On 4/25/2018 6:57 AM, [email protected] wrote:
On Wednesday, April 25, 2018 at 10:51:13 AM UTC, Bruce wrote: From: *Bruno Marchal* <[email protected] <javascript:>>On 22 Apr 2018, at 01:47, Bruce Kellett <[email protected] <javascript:>> wrote: From: *smitra* <[email protected] <javascript:>>On 22-04-2018 00:18, Brent Meeker wrote: On 4/21/2018 12:42 PM, smitra wrote: 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. Sure it does. The state vector itself is a function of spacelike separate events, which cause it to evolve into orthogonal components...whose statistics violated Bell's inequality. Brent There is no non-locality implied here unless you assume that the dynamics as predicted by QM is the result of a local hidden variables theory. SaibalThere is no need to suggest local (or non-local) hidden variables. The non-locality we are talking about is implied by the quantum state itself -- nothing to do with the dynamics.But that type of non-locality has never been questioned, neither in the MWI, or a fortiori in QM+collapse. But the MWI explains without the need of “mysterious” influence-at-a-distance, which would be the case in the mono-universe theory, or in Bohm-De Broglie pilot wave theory. Without dynamic we have “only” d’Espagnat type of inseparability. BrunoIt seems that you are starting to see it from my perspective. Non-locality is just another way of emphasizing the non-separablity of the quantum singlet state. As you say, this is true in MWI as in collapse theories. In my extended development of the mathematics in another recent post, I demonstrated that there is actually no difference between MWI and CI in this regard. All that we have is the non-separability of the state, which means that a measurement on one particle affects the result of measurements on the other -- they are inseparable. This is all that non-locality means, and this is not changed by MWI. An awful lot of nonsense has been talked about this -- people trying to find a "mechanism" for the inseparability -- but that is not necessary. Quantum theory requires it, and it has been totally vindicated by experiment. That is the way things are, in one world or many. BruceYou place great faith in the singlet wf. But how can you legitimately treat the system quantum mechanically if you assume zero uncertainty in the total spin AM? AG
Zero spin is insured by conservation of angular momentum. There are limitations imposed on the measurement by the uncertainty principle as shown by the WAY theorem, but the constraint isn't of practical significance for typical laboratory measurement because the apparatus is so big (in action) compared to the variable measured:
In 1952 Wigner [2 <http://iopscience.iop.org/article/10.1088/1367-2630/15/1/013057/meta#nj454968bib02>] provided analysis that showed that in the presence of a conservation law it is impossible to perform an ideal measurement of an observable/L/_/S/ that does not commute with the conserved quantity. Specifically, Wigner showed that if one has an additive conservation law of some quantity$N_{\mathrm{tot}}= N_S\otimes \mathbbm{1} + \mathbbm{1} \otimes N_{\mathrm {A}}$ over the composite system (such as angular momentum or baryon number), and an observable/L/_/S/ for which [/L/_/S/ ,/N/_/S/ ] ≠ 0, then there cannot exist a von Neumann–Lüders measurement that respects the conservation law with$[V,N_{\mathrm{tot}}]=0$ . *Wigner demonstrated, however, that an****/approximate/**measurement of**/L/**_/S/ **can be performed, with the error decreasing as a function of the size of the apparatus system.
*http://iopscience.iop.org/article/10.1088/1367-2630/15/1/013057/meta** **Brent** ** -- 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.
