Schrödinger’s cat

A thought experiment in which a cat’s fate is linked to a quantum event, exposing the tension between quantum superposition and definite macroscopic outcomes.

Quantum superposition: A quantum state formed from a combination of possible states, whose outcomes become definite upon measurement. The cat’s alive and dead alternatives represent the superposed possibilities in the experiment.
Erwin Schrödinger: A physicist who developed wave mechanics and helped establish quantum theory. He devised the cat example to challenge a literal reading of quantum superposition.
Many-worlds interpretation: An interpretation of quantum mechanics in which measurement outcomes occur in distinct branches of a universal quantum state. It treats the alternatives as branching outcomes rather than one state selected by collapse.
Schrödinger’s poison apparatus: The hypothetical detector-and-release mechanism in Schrödinger’s cat scenario, triggered by radioactive decay. It is the device that converts an atomic event into the cat’s possible death.
Quantum entanglement: A physical relation in which multiple systems share a joint quantum state that cannot be reduced to independent states. The cat’s state is correlated with the radioactive atom and the detector.
EPR paradox: A 1935 argument by Einstein, Podolsky, and Rosen that quantum mechanics may be incomplete because it permits puzzling correlations. Schrödinger introduced the cat discussion in the same debate about quantum theory’s completeness.
Objective-collapse theory: A family of theories in which quantum states spontaneously collapse through physical processes, without requiring measurement. These theories propose a physical limit to the cat’s superposition.
Wigner’s friend experiment: A thought experiment comparing an observer’s definite measurement outcome with an outside description of the observer and system as a superposition. It reformulates the cat’s measurement puzzle around an observer inside a laboratory.
Radioactive decay: The spontaneous transformation of an unstable atomic nucleus into a more stable configuration, emitting particles or radiation. A decay event triggers the mechanism that determines the cat’s fate.
Einstein–Bohr debates: A series of exchanges between Albert Einstein and Niels Bohr about the meaning and completeness of quantum mechanics. The cat became part of the broader dispute over whether quantum theory describes reality or measurement outcomes.

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