On Wed, Dec 22, 2021 at 12:53 AM smitra <[email protected]> wrote: > On 21-12-2021 07:12, Bruce Kellett wrote: > > On Tue, Dec 21, 2021 at 4:40 PM Jesse Mazer <[email protected]> > > wrote: > > > > I wasn't linking to the paper for the argument about semantics (there > > doesn't seem to be any agreed-upon definition of 'realism' distinct > > from local realism in physics, from what I've seen) but rather for the > > toy model they provide in section 5 with the experimenters being > > duplicated when they try to measure the entangled particle. The point > > is that Alice is locally duplicated when she measures her particle, > > and Bob is locally duplicated when he measures his, but there is no > > need for the universe to decide which copy of Bob inhabits the same > > "world" as a given copy of Alice, or vice versa, until there's been > > time for signals limited by the speed of light to pass between them > > (or to a third observer). This is not the sort of "local realist" > > theory that Bell was trying to refute (one of the implicit assumptions > > in his derivation was that each spin measurement produces exactly one > > of two possible outcomes), but the dynamics of such splitting can be > > perfectly local, and it can still be true that if you randomly select > > one of the copies of an observer in a Bell type experiment, the > > probabilities that your randomly selected copy will see various > > outcomes can be made to match the QM predictions that violate Bell > > inequalities. > > > > This seems to be the hand-waving way in which this is usually argued. > > I was asking for something a little more concrete. > > > > There is a fairly simple argument that shows that many worlds ideas > > can have no role to play in the violation of the Bell inequalities. In > > other words, there is an indirect no-go theorem for the idea that MWI > > makes these experiments completely local. > > > > The argument goes like this. Take Alice and Bob measuring spin states > > on members of entangled pairs of particles -- they are presumed to be > > distant from each other, and independent. Alice, say, measures a > > sequence of particles at random polarizer orientations, randomizing > > the polarizer angle between measurements. She records her results (up > > or down) in a lab book. After N such pairs have been measured, her lab > > book contains a sequence of N 0s or 1s (for up/down), with a record of > > the relevant polarizer angle for each measurement. If MWI is correct, > > there are 2^N copies of Alice, each with a lab book containing a > > similar binary sequence. Over the 2^N copies of Alice, all possible > > binary sequences are covered. Bob does the same, so he has a lab book > > with some binary sequence of 0s and 1s (and 2^N copies with different > > lab books). For each copy of Bob, and each lab book, all N > > measurements were necessarily made in the same world (because > > individuals cannot move between worlds). > > > > After all measurements are complete, Alice and Bob meet and compare > > their lab books in order to calculate the correlations between results > > for different relative measurement angles. Once Alice and Bob meet, > > they are necessarily in the same world. And since they carry their lab > > books with them, the measurements made in each lab book must all have > > been made in that same, single, world. The correlations that Alice and > > Bob calculate are shown to violate the Bell inequality. (That is > > experimentally verified). But this violation of the inequality takes > > place in just one world, as has been seen by the above construction. > > The alternative copies of Alice and Bob also meet to compare results. > > As before, all these meetings take place in the same worlds as all the > > relevant measurements were made. Consequently, the many-worlds > > analysis for each Alice-Bob pair is exactly the same as the single > > world analysis obtained if collapse is assumed. Many-worlds adds > > nothing to the analysis, so MWI cannot give any alternative > > explanation of the correlations. In particular, MWI cannot give a > > local account. > > > > Bruce > > It is the violation of the Bell inequality in each world that is the > evidence of the existence of the other worlds.
Huh????? The problem is with > comparing with collapse hypothesis and then saying that there is no > difference. If there is no difference, where is the problem? But the whole problem is that when Alice makes her > measurement that she gains some amount of information about what Bob is > going to find, even though they are spacelike separated. In general, that is not true. When both Alice and Bob set their polarizers randomly while the particles are in flight, the fact that Alice might get |up> tells her nothing about what Bob will get at some randomly different polarizer orientation. You seem to be stuck with thinking in terms of parallel polarizer orientations. In the MWI > there is no such mysterious gain of information due to the correlation > being caused by common cause when the entangled pair is created > Rubbish. If there were a common cause, then that would have to depend on the final polarizer orientations. And those are not known at the time of creation of the entangled pair. You are, then, back with some non-local influence (or retro-causation). 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/CAFxXSLTSjRuVO1hniau2n0WBGUBo7pGp0subA-SnLst-bwddKQ%40mail.gmail.com.
