A thought experiment that became a symbol of quantum paradox. How can something be simultaneously alive and dead? The explanation that changes everything.
🐱 A thought experiment born as satire
In 1935, the Austrian physicist Erwin Schrödinger published in the journal Naturwissenschaften a paper titled “The present situation in quantum mechanics.” Within it, he described a thought experiment that today is perhaps the most recognizable symbol of quantum physics. Schrödinger did not intend to showcase something wondrous — on the contrary, he wanted to demonstrate the absurdity of the interpretation that dominated his era.
The stimulus was a correspondence with Albert Einstein. Earlier that year, Einstein, together with Boris Podolsky and Nathan Rosen, had published the famous EPR (Einstein-Podolsky-Rosen) paper, which challenged the completeness of quantum mechanics. In a letter to Schrödinger, Einstein proposed an example: a keg of gunpowder that after a while would exist in a superposition — simultaneously exploded and unexploded. Schrödinger took this idea and turned it into something even more dramatic.
💡 The Central Paradox
According to quantum mechanics, a particle can exist in a superposition of states — simultaneously in two or more states. Schrödinger showed that if you link an atomic event to a macroscopic object (a cat), quantum logic leads to a cat that is simultaneously alive and dead.
📦 A cat in a steel box
Schrödinger's description is remarkably precise. A cat is placed inside a sealed steel chamber together with an “infernal device.” This device includes: a Geiger counter, a tiny amount of radioactive substance — so small that in one hour perhaps one atom will decay, but perhaps none — and a flask of hydrocyanic acid (poison). If an atom decays, the Geiger counter detects it, activates a relay, a hammer shatters the flask, and the cat dies. If no atom decays, the cat remains alive.
Schrödinger noted that, according to quantum mechanics, the wave function (ψ-function) of the entire system would contain the living and the dead cat “mixed or spread out in equal parts.” Neither alive, nor dead — but both simultaneously. What he himself called a “completely burlesque case” (ganz burleske Fälle) in the original German text.
"An indeterminacy originally restricted to the atomic domain turns into a macroscopic indeterminacy, which can then be resolved by direct observation."
— Erwin Schrödinger, 1935⚖️ Reductio ad absurdum: a critique, not a proposal
It is important to understand what exactly Schrödinger intended. He did not propose that the cat is literally simultaneously alive and dead. Instead, he used the method of reductio ad absurdum — proof by contradiction — to show that the Copenhagen interpretation, advocated by Niels Bohr and Werner Heisenberg, leads to absurd conclusions when applied to macroscopic objects.
Schrödinger himself was not pleased with the implications of quantum theory. In a characteristic statement, he declared: “I don't like it, and I'm sorry I ever had anything to do with it.” Einstein fully agreed.
"You are the only contemporary physicist, besides Laue, who sees that one cannot get around the assumption of reality."
— Albert Einstein, letter to Schrödinger, 1950⚛️ How superposition works: the root of the problem
In the quantum world, a particle can exist in a superposition of states (quantum superposition). An electron can “spin” simultaneously clockwise and counterclockwise. A photon can travel simultaneously along two paths. This is not a theoretical hypothesis — it is experimentally confirmed in every quantum physics laboratory in the world.
The problem appears at measurement. The moment we observe a particle, the superposition “collapses” into a single state. But what exactly constitutes a “measurement”? When does quantum behavior end and classical behavior begin? This is the measurement problem — one of the greatest open questions in physics.
🌍 The interpretations: from Copenhagen to many worlds
The Copenhagen interpretation, the most widespread, declares that the wave function does not represent a physical reality — it is a tool for calculating probabilities. The cat is not “really” alive and dead, but we simply cannot say anything until we open the box.
In 1957, Hugh Everett III proposed a radically different solution: the many-worlds interpretation. In this view, there is no collapse. At the moment of measurement, the universe branches — in one branch the cat is alive, in another dead. Both versions are equally real, but they cannot communicate with each other.
Decoherence, developed by H. Dieter Zeh in 1970 and Wojciech Zurek in the 1980s, offered a more practical explanation. When a quantum system interacts with its environment — other molecules, photons, thermal radiation — the superposition “dissolves” extremely rapidly. For an object the size of a cat, the decoherence time is so short that practically the cat is never in superposition. Decoherence explains why we don't see quantum properties in the everyday world, but — as both Zeh and Zurek acknowledge — it does not fully solve the measurement problem.
🔬 Experimental "cats": from ions to trillions of atoms
No one has ever placed a real cat in quantum superposition. But scientists have created “cat states” in increasingly larger systems. In 1996, Chris Monroe and colleagues at NIST managed to place a beryllium ion in superposition — simultaneously in two different physical positions.
Scientists built superconducting SQUID (Superconducting Quantum Interference Device) devices, where billions of electrons flow simultaneously in both directions of a loop. A piezoelectric “tuning fork” of 10 trillion atoms was placed in a superposition of vibrating and non-vibrating states. And in 2010, NIST researchers created “quantum cats” made of photons.
Along this direction, French physicist Serge Haroche and his team at the École Normale Supérieure in Paris quantitatively measured the decoherence process for the first time in 1996, sending individual rubidium atoms in superposition through a microwave cavity. This work led to the 2012 Nobel Prize in Physics, shared by Haroche with David Wineland.
💻 The cat in modern culture and technology
According to historian of science Robert P. Crease, Schrödinger's cat did not become widely known until the 1970s. Science fiction writer Ursula K. Le Guin discovered the thought experiment while researching material for her novel The Dispossessed (1974), and her short story “Schrödinger's Cat” introduced it to popular culture. Since then, the cat appears everywhere — in films, TV series, comics, music, poetry, and even memes.
In quantum technology, the term “cat state” is used today to describe GHZ (Greenberger-Horne-Zeilinger) states — superpositions of many qubits that simultaneously exist in |000...0⟩ and |111...1⟩. These states are fundamental for quantum error correction and testing quantum computers.
♾️ The cat that never dies
Ninety years after its publication, Schrödinger's thought experiment remains alive. There is still no agreement on when exactly quantum superposition “collapses” — or whether it collapses at all. Decoherence explains why we don't see macroscopic superpositions, but does not answer why only one outcome becomes reality. Every new interpretation of quantum mechanics — and there are over a dozen — must confront Schrödinger's cat.
Schrödinger himself, born in Vienna in 1887, was awarded the Nobel Prize in Physics in 1933 alongside Paul Dirac for wave mechanics. He died in January 1961. On his grave in Alpbach, Austria, above the nameplate, his equation is inscribed: iℏΨ̇ = HΨ. But in collective memory, we don't remember him for the equation. We remember him for a cat that no one ever put in a box.