Schrödinger's cat

Erwin Schrödinger introduced the cat example in his 1935 discussion of the foundations of quantum mechanics, following the Einstein–Podolsky–Rosen argument. In the simplified scenario, a cat is sealed in a chamber with a radioactive source, a detector and a mechanism that releases poison if a decay is registered. After a specified time, the radioactive atom has a probability of having decayed or not; the device links those alternatives to different outcomes for the cat.

Before the chamber is opened, quantum mechanics can describe the coupled atom-and-device system as a superposition of alternatives associated with decay and no decay. Because the apparatus connects the microscopic event with a macroscopic result, extending that description to the entire system appears to yield a superposition involving a living cat and a dead cat. Schrödinger called the example an absurd or ridiculous case: it highlighted the tension between the formal quantum description and the definite outcomes of ordinary experience.

The paradox raises questions about how quantum states relate to measurement outcomes, how interactions with measuring devices are described, and where a boundary between quantum and classical descriptions might be drawn. Different interpretations of quantum mechanics give different accounts of measurement, and the thought experiment by itself does not decide among them. It is not accurate to treat the phrase as an experimental finding that an ordinary cat has been observed to be alive and dead simultaneously.

Researchers have produced and studied superposition states in controlled quantum systems, including atoms, ions, photons and larger molecules. Such experiments investigate how quantum coherence can persist or be lost through interactions with an environment. They are not literal tests involving a cat in a poison-filled box; biological systems are extraordinarily complex and interact strongly with their surroundings. The cat remains a conceptual illustration, while laboratory work tests related quantum principles in carefully controlled systems.