hidden variables

Hidden variables are proposed extra parameters, not contained in the wavefunction, that would give every system a complete and definite state at all times. The hope behind them is the one Einstein voiced when he said God does not play dice: that quantum randomness is only apparent, a reflection of our ignorance of these underlying details, much as a coin toss looks random only because we cannot track the spinning coin precisely enough.

If such variables existed and were known, the future would in principle be fixed; the probabilities of quantum mechanics would then be ordinary statistical averages over the unknown values, restoring determinism beneath the surface. This is a deeply natural idea, and for a while it seemed merely a matter of finding the right hidden variables. The story took a sharp turn with John Bell.

Bell's theorem proved that no theory of local hidden variables — variables whose influence does not travel faster than light — can reproduce all the predictions of quantum mechanics. Experiments since the 1970s, increasingly free of loopholes, have come down firmly on the quantum side. The honest conclusion is not that all hidden variables are dead, but that any that survive, such as those in Bohmian mechanics, must be explicitly nonlocal.