Schrodinger's Cat

Schrödinger's cat is a thought experiment that Erwin Schrödinger devised in 1935 to expose what he saw as an absurdity in the prevailing interpretation of quantum mechanics. By chaining the life of a cat to a single random quantum event, he forced a question that physicists still debate: if microscopic systems can exist in superpositions, why don't we ever see everyday objects in them?

The setup

A cat is sealed in a box with a radioactive atom, a Geiger counter, and a flask of poison. If the atom decays, the counter triggers a hammer that breaks the flask and kills the cat; if it does not, the cat lives. Quantum mechanics describes the undecayed/decayed atom as a superposition, written

|ψ⟩ = (1/√2)( |no decay⟩|alive⟩ + |decay⟩|dead⟩ )

Taken literally, this couples the atom's superposition to the cat, implying the cat is itself in a superposition of alive and dead until someone opens the box. Schrödinger's point was that this conclusion seems plainly ridiculous — and that the ridiculousness is a problem for the theory, not the cat.

The measurement problem

The puzzle is genuine and has a name: the measurement problem. The Schrödinger equation evolves states smoothly and never, by itself, picks out a single outcome. Yet every observation we make returns one definite result. What converts a superposition of possibilities into one actual fact — and at what point in the chain from atom to counter to cat to observer — is exactly what the thought experiment dramatises.

How the interpretations answer it

Different interpretations of quantum mechanics give different resolutions. In the Copenhagen view, measurement by a macroscopic apparatus collapses the wave function to one outcome. In the many-worlds interpretation, no collapse occurs; both branches are equally real and the observer simply splits with them. The modern workhorse is decoherence: a cat is constantly interacting with ~10²⁷ air molecules, photons, and its own internal heat, so any superposition of "alive" and "dead" leaks into the environment and becomes unobservable in a tiny fraction of a second. Decoherence explains why we never see the superposition, though interpretations still differ on whether one outcome is then truly selected.

A common misconception

The cat is not "in superposition because no conscious mind is watching." Consciousness plays no role in the physics; an interaction with any sufficiently complex system — the Geiger counter alone — already destroys the delicate superposition through decoherence. Laboratory experiments have placed atoms, molecules, and even small superconducting circuits into genuine superpositions, but scaling that to a macroscopic cat is prevented by decoherence, not by the absence of an observer.