On Tue, Sep 5, 2023 at 8:34 PM John Clark <johnkcl...@gmail.com> wrote:
> On Mon, Sep 4, 2023 at 8:14 PM Bruce Kellett <bhkellet...@gmail.com> > wrote: > >> On Tue, Sep 5, 2023 at 12:02 AM smitra <smi...@zonnet.nl> wrote: >> > > > >> Bell's theorem is about local hidden variables theories >> >> >> > *It is difficult to know how to respond to this absurd idea. I have >> read quite extensively on Bell's theorem and locality in quantum mechanics >> and I have never met this contention before.* >> > > Huh? How can you "*have **read quite extensively on Bell's theorem and > locality*" and not know that Bell's theorem is a test to see if any > theory that assumes* local realism* can account for experimental > observations? Hell if you did nothing but skim the Wikipedia article on Bell's > theorem you should know that because the very first sentence is: > > *"Bell's theorem is a term encompassing a number of closely related > results in physics, all of which determine that quantum mechanics is > incompatible with local hidden-variable theories"* > > And just a few sentences later Wikipedia says: > > *"Its derivation here depends upon two assumptions: first, that the > underlying physical properties and exist independently of being observed or > measured (sometimes called the assumption of realism); and second, that > Alice's choice of action cannot influence Bob's result or vice versa (often > called the assumption of locality)"* > Unfortunately, Wikipedia is not an authoritative source. The derivation of the Bell inequality that you refer to in Wikipedia is not the derivation given by Bell in his original papers. Bell's own derivation appears later in the article, and you can see that Bell does not make the realism assumption. Since the inequality can be derived without this assumption, violating relaasm makes no difference to the overall result. The correlations in any local theory must satisfy the inequality. Bell shows that the quantum mechanical correlations violate the inequality, so quantum mechanics cannot be a local theory, and any hidden variable completion of QM must also be non-local. Other people have claimed that Bell made a whole range of other assumptions that their pet theories violate, thus rendering Be;ll's theorem toothless. But one is hard-pressed to see where any of these supposed additional assumptions come in. In fact, the range of things sometimes said to be assumed are often contradictory. The important point is that Bell used a particular implementation of the idea of locality for his theorem, and few other assumptions (the main one being the absence of superdeterminism), leaving the consequence of violations of the inequality pretty clear -- any such theory must be non-local. Quantum mechanics violates the inequality, therefore quantum mechanics is intrinsically non-local. Experiment confirms the quantum mechanical predictions. But since the inequality itself does not depend on any assumption of realism, the observed violations cannot be explained by claiming that the theory is local but non-realistic -- as I have said several times, "realism" has nothing to do with it. The Wikipedia article is quite misleading in this respect because it does not make clear that the result can also be derived without assuming realism (measurement results exist in the state *before* the measurement is performed.) In fact, the assumption of realism is pretty meaningless because QM itself does not have this property -- it is intrinsically probabilistic and non-realist. 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 view this discussion on the web visit https://groups.google.com/d/msgid/everything-list/CAFxXSLTUV-9kzw6tWaGh0t2DdQ45Uzr2guq435kR7wrSyyxJjA%40mail.gmail.com.
