.google.com/d/msgid/everything-list/CAFxXSLSeXr%3DcBeWuaRaLusZ4jyPpBJ%2Bzu%3DnGEMjn0QyXuszKFQ%40mail.gmail.com.
