Science › Quantum world
Schrödinger's cat
1935· Idea
ProvenTested many times; part of working physics.
Schrödinger's 1935 thought experiment showing how absurd quantum superposition sounds when applied to an everyday object.
See it on the Science map →How it works
A cat is sealed in a box with a radioactive atom, a Geiger counter and poison. If the atom decays, the poison is released. Quantum rules alone put the atom in a blend of decayed and not decayed, so the cat would be in a blend of dead and alive until someone looks. Schrödinger meant it as criticism: something must be missing from the theory. No real cat was involved.
Why it matters
It became the iconic image of the measurement problem, forcing the question of where the quantum world ends and the everyday world begins.
How we know
It is a thought experiment, not a test. But labs now make 'cat states', superpositions of light fields, ions and tiny vibrating crystals; Serge Haroche and David Wineland shared the 2012 Nobel for such work.
Still a mystery
Exactly where and why the blend turns into one outcome. Decoherence explains why we never see a blurred cat, but not, by itself, why one result is experienced.
Connections (3)
▸Erwin Schrödinger proposed Schrödinger's catProven
The relationship
Schrödinger connected a single atom's superposition to a cat's life through a chain of devices. If the quantum rules apply all the way up, the cat ends up in a blend of alive and dead. He meant this as a reductio, a way to show the standard view leads to nonsense.
How the link was found
Schrödinger, in a 1935 paper in Naturwissenschaften, written after exchanging letters with Einstein about EPR.
Why it's proven
The historical link is documented. As a thought experiment it proves nothing about nature, but it pinpoints a real gap in the theory.
Open questions and mysteries
Schrödinger's challenge, where the quantum blend ends and definite outcomes begin, still has no agreed answer.
▸Superposition: many states at once is scaled up to absurdity in Schrödinger's catProven
The relationship
Superposition is routine for atoms. Schrödinger linked one atom's superposition, decayed and not decayed, to a poison trigger and a cat. If superposition applies to everything, the cat must be in a blend of alive and dead. Scaling up shows how bizarre the rule looks for large objects.
How the link was found
Schrödinger, 1935, prompted by EPR and his correspondence with Einstein, who had suggested a similar example involving gunpowder.
Why it's proven
Superpositions of ever-larger systems, from big molecules to a 16-microgram crystal (2023), have been made in labs. No cat-sized superposition has, because decoherence destroys it almost instantly.
Open questions and mysteries
Is there a size limit where superposition fails? Some collapse theories say yes; experiments are still searching.
▸Decoherence: why big things look normal explains why we never see a live-and-dead Schrödinger's catProven
The relationship
A real cat constantly exchanges heat, air molecules and light with its surroundings. Any alive-and-dead blend would spread into the environment within a tiny fraction of a second, after which the two possibilities could never interfere again. The box effectively holds either a live cat or a dead one, never an observable mix.
How the link was found
Developed by Zeh, Zurek, Joos and others from 1970 through the 1990s; Zurek's 1991 Physics Today article brought it to a wide audience.
Why it's proven
Experiments with deliberately built 'cat states' of light show their blends degrade faster the larger they are, as decoherence predicts (Haroche's group, 1996).
Open questions and mysteries
Decoherence says what we will see, not why the cat is definitely one or the other. That last step still depends on interpretation.