From: <agrayson2...@gmail.com <mailto:agrayson2...@gmail.com>>
On Sunday, August 19, 2018 at 11:36:47 AM UTC, Bruce wrote:From: *Bruno Marchal* <mar...@ulb.ac.be>Yes. My feeling is that you do introduce some preferred base.Yes, your feelings are very much at fault here. If you thought a bit rather than go with feelings, we might be better off. I do not introduce a preferred basis. Where do you think I do that? You argue against a straw man, as usual. A typical base is not a preferred base. *Since you could have chosen u - d and u + d as the basis, cannot u and d be considered a preferred basis? AG*
The (u-d), (u+d) base is just a 90 degree rotation of the u,d base. It could be relevant if you oriented your magnets that way rather than in the u,d direction. As usual, Bruno has muddied the waters here by introducing the idea that |u> and |d> form a preferred basis. That is not the case, because the usual "preferred basis problem" of the MWI concerns the stability of any particular basis under environmental decoherence. Here the basis is chosen according to the orientation of the magnets -- there is no idea of preference under decoherence involved; the only consideration is that the basis in the direction of the magnets is the one in which you can most easily make verifiable predictions. You could work in some other basis if you are so stupid, but that has no effect other than to complicate you calculations. It would not affect the physics.
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