Bell's theorem, also called "Bell's inequality," is a thought experiment joined with real experiments designed to show that there are no hidden variables that can explain some of the unexpected consequences of quantum mechanics, unless there is "spooky action at a distance." This study, which is closely related to Quantum mechanics, was produced by John Stewart Bell. The following analogy has been provided by Brian Greene:
Pairs of boxes have been prepared and sent to recipients on Earth and on Vulcan. There is some mechanism 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.
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, unless there is "spooky action at a distance" faster than the speed of light. This "spooky" alternative is because, for example in the G-G case, the mechanism on Vulcan could flip a coin to reset the light when the G door on Earth is opened; that results in a 50% success rate instead of 55%, but would require that somehow the box on Vulcan immediately "knew" what happened on Earth.
The analogy given here follows closely the one used by Brian Greene in The Elegant Universe, p. 107ff.
Outside resources[change | change source]
- Bell, J.S., 1964, “On the Einstein-Podolsky-Rosen Paradox”, Physics, 1:195-200, reprinted in Bell 1987.
- Bell, J.S., 1987, Speakable and Unspeakable in Quantum Mechanics, New York: Cambridge University Press.