In quantum mechanics, Schrödinger’s cat is a thought experiment concerning quantum superposition.
In the thought experiment, a hypothetical cat may be considered simultaneously both alive and dead, while it is unobserved in a closed box, as a result of its fate being linked to a random subatomic event that may or may not occur.
The experiment viewed this way is described as a paradox.
This thought experiment was devised by physicist Erwin Schrödinger in 1935 in a discussion with Albert Einstein to illustrate what Schrödinger saw as the problems of the Copenhagen interpretation of quantum mechanics.
In Schrödinger’s original formulation, a cat, a flask of poison, and a radioactive source are placed in a sealed box. If an internal radiation monitor (Geiger counter) detects radioactivity (a single atom decaying), the flask is shattered, releasing the poison, which kills the cat.
The Copenhagen interpretation implies that, after a while, the cat is simultaneously alive and dead. Yet, when one looks in the box, one sees the cat either alive or dead, not both alive and dead. This poses the question of when exactly quantum superposition ends and reality resolves into one possibility or the other.
Although originally a critique on the Copenhagen interpretation, Schrödinger’s seemingly paradoxical thought experiment became part of the foundation of quantum mechanics. The scenario is often featured in theoretical discussions of the interpretations of quantum mechanics, particularly in situations involving the measurement problem. As a result, Schrödinger’s cat has had enduring appeal in popular culture.
The experiment is not intended to be actually performed on a cat, but rather as an easily understandable illustration of the behaviour of atoms. Experiments at the atomic scale have been carried out, showing that very small objects may exist as superpositions; but superposing an object as large as a cat would pose considerable technical difficulties.
Fundamentally, the Schrödinger’s Cat experiment asks how long quantum superpositions last and when (or whether) they collapse. Different interpretations of the mathematics of quantum mechanics have been proposed that give different explanations for this process.
Schrödinger intended his thought experiment as a discussion of the EPR article, named after its authors Einstein, Podolsky, and Rosen in 1935. The EPR article highlighted the counterintuitive nature of quantum superpositions, in which a quantum system for two particles does not separate even when the particles are detected far from their last point of contact. The EPR paper concludes with a claim that this lack of separability meant that quantum mechanics as a theory of reality was incomplete.
Schrödinger and Einstein exchanged letters about Einstein’s EPR article, in the course of which Einstein pointed out that the state of an unstable keg of gunpowder will, after a while, contain a superposition of both exploded and unexploded states.
To further illustrate, Schrödinger described how one could, in principle, create a superposition in a large-scale system by making it dependent on a quantum particle that was in a superposition.
He proposed a scenario with a cat in a closed steel chamber, wherein the cat’s life or death depended on the state of a radioactive atom, whether it had decayed and emitted radiation or not.
According to Schrödinger, the Copenhagen interpretation implies that the cat remains both alive and dead until the state has been observed.
Note: No cats were actually used in his experiment.
Schrödinger did not wish to promote the idea of dead-and-live cats as a serious possibility; on the contrary, he intended the example to illustrate the absurdity of the existing view of quantum mechanics and thus he was employing reductio ad absurdum.
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