Schrödinger's Cat Explained Simply

The Original Thought Experiment

Erwin Schrödinger published his cat thought experiment in 1935, in a paper titled "The Present Situation in Quantum Mechanics." The setup:

"A cat is placed in a sealed box with a small amount of radioactive substance with a 50% probability of decaying in one hour. If decay occurs, a Geiger counter triggers a hammer that shatters a flask of hydrocyanic acid, killing the cat. If no decay, the cat lives."

The radioactive atom is a quantum system — before observation, it is in a superposition of "decayed" and "not decayed." According to quantum mechanics, the entire system (atom + detector + poison + cat) becomes entangled into a superposition of states: the cat is simultaneously dead and alive until the box is opened and the system is observed.

What Schrödinger Was Actually Arguing

Here is the most important thing to understand: Schrödinger invented this thought experiment to argue against the Copenhagen interpretation — not to celebrate it.

His point was a reductio ad absurdum. The Copenhagen interpretation (Bohr, Heisenberg) holds that quantum systems exist in superposition until measured, and that the wave function is a complete description of reality. Schrödinger was saying: "If you take this seriously, you're forced to conclude that a cat can be simultaneously dead and alive — which is obviously ridiculous. Therefore, there must be something wrong with how we're interpreting quantum mechanics."

He was right that there's a problem. The measurement problem — when and why does the quantum world of superpositions give rise to the definite classical world we observe? — remains one of the deepest open questions in the foundations of physics.

The Wave Function and Superposition

In quantum mechanics, the state of a system is described by its wave function ψ. The probability of finding a particle in a particular state is |ψ|². Before measurement, a radioactive atom in superposition is described by:

If the cat's fate is coupled to the atom, the total state becomes entangled:

This is mathematically exact — the formalism of quantum mechanics produces it directly. The question is what it means. Different interpretations of quantum mechanics give radically different answers.

How Different Interpretations Solve It

1. Copenhagen Interpretation (Bohr, 1927)

The wave function is a calculational tool, not a description of reality. Physical properties are only defined at the moment of measurement by a "classical" observer or apparatus. Before measurement, it makes no sense to ask whether the cat is alive or dead — the question is meaningless. "Shut up and calculate."

Problem: What counts as a "measurement"? What is the boundary between quantum and classical? This is never precisely defined.

2. Many-Worlds Interpretation (Everett, 1957)

The wave function never collapses. Instead, when the box is opened, the observer splits into two branches of the universe: one where they see a live cat, one where they see a dead cat. Every quantum event branches the universe into all its possible outcomes. All branches are equally real.

Strength: Mathematically simple — just unitary quantum mechanics, no collapse postulate needed. Problem: What determines which branch "we" end up in? The Born rule (probability rule) has to be derived, not postulated — this remains controversial.

3. Pilot Wave Theory / Bohmian Mechanics (de Broglie, Bohm)

Particles always have definite positions and velocities, guided by a real wave. The cat is always definitely alive or definitely dead — but our ignorance of the initial conditions makes quantum probabilities necessary. No collapse, no branching.

Problem: Non-local by construction (as proven by Bell's theorem). Controversial to extend to relativistic quantum field theory.

4. Objective Collapse Theories (GRW, Penrose)

The wave function does physically collapse — but through a real, objective physical process, not just through "observation." GRW theory adds stochastic collapse terms to the Schrödinger equation. Penrose proposes that collapse occurs when two branches have sufficiently different spacetime geometries (quantum gravity). Macroscopic objects collapse essentially instantly; individual particles almost never.

Quantum Decoherence — The Modern Resolution

The most widely accepted practical resolution (though not a final interpretation) is quantum decoherence, developed by Dieter Zeh, Wojciech Zurek, and others from the 1970s onwards.

A macroscopic cat contains on the order of 10²⁸ particles, each constantly interacting with air molecules, photons, and each other. These interactions entangle the cat's quantum states with the environment at an extraordinary rate. The timescale for this environmental entanglement to make quantum superpositions unmeasurably small is:

where λ_dB is the thermal de Broglie wavelength and d is the object's size. For a cat-sized object at room temperature, decoherence occurs in roughly 10⁻³⁰ seconds — far too fast for any macroscopic superposition to persist.

Decoherence does not solve the measurement problem philosophically — it explains why we never see dead-and-alive cats in practice, but it doesn't explain why we see one definite outcome rather than another. That remains the hard problem.

Real Schrödinger Cat Experiments

Counterintuitively, physicists have created actual Schrödinger cat states — not with literal cats, but with mesoscopic quantum systems large enough to straddle the quantum-classical boundary:

• 1996 — Haroche group (ENS Paris): Photon fields in microwave cavities placed in superpositions of two distinct classical states. Won the 2012 Nobel Prize (Haroche and Wineland).
• 2000 — NIST: Trapped beryllium ions put in superpositions of two distinct motional states ("left" and "right" positions), separated by 80 nanometres — measurable by 2010 standards as a genuinely macroscopic quantum superposition.
• 2018 — University of Vienna: Molecules of 2,000 atoms (fullerene-based compounds) demonstrated quantum interference — the largest objects to show wave-particle duality to date.
• 2021 — MIT (nanomechanical resonators): Macroscopic mechanical oscillators (~10¹³ atoms) cooled to quantum ground states and entangled.

Each of these experiments probes the quantum-to-classical transition, providing increasingly stringent tests of where quantum mechanics breaks down (if it does) — or confirming that it doesn't, right up to larger and larger scales.

Frequently Asked Questions

Is Schrödinger's cat alive or dead?

Standard QM says it's in superposition until observed — but this was Schrödinger's point about the absurdity of that answer. Decoherence theory says the cat effectively collapses in 10⁻³⁰ s, so in practice it's always one or the other.

Was Schrödinger's cat meant to be taken seriously?

No — it was a reductio ad absurdum argument against the Copenhagen interpretation. Schrödinger was criticising quantum mechanics, not proposing that cats can be both dead and alive.

What resolution do most physicists accept today?

Most working physicists use Copenhagen interpretation operationally ("shut up and calculate"), accept decoherence as the practical explanation for why we don't see macroscopic superpositions, and acknowledge the measurement problem as philosophically unresolved. Many-worlds has growing theoretical support.