Heisenberg cut

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Quantum mechanics
${\displaystyle i\hbar {\frac {d}{dt}}|\Psi \rangle ={\hat {H}}|\Psi \rangle }$ Schrödinger equation
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Background Classical mechanics Old quantum theory Bra–ket notation Hamiltonian Interference
Fundamentals Complementarity Decoherence Entanglement Energy level Measurement Nonlocality Quantum number State Superposition Symmetry Tunnelling Uncertainty Wave function Collapse
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Equations Dirac Klein–Gordon Pauli Rydberg Schrödinger
Interpretations Bayesian Consciousness causes collapse Consistent histories Copenhagen de Broglie–Bohm Ensemble Hidden-variable Many-worlds Objective-collapse Quantum logic Superdeterminism Relational Transactional
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In quantum mechanics, a Heisenberg cut, is the hypothetical interface between quantum events and an observer's information, knowledge, or conscious awareness. Below the cut everything is governed by the wave function and Schrödinger equation; above the cut a classical description is used. The Heisenberg cut is a theoretical construct; it is not known whether actual Heisenberg cuts exist, where they might be found, or how they could be detected experimentally. However, the concept is useful for analysis.

The cut is named after Werner Heisenberg who first raised the idea during the 1929 Como Conference, during the discussions that followed Niels Bohr's introduction of the principle of complementarity.

The Heisenberg cut is associated with the Copenhagen interpretation of quantum mechanics, which requires a wave function collapse. Interpretations of quantum mechanics that do not recognise wave function collapse (such as De Broglie–Bohm or many-worlds interpretations) do not require Heisenberg cuts.