... according to quantum mechanics one can arrange a pair of particles so that they are precisely two feet apart and yet neither particle on its own has a definite position.Albert and Galchen focus on the problem quantum theory poses for special relativity, pointing out that the problem has long been evaded.
Furthermore, the standard approach to understanding quantum physics, the so-called Copenhagen interpretation - proclaimed by the great Danish physicist Niels Bohr early last century and handed down from professor to student for generations - insists that it is not that we do not know the facts about the individual particles' exact locations; it is that there simply aren't any such facts. To ask after the position of a single particle would be as meaningless as, say, asking after the marital status of the number five. The problem is not epistemological (about what we know) but ontological (about what is).
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But entanglement also appears to entail the deeply spooky and radically counterintuitive phenomenon called nonlocality - the possibility of physically affecting something without touching it or touching any series of entities reaching from here to there. Nonlocality implies that a fist in Des Moines can break a nose in Dallas without affecting any other physical thing (not a molecule of air, not an electron in a wire, not a twinkle of light) anywhere in the heartland.
But it strikes me that quantum theory poses other problems as well.
One frequently encounters claims that action at a distance cannot happen when it might be more wisely described as statistically improbable (if you are somewhat largerthan an elementary particle). Statistical improbability is the basis on which lottery fraud is detected, for example. Few will believe that the lucky rabbit's foot did it.
The authors discuss the work of Irish physicist John Bell, who wanted to know whether non-local behaviour was real or only apparent. (One way of dealing with the conflict between special relativity and quantum theory has been to say, with Einstein, that quantum theory is incomplete.) However,
Bell seems to have been the first person to ask himself precisely what that question means. What could make genuine physical nonlocalities distinct from merely apparent ones? He reasoned that if any manifestly and completely local algorithm existed that made the same predictions for the outcomes of experiments as the quantum-mechanical algorithm does, then Einstein and Bohr would have been right to dismiss the nonlocalities in quantum mechanics as merely an artifact of that particular formalism. Conversely, if no algorithm could avoid nonlocalities, then they must be genuine physical phenomena. Bell then analyzed a specific entanglement scenario and concluded that no such local algorithm was mathematically possible.For the most part, work that demonstrated that fact was ignored, but Albert and Galchen argue that that is changing:
And so the actual physical world is nonlocal. Period.
It took yet another 30 years after the publication of Bell's paper for physicists to finally look these issues squarely in the face. The first clear, sustained, logically flawless and uncompromisingly frank discussion of quantum nonlocality and relativity appeared in 1994, in a book with precisely that title by Tim Maudlin of Rutgers University. His work highlighted how the compatibility of nonlocality and special relativity was a much more subtle question than the traditional platitudes based on instantaneous messages would have us believe. Maudlin's work occurred against the backdrop of a new and profound shift in the intellectual environment.One hopes that this shift will lead to open-minded examination instead of reassertions of truisms.
See also: Physics: A peek behind the veil of reality earns physicist Templeton Prize