Today's post, Bell's Theorem: No EPR "Reality" was originally published on 04 May 2008. A summary (taken from the LW wiki):
(Note: This post was designed to be read as a stand-alone, if desired.) Originally, the discoverers of quantum physics thought they had discovered an incomplete description of reality - that there was some deeper physical process they were missing, and this was why they couldn't predict exactly the results of quantum experiments. The math of Bell's Theorem is surprisingly simple, and we walk through it. Bell's Theorem rules out being able to locally predict a single, unique outcome of measurements - ruling out a way that Einstein, Podolsky, and Rosen once defined "reality". This shows how deep implicit philosophical assumptions can go. If worlds can split, so that there is no single unique outcome, then Bell's Theorem is no problem. Bell's Theorem does, however, rule out the idea that quantum physics describes our partial knowledge of a deeper physical state that could locally produce single outcomes - any such description will be inconsistent.
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Well, maybe my complaint about authority is just be hindsight talking. This is because it's not like entanglement has never again been part of scientific research - quantum computers are made of the stuff. Electrons are just not classical objects.
And I think that, if we treat the universe as based on causality (a la Judea Pearl), the hidden variable route ( P(A | B a b) = P(A | a b) ) really is the only relativistic one, if we avoid many worlds. There are three ways for events to be linked: direct causally linked (faster than light), both descendants of a node we know about (hidden variable), or both ancestors of a node we know about (faster than light).