On Tue, Apr 26, 2016 at 6:45 AM, Bruce Kellett <bhkell...@optusnet.com.au> wrote: > > > You think that "the state of the other particle" refers to the quantum > state that would be assigned to B given only knowledge of the state of A > (as well as knowledge of how they were entangled originally). Actually, > that is the interpretation I gave the words, except I teased out what that > actually meant. From the entangled state, given A's state (result, say > |+>), you would assign a state |-> to B. But this is wrong for spacelike > separations -- the state B actually measures is exactly the same as the > state A measured: |psi> = (|+>|-> - |->|+>)/sqrt(2). >
You use the full state if you just want to generate the total probabilities for various possible *joint* outcomes. But if you want a conditional probability of various outcomes *just for B* given knowledge of what measurement A got, this can be done in QM, in the Schroedinger picture you could project |psi> onto on eigenstate that corresponds to whatever definite outcome was measured on A, resulting in a different state vector for the combined system |psi'> which may lead to different probabilities of getting various results for B, but which does not assume any knowledge of what measurement was actually performed on B. I assume something similar is possible in the Heisenberg picture which Rubin is using, so I was speculating that he meant something like this when he talked about a label on one particle which says something about the state of the other particle. There is also another possibility along the same lines, which is that for any entangled system in a pure state, you can construct a "reduced density matrix" for some subsystem, which gives the probabilities of various outcomes for measurements just on the subsystem alone (and the subsystem could just be one particle in a multiparticle entangled system). This is important in the analysis of decoherence, for example, where the approach apparently involves treating both the subsystem and its environment as being in a pure state, and then looking at how interactions between subsystem and environment change the reduced density matrix for the subsystem. That is clearly wrong, so the details are irrelevant. If you think like a > physicist, rather than as a mathematician, you look for the physics of what > a paper is saying. > > It isn't obviously wrong in my interpretation above, and I think it's wrongheaded to imagine you can be confident about the interpretation of any verbal statement by a physicist if you don't have a detailed grasp on the mathematics of the model the physicist is talking about--if you don't you may miss possible interpretations, like the ones above that you don't seem to have considered. Also, do you plan to respond to the rest of my comment? In particular, do you think you can come up with any simple numerical examples that show a local-copies-with-matching model can't correctly reproduce some observed statistics at a given location if we assume that location has been "shielded" from any physical influences from Alice or Bob (and assuming 'matching' between copies of Alice and copies of Bob can only be done in regions that have received measurable physical signals from them), as you seemed to claim earlier? Jesse -- 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 firstname.lastname@example.org. Visit this group at https://groups.google.com/group/everything-list. For more options, visit https://groups.google.com/d/optout.