Schrodinger's Cat Explained: The Thought Experiment, and What It Really Means

The thought experiment, step by step

Erwin Schrodinger published it in 1935. Seal a cat in a steel chamber with a tiny amount of radioactive material, a Geiger counter, a hammer and a flask of poison. Over the course of an hour there is a fifty per cent chance that a single atom decays. If it decays, the counter trips, the hammer smashes the flask and the cat dies. If it does not, the cat lives.

Quantum theory describes the undecayed atom as a superposition: decayed and not decayed at once, with no definite fact of the matter until it is measured. The chain of machinery links that microscopic superposition to something the size of a cat. If you take the mathematics literally, and nothing collapses it, the wave function of the whole box now describes a cat that is neither alive nor dead but a combination of both.

Schrodinger was not selling the idea, he was attacking it

This is the part most retellings drop. Schrodinger was not proposing that cats exist in blurred states. He was pointing out that the standard account of measurement, taken at face value, produces a conclusion nobody can accept. His own words were that the wave function would express this by having the living and dead cat mixed in equal parts, which he called ridiculous.

"One can even set up quite ridiculous cases. The wave function of the entire system would express this by having in it the living and the dead cat mixed or smeared out in equal parts."Erwin Schrodinger, 1935

In other words: if your theory says this, your theory is incomplete. Einstein, who was corresponding with Schrodinger that year, made the same complaint with a barrel of gunpowder. Both men were pressing on the measurement problem, the gap between smooth waves of possibility and single definite outcomes.

So is the cat really alive and dead?

In practice, no, and physicists have a good account of why. A cat is not an isolated system. It is warm, it breathes, it exchanges billions of particles with its surroundings every instant. That constant interaction, called decoherence, destroys the delicate phase relationships that superposition depends on almost immediately. The interference between the alive branch and the dead branch becomes unobservable in a fraction of a nanosecond.

Decoherence explains why we never see large objects in superposition. What it does not do is explain why we experience exactly one outcome rather than both. That question is still open, and it is where the interpretations differ.

What each interpretation says about the cat

• Copenhagen: the wave function is a calculating device. The cat is alive or dead; asking what was true before you opened the box is not a question the theory answers.
• Many worlds: nothing collapses. Both branches are real and the world splits. There is a version of you that finds a living cat and a version that finds a dead one.
• Pilot wave, or Bohmian mechanics: the atom either decayed or it did not, all along. Particles always have definite positions guided by a real quantum field, so the cat is never in two states, only our knowledge is incomplete.
• Objective collapse: collapse is a real physical process triggered by mass or gravity. A cat is far too large to stay superposed, so the state resolves long before anyone looks. This version makes testable predictions.
• QBism and information first views: the wave function encodes an observer's expectations, not the cat. Opening the box updates the observer, not reality.

All of these reproduce the same laboratory predictions. The disagreement is about what exists, not about what the instruments read.

David Bohm's answer, and why it matters

David Bohm's 1952 papers showed that a fully coherent quantum theory can be written in which particles always have definite positions and outcomes are never ambiguous. On that account, Schrodinger's cat has a fate from the first moment, no observer required. The price is nonlocality: the guiding field connects the whole system at once.

Bohm read that nonlocality as a clue rather than an embarrassment. It suggested to him that separateness is the appearance and wholeness the deeper fact, the idea he later developed as the implicate order. Seen that way, the cat paradox is not a puzzle about cats. It is a symptom of trying to describe an undivided universe with a language built out of separate parts.

Has anything like it been done in a lab?

Not with cats, but the boundary keeps moving. Interference has been demonstrated with electrons, atoms, and large organic molecules containing hundreds of atoms. Superconducting circuits have held billions of electrons in coherent superposition, and vibrating drums and mirrors visible under a microscope have been placed in quantum states. No experiment has yet found a size at which quantum mechanics stops working.

Common questions

Who was Erwin Schrodinger?

An Austrian physicist who wrote the wave equation at the heart of quantum mechanics in 1926 and shared the 1933 Nobel Prize in Physics. He was also deeply interested in Vedanta and wrote about consciousness as fundamentally singular.

Does observing something change it?

In physics, an observation is any irreversible interaction that records information. A detector works perfectly well in an empty room. Nothing in the experiment requires a conscious mind, which is where most popular versions go wrong.

Was the cat harmed?

There was no cat. It is a thought experiment, and Schrodinger chose a deliberately shocking example precisely to make the absurdity land.

Where to go next

• Read our explainer on the double slit experiment for the real experiment behind the paradox.
• Read our guide to pilot wave theory for Bohm's version, in which the cat always has a fate.
• Read our guide to the observer effect for what measurement does and does not mean.
• Read our beginner's guide to quantum physics for the wider picture.