What is Schrödinger's cat? The quantum puzzle, explained

Schrödinger's cat, often typed as Schrodinger's cat, is a thought experiment the Austrian physicist Erwin Schrödinger described in 1935. A cat is sealed in a box with a device that kills it if a single radioactive atom decays. Quantum theory, taken literally, seems to describe the cat as neither alive nor dead until someone looks. Schrödinger didn't believe that. He invented the cat to show that something in the theory's usual reading had to be wrong.

In short

• The cat was a criticism, not a claim: Schrödinger used it to show how absurd quantum rules look when stretched to everyday objects.
• It dramatizes the measurement problem: the theory describes blends of possibilities, yet every measurement gives one definite result.
• Physicists still disagree about what really happens in the box; the main interpretations give different answers.

What exactly happens in the box?

Schrödinger's own setup, in John Trimmer's 1980 translation of his paper, is short and grim. A cat is shut in a steel chamber with a Geiger counter (a detector of radiation) and "a tiny bit of radioactive substance, so small, that perhaps in the course of one hour one of the atoms decays, but also, with equal probability, perhaps none." If an atom decays, the counter fires, a hammer breaks a flask of hydrocyanic acid, and the cat dies.

Leave the box closed for an hour. The atom is a quantum object, and quantum theory doesn't say whether it has decayed; it gives a 50% chance for each. Its mathematical description, the wave function, holds both possibilities at once, a state called a superposition. The trouble is that the atom isn't alone. The counter, the hammer, the flask and the cat are all linked to it. Follow the theory's equations and the wave function of the whole box ends up, in Schrödinger's words, "having in it the living and dead cat (pardon the expression) mixed or smeared out in equal parts."

What was Schrödinger trying to show?

He opened the passage with "One can even set up quite ridiculous cases." The point was that an uncertainty confined to one atom becomes, through a simple device, an uncertainty about a cat, and nobody believes a real cat is smeared between life and death. Opening the box would settle it, so the blur couldn't be a picture of reality, only a gap in the description. Schrödinger put it with a photographic image: there is a difference "between a shaky or out-of-focus photograph and a snapshot of clouds and fog banks."

The paper, The Present Situation in Quantum Mechanics, appeared in three parts in the German journal Die Naturwissenschaften in late 1935. It was a reply to a paper that Albert Einstein and two colleagues had published that May, arguing that quantum theory was incomplete. Einstein and Schrödinger had been writing to each other about it all summer; Einstein had offered a similar example, a keg of gunpowder that might or might not explode. The cat is the version that stuck.

How does the cat show the measurement problem?

Quantum mechanics has two rules that sit awkwardly together. Left alone, a system's wave function changes smoothly and predictably, and superpositions stay superpositions. When it is measured, though, you always get one definite answer, with odds set by the wave function. The theory doesn't say what counts as a measurement, or where the smooth rule stops and the jump to one answer starts. That gap is called the measurement problem, as the Stanford Encyclopedia of Philosophy sets out.

The numbers show how the theory describes the box without saying what is in it. With Schrödinger's even odds per hour, the chance the cat is alive halves every hour:

Hours the box stays shut	Chance no atom has decayed	Chance the cat is found alive
0	100%	100%
1	50%	50%
2	25%	25%
3	12.5%	12.5%

Every interpretation agrees on this table, and every experiment confirms odds like these. What they disagree about is the story behind the numbers: is the cat already alive or dead before anyone looks, and if not, what makes it one or the other?

Is the cat really both alive and dead?

No experiment says so, and no mainstream interpretation says a real cat is "half dead" in the everyday sense. Superposition describes how quantum possibilities combine and interfere, not an object being in two states the way a coin could be half painted. The main interpretations tell the story differently:

Interpretation	What happens in the box	Main objection
Copenhagen (Bohr, Heisenberg, 1920s)	The wave function is a tool for predicting results; asking what the cat is "really" doing before measurement is not a meaningful physics question	It never says where measurement begins
Many worlds (Everett, 1957)	Nothing collapses: the box, and the observer who opens it, split into a branch with a living cat and a branch with a dead one	Hard to make sense of probability when every outcome happens
Pilot wave (de Broglie, Bohm, 1952)	The cat is always definitely alive or dead; particles have real positions, guided by the wave	Needs instant links across distance, and is hard to extend to relativity
Objective collapse (Ghirardi, Rimini, Weber, 1986)	Wave functions collapse on their own, very fast for large objects, so the cat is never in a superposition	Changes the theory, and the extra collapse hasn't been detected

Most working physicists add one more idea that isn't an interpretation in itself: decoherence. A cat is warm and touches air, light and the box, and each contact entangles it with its surroundings. That spreads the superposition into the environment within a tiny fraction of a second, so the two possibilities stop interfering and behave, for any practical test, like ordinary odds. Decoherence explains why nobody ever sees a cat in superposition. Whether it fully solves the problem, or only shows why the jump looks sudden, is still debated.

Still argued

There is no agreed answer to what happens in the box. Copenhagen-style views, many worlds, pilot-wave theory and collapse theories all predict the same results in the experiments done so far, so evidence hasn't picked a winner. When Nature surveyed more than a thousand researchers in 2025, Copenhagen-style views were the most widely held, yet only 24% were willing to call their preferred interpretation correct, as CERN Courier reported. The Stanford Encyclopedia's entry on many worlds sets out how its supporters and critics still argue over what probability means in it.

Have scientists made real "cat states"?

Yes, on a small scale. Physicists now call any superposition of two clearly different states of a larger system a "cat state." In the 1990s, Serge Haroche trapped light inside a cavity in a superposition of two opposite states, and David Wineland did the same with single trapped ions, then watched the superpositions decay as the systems touched their surroundings. They shared the 2012 Nobel Prize in Physics, and the Nobel committee's account describes their work as putting quantum particles in "cat-like" states. In 2025, John Clarke, Michel Devoret and John Martinis won the prize for showing quantum behavior in an electric circuit big enough to hold in your hand, the effect behind many of today's quantum computers.

These states are fragile because of decoherence, which is why many quantum computers are kept colder than outer space and shielded from stray signals. No one has put anything as large and warm as a cat in a superposition, and doing so may never be possible in practice.

Why does a thought experiment still matter?

A thought experiment tests an idea by following it to its end in the imagination, the way the Ship of Theseus tests what makes an object the same over time. Schrödinger's cat does that for quantum theory. It doesn't break the theory's predictions, which have passed every test. It shows that the theory, read naively, gives no clear picture of reality at the scale of cats and people. That is still an open question in science, and more than 90 years on, the cat remains the shortest way to state it.

Check yourself

• Without looking back, say what Schrödinger was criticizing with the cat, and why a cat rather than an atom.
• What is the measurement problem, in two sentences?
• How do the many-worlds and pilot-wave views differ about what is in the box before it opens?

Sources

• The Present Situation in Quantum Mechanics, translated by John D. Trimmer — Erwin Schrödinger, 1935, Proceedings of the American Philosophical Society, 1980
• Measurement in quantum theory — Stanford Encyclopedia of Philosophy
• Many-worlds interpretation of quantum mechanics — Stanford Encyclopedia of Philosophy
• The measurement problem, measured — CERN Courier, 2025
• The Nobel Prize in Physics 2012, popular information — Royal Swedish Academy of Sciences, 2012