Hidden-variable theory

A theory that supplements quantum mechanics with unobserved variables to account for measurement outcomes and recover definite underlying properties.

Pilot-wave theory: An interpretation of quantum mechanics in which particles have definite positions guided by a wavefunction. It gives hidden variables a concrete role: particle positions evolve under wavefunction guidance.
Louis de Broglie: A French physicist who proposed the pilot-wave interpretation of quantum mechanics in the 1920s. His proposal was the first major wave-guided hidden-variable theory.
Copenhagen interpretation: A family of interpretations emphasizing quantum probabilities, measurement, and limits on assigning classical properties. Unlike hidden-variable accounts, it does not generally posit definite unobserved values for every measured quantity.
Kochen–Specker theorem: A theorem showing that quantum observables cannot all have context-independent predetermined values consistent with quantum structure. It rules out a broad class of noncontextual hidden-variable models.
Superdeterminism: The hypothesis that measurement settings and physical systems share prior causes that undermine statistical independence. It offers a way to evade Bell's conclusion by rejecting independent choice of settings.
Bohmian mechanics: A deterministic formulation of quantum mechanics with particles following trajectories guided by the wavefunction. It is the best-known explicit hidden-variable theory, with a precise guidance equation.
David Bohm: An American theoretical physicist who developed a deterministic hidden-variable interpretation of quantum mechanics. His 1952 papers revived pilot-wave ideas as a complete alternative formulation.
Many-worlds interpretation: An interpretation in which the universal wavefunction evolves without collapse and measurement outcomes occur in branching worlds. It addresses measurement outcomes through branching rather than additional variables.
Contextuality: The dependence of a measurement outcome or assigned value on which compatible measurements are performed alongside it. Many hidden-variable theories must allow values to depend on measurement context.
Retrocausality: The hypothesis that future measurement choices can influence earlier physical states or events. It is another proposed route to Bell-compatible hidden-variable models without ordinary nonlocality.

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