Schrödinger’s cat
Schrödinger’s cat is a thought experiment the physicist Erwin Schrödinger published in 1935: a cat shut in a steel chamber that quantum mechanics, taken literally, would describe as alive and dead at once until someone looks. He offered it as a ridiculous case, to question the theory; it has since become a popular emblem for anything undecided.
Meaning
The cat takes up one paragraph of Schrödinger’s three-part paper “Die gegenwärtige Situation in der Quantenmechanik” (“The present situation in quantum mechanics”), published in Die Naturwissenschaften in 1935 and translated into English by John D. Trimmer in 1980. Schrödinger introduces it with “One can even set up quite ridiculous cases.” A cat is penned in a steel chamber with a Geiger counter, a speck of radioactive substance that has an even chance of one atom decaying within the hour, and a device that would then kill the cat. The theory’s description of the whole system, its ψ-function, would then contain “the living and the dead cat (pardon the expression) mixed or smeared out in equal parts.” Paul Halpern, writing for PBS’s NOVA, calls it a deliberate misapplication of rules made for subatomic particles to a large living being. Today the cat belongs to physics teaching and to popular culture.
Origin and history
Schrödinger had shared the 1933 Nobel Prize in Physics with Paul Dirac for his wave equation. The Nobel Foundation’s biography says he disliked the accepted description of matter in terms of both waves and particles, with a statistical reading of the waves, and that this led him into controversy with other leading physicists. Earlier in 1935 Einstein, Podolsky and Rosen had published their paper on quantum mechanics, and Schrödinger says in a footnote that it prompted his own.
The cat was an objection. Straight after it, Schrödinger draws the point: in such cases “an indeterminacy originally restricted to the atomic domain becomes transformed into macroscopic indeterminacy, which can then be resolved by direct observation. That prevents us from so naively accepting as valid a ‘blurred model’ for representing reality.” Halpern notes that it echoed an earlier idea of Einstein’s, a keg of gunpowder in a mixed state between exploded and unexploded.
For a quarter of a century the cat went almost unnoticed. Halpern writes that from 1935 to Schrödinger’s death in 1961 it was scarcely mentioned in the literature, that Schrödinger himself rarely brought it up, and that none of his lengthy obituaries mentioned it. It lived on as an example in classrooms and seminars. The philosopher Hilary Putnam discussed it in 1965, and Scientific American’s review of the book containing his essay may have been the cat’s first appearance in the popular press.
What later physicists made of it
The cat outlived Schrödinger’s objection by becoming a test case. Halpern gives three answers. Eugene Wigner’s 1963 essay on quantum measurement added a colleague waiting outside the laboratory door, to ask when an observation settles the outcome. In the many-worlds interpretation the cat survives in one branch of the universe and dies in another. Physicists who follow decoherence argue that a system’s interactions with its surroundings settle it into a single state with no observer needed; on that view the cat is either dead or alive, depending on what the radioactive sample did.
How the cat became a symbol
Science fiction took the cat first: Halpern names Ursula K. Le Guin, Robert Anton Wilson and Robert Heinlein, and John Gribbin’s popular book on quantum physics followed. By 2015 it had been mentioned on The Big Bang Theory and Futurama, had a Google doodle and was printed on countless T-shirts. Halpern calls it “a popular emblem for ambiguity”.
Where you’ll see it today
whatdoesthissymbolmean.comPsiSchrödinger's cat is written into the ψ-function of the whole systemSee also
Sigils, Infinity symbol (∞), Autism infinity symbol, Neurodiversity symbol, Semicolon tattoo.
Sources
- The Present Situation in Quantum Mechanics: A Translation of Schrödinger’s “Cat Paradox” Paper, John D. Trimmer, Proceedings of the American Philosophical Society 124 (5), 1980, pp. 323–338, American Philosophical Society (copy hosted by the Universidade Estadual de Campinas), Introduction and §5
- Schrödinger’s Cat Lives On (Or Not) at the Age of 80, Paul Halpern, 10 November 2015, NOVA, PBS
- Erwin Schrödinger – Biographical, From Nobel Lectures, Physics 1922–1941 (Elsevier, 1965), Nobel Prize Outreach
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Last reviewed September 30, 2026 by Rory Hansen. How we research