Bell's theorem

Bell's theorem, also called "Bell's inequality," is a thought experiment. When joined with real experiments, it shows there are no hidden variables which can explain some of the consequences of quantum mechanics. This study, closely related to quantum mechanics, was done by John Stewart Bell.Bell J.S. 1987. Speakable and unspeakable in quantum mechanics. Cambridge University Press. 

 Brian Greene's analogy 
The following analogy has been provided by Brian Greene:
Pairs of boxes have been prepared and sent to Earth and on Vulcan. There is something inside that shows a light when a door is opened. If the same doors are opened on both members of the pair, then both lights will be blue or both lights will be red. What if different doors are opened? Maybe the chances that a red light or a blue light shows up were set by the maker of the boxes. Or maybe whether a red light or a blue light shows up depends on an individual coin flip in each box. How can we tell?

In a system that works the way quantum mechanics does, there is a 50% chance that opening any pair of doors (on the gray side, the white side, or the black side of each of the boxes in the schematic diagram) will give rise to a match. For instance, Earth could open the door in the gray side of its box, and Vulcan could open the door in the black side of its box. Quantum physics says that half the time they should get a match. But if the decision to produce a certain color for each of the doors opened is programmed into both boxes, then there is a 55% or greater chance of getting a match. 

   G-W-B Choose Choose Choose Choose Choose Choose Choose Choose Choose % matched Preset B-B-R G-G G-W G-B W-G W-W W-B B-G B-W B-B Match?  yes yes no yes yes no no no yes 55% Preset B-R-B G-G G-W G-B W-G W-W W-B B-G B-W B-B  Match  yes no  yes no  yes no  yes no  yes 55% Preset B-R-R G-G G-W G-B W-G W-W W-B B-G B-W B-B  Match  yes no  no no  yes yes  no yes  yes 55% Preset R-B-B G-G G-W G-B W-G W-W W-B B-G B-W B-B  Match  yes no  no no  yes yes  no yes  yes 55% Preset R-B-R G-G G-W G-B W-G W-W W-B B-G B-W B-B  Match  yes no yes no yes no yes no yes 55% Preset R-R-B G-G G-W G-B W-G W-W W-B B-G B-W B-B  Match  yes yes no yes yes no no no yes 55%

Bell argues that because in actual experiments doing the equivalent of "opening different doors" only yields a 50% success rate, there can be no hidden variables in operation that would have "preselected" states for all six alternatives.

The analogy given here follows closely the one used by Brian Greene in The Fabric of the Cosmos, p. 107ff.

 References 

 Other sources 
Bell J.S. 1964. On the Einstein-Podolsky-Rosen Paradox. Physics'', 1:195-200, reprinted in Bell 1987.

 Related pages 
EPR paradox

Category:Quantum mechanics