November 4 marked the 50th year since John Bell submitted one of the most famous papers in physics: On the Einstein Podolsky Rosen Paradox [PDF]. In this paper he addressed the paradox pointed out by Albert Einstein, Boris Podolsky, and Nathan Rosen (EPR) in their famous Gedanken experiment.
Quantum Mechanics (QM) does not permit one to simultaneously measure pairs of mutually non-commuting observables, i.e., if one precisely measures the spin of a particle on its x-axis, one cannot also know the spins on its y- and z-axes. EPR envisioned an experiment using entangled particles where one could simultaneously determine values for non-commuting observables and thus argued that this means QM is an incomplete theory and there must be yet unknown "hidden" variables that would make it complete (and on top of that, these hidden variables are local variables, meaning that there is no need for faster-than-light information transmission, also known as that spooky action at a distance ).
Bell's paper presented an inequality relationship that, if not satisfied, means for the EPR Gedanken experiment one must give up the idea of hidden variables or accept that spooky action at a distance. Bell's Inequality has laid the foundation of quantum information theory and quantum cryptography [PDF], given license to people to metaphysically run amok with QM, and fifty years later we're still arguing about what it really tells us about QM and what our experiments so far really mean. Some of the best explanations of Bell's work come from his own [PDF] words [PDF].
(Score: 4, Insightful) by cosurgi on Thursday November 06 2014, @04:05PM
Major problem with people who do not understand Bell's theorem is that they do not understand counterfactual definiteness, and therefore assume that it is allowed.
Historians are not allowed to write historical books about what happens to Europe if there is no World War.
In Quantum Mechanics you are not allowed to talk about experiment that never happened. Problem with QM is that you have probability, so it seems plausible to you to say "If I had measured it on the other end, I would get this". That's the source of all paradoxes that you get here.
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(Score: 2) by HiThere on Thursday November 06 2014, @07:42PM
Perhaps they understand it can refuse to accept that it can't be applied?
Not everyone has the same problem with QM. (Personally I tend to use the EGW-multiworld interpretation, so I don't find myself bothered by that, instead I'm bothered by the question of how quickly a split in the universe propagates, and if two different worlds have subsequent worlds that are identical, are they exactly the same world. [My tentative answer is "yes"..but the speed question has me stumped. I also wonder if there's a distance dependent resolution, as I have trouble with the idea that the emission of a photon in a galaxy beyond the observable limit of the universe would cause this world to bifurcate.])
Javascript is what you use to allow unknown third parties to run software you have no idea about on your computer.
(Score: 2) by hubie on Thursday November 06 2014, @09:35PM
I was going to work in a nice link to the many worlds theory [pbs.org] as an example of a way around Bell's Inequality, and of course I'd have to include mention of collapsing wavefunctions and other alternative interpretations, and though it would have been a lot of fun to research links and write the article summary, it probably would have been much much too long.