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In quantum mechanics, Schrödinger's cat is a thought experiment concerning quantum superposition.

In the thought experiment, a hypothetical cat inside a closed box may be considered to be simultaneously both alive and dead while it is unobserved, as a result of its fate being linked to a random subatomic event that may or may not occur. 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 Niels Bohr and Werner Heisenberg's philosophical views on quantum mechanics. In Schrödinger's original formulation, a cat, a flask of poison, and a radioactive source are placed in a sealed box. Yet, when one looks in the box, one sees the cat either alive or dead, not both alive and dead. Although originally a critique of Bohr and Heisenberg, Schrödinger's seemingly paradoxical thought experiment became part of the foundation of quantum mechanics. As a result, Schrödinger's cat has had enduring appeal in popular culture. Fundamentally, the Schrödinger's cat experiment asks how long quantum superpositions last and when (or whether) they collapse.

According to Schrödinger, the position taken by Bohr and Heisenberg would be that the cat remains both alive and dead until the state has been observed. Since Schrödinger's time, various interpretations of the mathematics of quantum mechanics have been advanced by physicists, some of which regard the "alive and dead" cat superposition as quite real, while others do not. Intended as a critique of ideas prevalent in 1935, the Schrödinger's cat thought experiment remains a touchstone for modern interpretations of quantum mechanics and can be used to illustrate and compare their strengths and weaknesses.

The wave function description of the complete cat system implies that the reality of the cat mixes the living and dead cat.

In modern terms Schrödinger's hypothetical cat experiment describes the measurement problem: quantum theory describes the cat system as a combination of two possible outcomes but only one outcome is ever observed.

Since Schrödinger's time, other interpretations of quantum mechanics have been proposed that give different answers to the questions posed by Schrödinger's cat of how long superpositions last and when (or whether) they collapse.

The superposition itself had no physical meaning to Bohr: Schrödinger's cat would be either dead or alive long before the box is opened but the cat and box form an inseparable combination.

In other words, when the box is opened, the observer and the possibly-dead cat split into an observer looking at a box with a dead cat and an observer looking at a box with a live cat. It examines the Schrödinger's cat experiment from the point of view of the cat, and argues that by using this approach, one may be able to distinguish between the Copenhagen interpretation and many-worlds.

The cat can be considered an observer of the apparatus; meanwhile, the experimenter can be considered another observer of the system in the box (the cat plus the apparatus).

All the superconducting electrons in the SQUID flow both ways around the loop at once when they are in the Schrödinger's cat state."

Le Guin learned about it while researching quantum mechanics for her novel The Dispossessed; Crease credits her 1974 short story "Schrödinger's Cat" with bringing the concept into popular culture. Schrödinger's cat has been a motive in many science fiction works, and used as a title of a number of them, including Greg Bear's "Schrödinger's Plague" (Analog, 29 March 1982), George Alec Effinger's "Schrödinger's Kitten" (Omni, September 1988), F. Gwynplaine MacIntyre's "Schrödinger's Cat-Sitter" (Analog, July/August 2001), Rudy Rucker's "Schrödinger's Cat" (Analog, 30 March 1981), and Robert Anton Wilson's Schrödinger's Cat Trilogy (1988), illustrating various interpretations of quantum physics. "The Present Situation in Quantum Mechanics: A Translation of Schrödinger's "Cat Paradox" Paper". Reduction of the Schrödinger's cat to a simple quantum circuit. Schrödinger's Cat - a podcast produced by Sift.

Quick Facts

• Gwynplaine MacIntyre's "Schrödinger's Cat-Sitter" (Analog, July/August 2001), Rudy Rucker's "Schrödinger's Cat" (Analog, 30 March 1981), and Robert Anton Wilson's Schrödinger's Cat Trilogy (1988), illustrating various interpretations of quantum physics.
• Schrödinger's cat has been a motive in many science fiction works, and used as a title of a number of them, including Greg Bear's "Schrödinger's Plague" (Analog, 29 March 1982), George Alec Effinger's "Schrödinger's Kitten" (Omni, September 1988), F.
• Since Schrödinger's time, other interpretations of quantum mechanics have been proposed that give different answers to the questions posed by Schrödinger's cat of how long superpositions last and when (or whether) they collapse.
• In Schrödinger's original formulation, a cat, a flask of poison, and a radioactive source are placed in a sealed box.
• The superposition itself had no physical meaning to Bohr: Schrödinger's cat would be either dead or alive long before the box is opened but the cat and box form an inseparable combination.

Source material: Wikipedia - "Schrödinger's cat". Adapted and summarized for DiscoverScroll. Original contributors are credited through the linked Wikipedia article. Read original on Wikipedia. CC BY-SA 4.0. Changes were made from the original.