On Saturday, April 21, 2018 at 7:27:33 PM UTC-4, Bruce wrote: > > From: smitra < <javascript:>smi...@zonnet.nl <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. >
*You say it is "intrinsically non local", but isn't that because you are assuming instantaneous collapse? Wouldn't it be more accurate to say the singlet state is established as, or proven to be non local by the fact that in experiments Bell's inequality is violated? AG* > 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. > > 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 everything-list+unsubscr...@googlegroups.com. To post to this group, send email to everything-list@googlegroups.com. Visit this group at https://groups.google.com/group/everything-list. For more options, visit https://groups.google.com/d/optout.
