On Saturday, April 21, 2018 at 7:27:33 PM UTC-4, Bruce wrote: > > From: smitra < <javascript:>[email protected] <javascript:>> > > On 20-04-2018 04:54, Brent Meeker wrote: > >> >> So a measurement on one can, assuming some conserved quantity >> entangling them, will have an effect on the other, even if the all the >> details of measurement and decoherence are included and the >> measurement is treated as Everett does. It still zeroes out cross >> terms in the density matrix that correspond ot violation of the >> conservation law and that entails changing the wave function at remote >> places. >> >> Brent >> > > That's then an artifact of invoking an effective collapse of the > wavefunction due to introducing the observer. The correlated two particle > state is either put in by hand or one has shown how it was created. In the > former case one is introducing non-local effects in an ad-hoc way in a > theory that only has local interactions, so there is then nothing to > explain in that case. In the latter case, the entangled state itself > results from the local dynamics, one can put ALice and Bob at far away > locations there and wait until the two particles arrive at their locations. > The way the state vectors of the entire system that now also includes the > state vectors of Alice and Bob themselves evolve, has no nontrivial > non-local effects in them at all. > > Saibal > > > I think the confusion arises from a failure to distinguish between 'local > interactions' and 'non-local quantum states'. In the entangled singlet case > we have a non-local state since it involves two particles that are > correlated by angular momentum conservation no matter how far apart they > are, or whether measurements on the separate particles are made at > time-like of space-like separations. No one has ever denied that the > interactions involved in the separate measurements on the two particles are > all local, or that decoherence effects that entangle the particles with > environmental degrees of freedom are all local, unitary interactions. > Decoherence leads to the effective diagonalization of the density matrix, > and the effective separation of copies of the experimenters that obtained > different results, but this effective collapse of the wave-function is > brought about by purely local interactions. > > The usual many-worlds argument for the absence of non-local effects points > to the fact that all the interactions involved in measurement and > decoherence are purely local to argue that there is no non-locality. But > this entirely misses the fact that the original singlet state: > > |psi> = (|+>|-> - |->|+>)/sqrt(2) > > is intrinsically non-local. It refers to correlations due to angular > momentum conservation that persist over arbitrary separations, and these > correlations are neither enhanced nor destroyed by any number of purely > local interactions. > > So many-worlds or many-minds interpretations of quantum theory do not > obviate the need for non-locality: they cannot, because the basic state > that is talked about in all interpretations is non-local. The point to be > made is that in no theory, either a collapse or a non-collapse theory, are > there any non-local interactions: all interactions in measurement and > decoherence are local. But that does not mean that what one does to one > particle of the singlet does not affect the other particle -- directly and > instantaneously. It is just that this effect is not instantiated by a local > (or non-local) hidden variable. There are no faster-than-light physical > transfers of information. That would involve a local hidden variable, and > there are none such. >
*So you have two subsystems that are "non-separable", yet the measurements of each are indeed, undeniably spatially separated. Is this a problem, or do we simply sweep it under the rug by saying there is no physical transfer of information? AG * > > The point is that quantum mechanics is weirder that you think in that it > is intrinsically non-local, even though all physical interactions are > necessarily local. Thinking of the 6 spatial dimensions of the separated > singlet particles as forming a single point in configuration space may help > one to visualize this. Alternatively, one can note that the tensor product > Hilbert space of the two spin states is independent of spatial separation. > > 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 post to this group, send email to [email protected]. Visit this group at https://groups.google.com/group/everything-list. For more options, visit https://groups.google.com/d/optout.
