Measurement problem
For a specified ontology and dynamics, give a precise account of measurement records and their probabilities without an undefined system–observer cut; identify experimentally distinguishable predictions or explain why competing accounts are operationally equiv
Problem statement
For a specified ontology and dynamics, give a precise account of measurement records and their probabilities without an undefined system–observer cut; identify experimentally distinguishable predictions or explain why competing accounts are operationally equivalent.
Lean 4
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Background
The measurement problem is the tension between the linear, deterministic unitary evolution of the Schrödinger equation and the apparent occurrence of single, definite measurement outcomes with Born-rule probabilities. Unitary dynamics applied to system + apparatus produces an entangled superposition of pointer states, not one outcome; the standard 'collapse' postulate patches this by hand, without specifying when or why it applies (the Heisenberg cut).
What is known: The problem is sharply framed and there are several mature research programmes, but they disagree even on what would count as a solution. Objective-collapse theories (GRW, CSL, Diósi–Penrose) modify the dynamics with stochastic nonlinear terms and are experimentally testable — and increasingly constrained by experiment. Everettian / many-worlds interpretations keep unitarity and deny unique outcomes, relocating the problem to deriving probabilities (F2) and the preferred basis (F5). Bohmian mechanics adds definite particle positions with a guiding equation. Relational and QBist views reinterpret the state as information. Decoherence (F5) explains the suppression of interference and the preferred basis but not, by itself, the selection of a single outcome.
This remains a conceptual programme, not a single theorem awaiting a consensus proof. Unitarity and unique absolute outcomes cannot both be imposed as neutral requirements while also including Everett and objective-collapse models. For a chosen framework, specify its dynamics, ontology and record statistics; assess empirical tests of modified dynamics separately from interpretational comparisons of operationally equivalent theories.
The Everettian probability question from archived F2 belongs here: make branch ontology and rationality/probability assumptions explicit and determine what they establish, rather than asking for a derivation that wins universal assent. Axiomatic uniqueness of the Born rule is tracked under F3. Related: F3, F4, F5.
Progress & source assessment
Source catalogue review: 4 September 2026. Improved indicates partial progress, including qualified preprint claims.
- [1]Bassi, Lochan, Satin, Singh, Ulbricht, 'Models of wave-function collapse, underlying theories, and experimental tests', Rev. Mod. Phys. 85, 471 (2013) link
- [2]Bell, 'Against measurement', Physics World 3(8), 33–40 (1990) link
- [3]Bell, 'Speakable and Unspeakable in Quantum Mechanics' (CUP, 2nd ed., 2004) link
Recorded progress
Add yours →Nothing recorded yet. A pointer to a paper the entry is missing counts.
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Discussion
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