Category: physics
Associated thinker(s): Erwin Schrödinger
Original publication or approximate period: “Die gegenwärtige Situation in der Quantenmechanik” (1935)
Difficulty: Intermediate
Type: thought experiment
Themes: measurement; superposition; interpretation
Related experiments: wigners-friend; epr
Imagine a sealed chamber containing a cat, a radioactive source, a detector, and a mechanism that releases poison if the detector registers a decay. Choose the interval so that a decay has a substantial chance of occurring. Treat the source, detector, apparatus, and cat as one quantum system. If its evolution is entirely linear, the description correlates the undecayed component with an unharmed cat and the decayed component with a dead cat.
The crucial issue is the relation between that combined quantum state and the single definite result we ordinarily observe. Merely closing a lid does not produce a coherent cat state in a real laboratory. Isolation and control are idealizations, not a recipe for a classroom demonstration.
How can a theory with linear superposition account for definite macroscopic outcomes?
The scenario turns an abstract problem about measurement into a concrete question about an apparatus and its record. If measuring instruments are also physical systems, it is difficult to explain why one set of rules governs the atom while another governs the pointer. A proposed solution must say what changes, rather than only announcing that the situation is strange.
Collapse approaches add or invoke a state reduction yielding a definite outcome. Dynamical collapse theories make this a physical process with potentially testable consequences. Everettian approaches retain linear dynamics and explain outcomes through branching and relative records. Hidden-variable approaches add a more detailed ontology. These are competing accounts of a shared predictive success, not three interchangeable descriptions of an established cat experiment.
A superposition is not just ignorance about which ordinary state already obtains. Looking with human eyes is not universally accepted as the trigger of a physical collapse. Decoherence explains the suppression of accessible interference but, by itself, does not select a unique outcome in every interpretation.
Wigner’s friend replaces the cat with an observer. Laboratory “cat states” involve controlled physical degrees of freedom; they should not be advertised as literal realizations of the entire original story.
Schrödinger’s 1935 publication introduced the cat as a challenge concerning quantum description. The linked EPR history supports that context; the quantum overview supports the interpretive distinctions. The account is a schematic reconstruction, not a quotation.
Verified on 2026-10-30. Source coverage: [1] Measurement problem, Wigner scenario, competing theories; historical bibliography. [2] 1935 authorship, original position/momentum argument, Bohm variation and Bell developments. Reflection questions and illustrative numerical examples are original editorial material.