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Quantum Weirdness 107: Bell’s Inequality

Note:  I said in Quantum Weirdness 106 that I was done with this series for now.  There are two possibilities here.  Either my definition of “for now” is a very short time, or I have branched off into an alternate universe where the term “done for now” has no meaning.**  Then again, I could have branched off into an alternative universe where, instead of writing this post, I would be lying on a Mediterranean beach next to a super-model in a string bikini.   I wish.

**Okay, I might just have have lied.

“God does not play dice.”–Albert Einstein

“Quit telling god what to do.”–Niels Bohr

It’s complicated.  And this just about reaches the limit of my own understanding.

The whole point of Einstein’s comment is that he could not accept the random nature of the quantum world.  He could not accept that quanta of matter and energy, and all their itinerant properties, only exist as probabilities until we observe  them.  He felt that there must be hidden variables that gave them these properties whether anyone was watching or not.  “I’d like to think the moon is there whether I am looking or not,” he said.

He was wrong.  Well, I don’t know about the moon, as that invokes the infamous Schrödinger’s Cat problem and it’s obfuscation of the Copenhagen Interpretation.  But for those tiny little quantum bits of stuff, it seems as if he blew it.

It all boils down to two papers.  The first was a 1935 paper by Einstein, along with colleagues Nathan Rosen and Boris Podalsky that proposed a thought experiment to demonstrate that there are only two possible explanations for certain properties of quantum mechanics: either there are hidden variables governing the quantum world, or else, as Einstein called it “spooky action at a distance.”  This has become known as the EPR paradox.

The second was a 1964 paper by John S. Bell, proposing an equation and related experiment that could be used to determine which of the alternatives is correct.  This became known as Bell’s inequality.

The technology did not yet exist, though, to make the measurements required to determine the solution to Bell’s equation. That did not occur until Alain Aspect, et al, performed an experiment in 1981 that proved, finally, that Einstein was wrong: no hidden variables exist; it’s spooky action at a distance.  At least, that is,  until further notice.

A  fairly facile explanation of the concepts and history is available here (including a brief touching on their relationship to Schrödinger’s Cat) and some subsequent contrary opinions here.  Or for those who can’t (or prefer not to) read, see the video that follows.  Confused?  One of the greatest scientific minds of the 20th century, Richard Feynman, said that nobody understand quantum mechanics.  Boy, does that give me free rein to get crazy with conjecture #5: Quantum Solipsism.   There may actually be a universe where I finally write and post it.

Whew.

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