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No physical theory of local hidden variables can ever reproduce all of the predictions of quantum mechanics.>
>I would really like to understand what this means, and precisely what assumptions go along with Bell's conception of a "physical theory of local hidden variables", such that so sweeping an impossibility proof could be obtained. Let us not forget Bell's ironic quote about impossibility proofs.
Did you read the Wikipedia article con hidden variables? It seems to give some introductory explanation on this problem.
Difference in opinions hath cost many millions of lives: for instance, whether flesh be bread, or bread be flesh; whether whistling be a vice or a virtue; whether it be better to kiss a post, or throw it into the fire... (from Gulliver's Travels)