Editing Measurement problem (section)

== Schrödinger's cat ==
{{main | Schrödinger's cat}}
A thought experiment called  [[Schrödinger's cat]] illustrates the measurement problem. A mechanism is arranged to kill a cat if a quantum event, such as the decay of a radioactive atom, occurs. The mechanism and the cat are enclosed in a chamber so the fate of the cat is unknown until the chamber is opened. Prior to observation, according to quantum mechanics, the atom is in a [[quantum superposition]], a [[linear combination]] of decayed and intact states. Also according to quantum mechanics, the atom-mechanism–cat composite system is described by superpositions of compound states. Therefore, the cat would be described as in a superposition, a linear combination of two states an "intact atom–alive cat" and a "decayed atom–dead cat".  However, when the chamber is opened, the cat is either alive or it is dead: there is no superposition observed. After the measurement, the cat is definitively alive or dead.<ref name=BaggottStory>{{Cite book |last=Baggott |first=J. E. |title=The quantum story: a history in 40 moments |date=2013 |publisher=Oxford Univ. Press |isbn=978-0-19-965597-7 |edition=Impression: 3 |location=Oxford}}</ref>{{rp|154}} 

The cat scenario illustrates the measurement problem: how can an indefinite superposition yield a single definite outcome? It also illustrates other issues in quantum measurement,<ref name="Griffiths3rd">{{Cite book |last1=Griffiths |first1=David J. |last2=Schroeter |first2=Darrell F. |date=2018-08-16 |title=Introduction to Quantum Mechanics |url=https://www.cambridge.org/highereducation/product/9781316995433/book |publisher=Cambridge University Press |doi=10.1017/9781316995433 |isbn=978-1-316-99543-3}}</ref>{{rp|585}} including: When does a measurement occur? Was it when the cat was observed? How is a measurement apparatus defined? The mechanism for detecting radioactive decay? The cat? The chamber? What is the role of the [[Observer (quantum physics)|observer]]?