On Tue, Dec 21, 2021 at 4:40 PM Jesse Mazer <[email protected]> wrote:
> On Mon, Dec 20, 2021 at 8:10 PM Bruce Kellett <[email protected]> > wrote: > >> On Tue, Dec 21, 2021 at 11:53 AM Jesse Mazer <[email protected]> >> wrote: >> >>> >>> But one of the big selling points of the MWI is to give some sort of >>> objective picture of reality in which "measurements" have no distinguished >>> role, but are simply treated using the usual rules of quantum interactions. >>> >> >> At one time, that might have been a point on which to prefer MWI over >> Bohr's version of the CI, but that is no longer true. Modern collapse >> theories do not have to distinguish particular "measurement" events, and do >> not have to assume a classical superstructure . In modern fGRW, for >> example, everything can be treated as quantum, and the theory is completely >> objective. >> >> fGRW has the added advantage that it is an inherently stochastic theory. >> Probability is treated as a primitive notion that is not based on >> anything else. MWI struggles with the concept of probability, and while it >> has to reject a frequentist basis for probability, it cannot really supply >> anything else. Self-locating uncertainty does not, in itself, serve to >> define probability. You have to have some notion of a random selection from >> a set, and that is not available in either the Schrodinger equation or in >> self-locating uncertainty. >> > > What does fGRW stand for? > It is short for Flash-GRW, in which the random collapse interactions of GRW are replaced by "flashes". The point here is that this formulation is Lorentz invariant and completely relativistic. If it's stochastic, do you mean it's one of those theories that involves > stochastic spontaneous collapse? Such theories are usually in principle > experimentally distinguishable from QM, would that be true of this theory > as well? > In principle this collapse model is distinguishable from no-collapse models. The experiments to detect this might be outside current capabilities. If you have to say "OK, I believe in the MWI plus Born rule for >>> measurements" with there being no dynamical definition of what qualifies as >>> a measurement, where the moments we call 'measurements' are just something >>> we feed into the theory on a know-it-when-I-see-it basis, then this claim >>> to objectivity is lost and it's not clear what theoretical appeal it has >>> over the Copenhagen interpretation. >>> >>> Personally I still lean towards some version of the MWI being true >>> mainly because you can come up with a toy model with MWI-style splitting >>> that deals with Bell style experiments in a way that preserves locality >>> >> >> No you can't. >> >>> but doesn't require hidden variables (see >>> https://www.mdpi.com/1099-4300/21/1/87/htm ) but I see it as a sort of >>> work in progress rather than a complete interpretation. >>> >> >> They set up a contrast between realism and locality. >> > > 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 > As I said this can only be shown clearly in a toy model like the one in > section 5 of that paper, but a number of physicists including David Deutsch > do think that the full MWI would also respect a principle of "local > splitting", although even if this can be shown in terms of the quantum > formalism we still have the problem of deriving probabilities. The article > on the MWI by Lev Vaidman at > https://plato.stanford.edu/entries/qm-manyworlds/ discusses work on the > notion of local splitting in the MWI: > > 'Deutsch 2012 claims to provide an alternative vindication of quantum > locality using a quantum information framework. This approach started with > Deutsch and Hayden 2000 analyzing the flow of quantum information using the > Heisenberg picture. After discussions by Rubin 2001 and Deutsch 2002, > Hewitt-Horsman and Vedral 2007 analyzed the uniqueness of the physical > picture of the information flow. Timpson 2005 and Wallace and Timpson 2007 > questioned the locality demonstration in this approach and the meaning of > the locality claim was clarified in Deutsch 2012. Rubin 2011 suggested that > this approach might provide a simpler route toward generalization of the > MWI of quantum mechanics to the MWI of field theory. Recent works > Raymond-Robichaud 2020, Kuypers and Deutsch 2021, Bédard 2021a, clarified > the meaning of the Deutsch and Hayden proposal as an alternative local MWI > which not only lacks action at a distance, but provides a set of local > descriptions which completely describes the whole physical Universe. > However, there is a complexity price. Bédard 2021b argues that “the > descriptor of a single qubit has larger dimensionality than the Schrödinger > state of the whole network or of the Universe!”' > I did suggest that you made the argument yourself rather than giving a long list of references. B. -- 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/CAFxXSLSo_am0G4ncg2FqCNeULXNR2AdwqLsf%2B0tnUQwuVmbC5Q%40mail.gmail.com.
