Schrödinger's Cat: The Thought Experiment Physicists Misuse Most

In 1935, Erwin Schrödinger described a cat sealed in a box with a radioactive atom, a Geiger counter, a hammer, and a flask of cyanide. According to the dominant quantum theory of the day, until the box was opened, the cat was simultaneously alive and dead. Schrödinger proposed this scenario as an absurdity — a reductio ad absurdum against quantum theory, not a celebration of it. Somewhere between 1935 and the first time you heard the phrase, the punchline got lost.

What Schrödinger Was Actually Arguing

To understand the thought experiment, you need to understand what Schrödinger was fighting against: the Copenhagen interpretation.

Copenhagen interpretation — the view, developed primarily by Niels Bohr and Werner Heisenberg in the 1920s, that a quantum system does not have definite properties until it is measured. Before measurement, a particle exists in a superposition of all possible states. The act of measurement causes the wave function to "collapse" to a single outcome.

This view worked perfectly for calculating experimental results. Physicists could use it to predict the outcomes of particle experiments with extraordinary precision. The problem, as several of the interpretation's opponents saw it, was that it said almost nothing about what was actually happening between measurements. If the particle "had no definite state" until measured, what exactly was going on before the measurement? And who or what counted as a measurement?

Schrödinger, one of the architects of quantum theory (his wave equation is the central equation of quantum mechanics), found this philosophically intolerable. He and Albert Einstein exchanged a series of letters in the summer of 1935 critiquing the Copenhagen position.

Source: Schrödinger, E. (1935). Die gegenwärtige Situation in der Quantenmechanik. Die Naturwissenschaften, 23(48), 807–812. [Translation: "The Present Situation in Quantum Mechanics"]

Source: Britannica. Schrödinger's cat. [britannica.com]

In the letters, Einstein suggested an analogous argument using unstable gunpowder: if quantum mechanics applied to large objects, a box of gunpowder should be simultaneously exploded and unexploded. Schrödinger took this idea further and made it a cat.

The Mechanism Inside the Box

The apparatus Schrödinger described has four components:

1. A radioactive atom with a 50% probability of decaying within one hour.
2. A Geiger counter connected to the atom — if the atom decays, the counter registers the particle.
3. A hammer connected to the counter, held above a flask.
4. A flask of prussic acid (hydrogen cyanide) — if the hammer falls, the flask shatters and the cat dies.

The cat is sealed in the box with this apparatus for one hour. According to the Copenhagen interpretation, the atom exists in a superposition of decayed and undecayed states until it is observed. If the atom's state is undetermined, then the counter's state is undetermined. If the counter's state is undetermined, the hammer's state is undetermined. And if the hammer's state is undetermined, the cat's state is undetermined — it is, in the formalism of quantum mechanics, simultaneously alive and dead.

Schrödinger's argument was: this is clearly ridiculous. We know cats are either alive or dead. The theory must be missing something — either about when the wave function collapses, or about whether "superposition" refers to objective reality or only to our knowledge of the system.

Think of it like a coin in mid-air. While it is spinning, we cannot predict heads or tails — but the coin is not literally both heads and tails simultaneously. It has a definite state at every moment, we simply lack access to it. Schrödinger believed quantum mechanics was in the same position: the mathematics of superposition described our ignorance, not the world's actual state.

Source: Sciencenotes.org. Schrödinger's cat — A Thought Experiment in Quantum Mechanics. [sciencenotes.org]

The Einstein-Schrödinger Correspondence

Einstein and Schrödinger's 1935 letters are among the most important documents in the philosophy of physics. They were not published contemporaneously — they circulated privately — and they reveal both men's deep discomfort with quantum mechanics as a complete description of reality.

Einstein had already published, with Boris Podolsky and Nathan Rosen, the EPR paper that year — another argument that quantum mechanics must be incomplete, this time using entangled particles to show that the theory implied "spooky action at a distance."

Both Einstein and Schrödinger were committed to what physicists call local realism: the view that physical objects have definite properties at all times (realism) and that the influence of one object on another cannot travel faster than light (locality). Quantum mechanics, in their reading, violated at least one of these principles. The cat thought experiment was Schrödinger's contribution to the case against it.

Local realism — the philosophical position that the physical world has definite properties independent of observation, and that objects can only be influenced by their immediate surroundings, not by distant events.

Source: Quantumzeitgeist.com. Schrödinger's Cat: The Baffling Enigma That Scientists Have Debated for Almost a Century. [quantumzeitgeist.com]

The Paper: Die Naturwissenschaften, 1935

Schrödinger published his cat argument in a three-part paper titled "Die gegenwärtige Situation in der Quantenmechanik" ("The Present Situation in Quantum Mechanics") in the German journal Die Naturwissenschaften in November and December 1935. The paper is a sustained, technical philosophical critique of quantum mechanics — the cat appears near the end as an illustrative device, not as the paper's main subject.

The paper also introduced the concept of entanglement (Verschränkung in German) — Schrödinger coined the term in this paper, calling entanglement "the characteristic trait of quantum mechanics, the one that enforces its entire departure from classical lines of thought."

That is worth sitting with: the same paper that gave us Schrödinger's cat also gave us the word "entanglement," which is now one of the most important concepts in quantum computing and quantum communication.

Source: Schrödinger, E. (1935). Die gegenwärtige Situation in der Quantenmechanik. Die Naturwissenschaften, 23(48), 807–812.

The Timeline

Year	Event
1925–1926	Schrödinger develops wave mechanics; publishes his wave equation
1927	Copenhagen interpretation formalized at the Solvay Conference
1927	Heisenberg publishes the uncertainty principle
1935 (May)	Einstein, Podolsky, Rosen publish the EPR paper
1935 (summer)	Einstein and Schrödinger exchange private letters critiquing quantum completeness
1935 (Nov–Dec)	"Die gegenwärtige Situation in der Quantenmechanik" published in three parts; cat thought experiment introduced
1957	Hugh Everett proposes the many-worlds interpretation as an alternative to Copenhagen
1970	H. Dieter Zeh founds decoherence theory
1991	Wojciech Zurek develops einselection — explains why macroscopic objects appear classical
2019	Experimental test of Wigner's Friend scenario (Proietti et al.) published in Science Advances
2024	Quantum superposition maintained in objects containing ~2,000 atoms in laboratory conditions

What "Superposition" Actually Means

Modern physicists do not say the cat is literally alive and dead at the same time. That description comes from applying the mathematical formalism of quantum mechanics too literally to everyday language. A more careful statement is: before measurement, the system's wave function includes terms corresponding to both outcomes, and we have no experimental way to distinguish which outcome will result.

An analogy: imagine you have a friend in another city who flipped a coin an hour ago and has not told you the result. From your perspective, the coin's state is "heads or tails, unknown." From your friend's perspective, it is definitely one or the other. Schrödinger's point was that quantum mechanics seemed to be claiming something like the first description — objective indeterminacy, not mere ignorance — applied to physical systems. That, he thought, was absurd.

Whether the indeterminacy is objective (the system genuinely has no definite state) or epistemic (we simply lack information) is still one of the most contested questions in the foundations of physics.

Modern Interpretations of the Paradox

The cat has not left the building. The paradox it describes — the measurement problem, also called the quantum-to-classical transition — remains genuinely unsolved. Different interpretations of quantum mechanics handle it differently.

Measurement problem — the question of why and when a quantum system in superposition yields one definite outcome rather than all possible outcomes when measured.

Copenhagen Interpretation (Orthodox)

The wave function collapses upon measurement. The cat becomes definitely alive or definitely dead the moment the box is opened. What happens before opening is simply not a meaningful question — there is no "real" state to speak of. This position is pragmatically successful and philosophically uncomfortable.

Many-Worlds Interpretation (Everett, 1957)

The wave function never collapses. When the box is opened, the universe splits into two branches: one in which the cat is alive and the observer sees it alive, and one in which the cat is dead and the observer sees it dead. Both outcomes occur — in different branches of reality. There is no probability collapse, just splitting.

The many-worlds interpretation is internally consistent and mathematically clean. It requires no special "collapse" rule. It also implies an exponentially large number of constantly multiplying parallel universes, which many physicists find extravagant.

Decoherence (Zeh, Zurek, 1970s–1990s)

The most important modern framework for understanding the cat paradox is decoherence. Decoherence is the process by which a quantum system loses its quantum character through interaction with its environment. When a system becomes entangled with millions of environmental particles (air molecules, photons, walls), interference terms between different states effectively vanish — the system behaves classically.

Decoherence explains why cats look classical even if they are, at some level, quantum systems. The cat interacts with so many environmental particles per second that quantum superpositions of "alive" and "dead" decohere on a timescale far shorter than any possible measurement — roughly 10^-23 seconds. This is not collapse; it is spreading of quantum correlations into the environment at such a scale that they become practically undetectable.

Decoherence theory does not fully solve the measurement problem — it explains why we don't see superpositions at the cat scale, but it does not explain why any individual measurement has one definite outcome. That residual mystery remains.

Source: Decoherence of a Macroscopic Quantum Superposition. [arxiv.org: 0809.3889]

QBism (Quantum Bayesianism)

A more recent interpretation, QBism treats the wave function not as a description of physical reality but as an agent's personal probability assignments for future experiences. In this view, the cat question doesn't arise — the wave function is not a description of the box's contents, it is a description of what the experimenter should expect upon opening the box. The cat is always either alive or dead; the wave function describes beliefs, not facts.

Objective Collapse Theories

Proposals by Roger Penrose and GianCarlo Ghirardi suggest a physical mechanism that spontaneously collapses wave functions when systems become large enough. These theories make testable predictions — they predict that superpositions of large objects should collapse faster than quantum mechanics expects. So far, experiments have not confirmed the additional collapse, but they have not definitively ruled it out either.

Source: Builtin.com. What Is Schrödinger's Cat? [builtin.com]

The Wigner's Friend Extension

In 1961, physicist Eugene Wigner proposed a related thought experiment that deepens the cat paradox. Imagine a friend performs a measurement inside a sealed lab, records the result, then the lab is sealed again. From the outside, before you open the lab, is your friend in a superposition of "saw alive" and "saw dead"?

The Wigner's Friend scenario shifts the measurement problem from quantum particles to conscious observers. In 2019, Massimiliano Proietti and colleagues at Heriot-Watt University published an experiment in Science Advances that operationalized this scenario using photons and two "observers" (detectors) in separated labs. The results were consistent with quantum mechanics and inconsistent with the assumption that different observers always agree on a single objective measurement outcome — a challenge to certain realist interpretations.

Source: Proietti, M. et al. (2019). Experimental test of local observer-independence. Science Advances, 5(9), eaaw9832.

Macroscopic Superposition Today

Experimental physicists have been progressively pushing quantum superpositions toward larger and larger objects, testing where the quantum-classical boundary actually lies. Recent milestones:

• 2011: Quantum superpositions demonstrated in molecules containing 430 atoms (University of Vienna, Markus Arndt's group).
• 2019: Superpositions demonstrated in objects of approximately 25,000 atomic mass units.
• Ongoing: Proposals to test for superposition in viruses, micron-scale mechanical oscillators cooled to their quantum ground state.

None of these experiments involve cats. But they establish that quantum superpositions can persist in progressively larger systems, provided decoherence is suppressed by isolating the object from environmental noise. The question remains: is there a fundamental size limit at which superposition becomes impossible, or can quantum mechanics in principle apply to any scale given sufficient isolation?

Source: Schrödinger's cat — Grokipedia. [grokipedia.com]

What the Cat Paradox Reveals

Schrödinger's cat has survived 90 years not because it is a clean, resolved problem, but because it points at a genuine gap in our understanding. Quantum mechanics is the most precisely tested theory in the history of science — its predictions match experiment to 12 decimal places in some tests. And yet physicists still do not agree on what the theory is describing.

The cat paradox is essentially asking: what is the relationship between the mathematical machinery of quantum mechanics and the physical world it describes? That is a question about the interpretation of a scientific theory, not about the theory's accuracy. On the accuracy, there is no dispute. On the interpretation, the debate Schrödinger started in 1935 is still running.

See also Erwin Schrödinger for more on the physicist's broader contributions to wave mechanics.

Source: Predictivehistory.com. Schrödinger's Cat: The Thought Experiment That Broke Physics. [predictivehistory.com]

From Thought Experiment to Real Macroscopic Superposition

Schrödinger designed his cat as a thought experiment because putting a macroscopic object into genuine quantum superposition seemed obviously impossible in 1935. In the decades since, experimental physicists have been methodically proving that assumption wrong — not with cats, but with progressively larger molecules, and eventually with objects visible to the naked eye under the right conditions.

The relevant constraint is decoherence. When a quantum system interacts with its environment — air molecules, photons, thermal vibrations — the quantum coherence between its superposed states disperses into those environmental interactions on timescales that scale inversely with the system's mass. For a dust grain in open air, decoherence is effectively instantaneous. For a carefully isolated molecule in a high-vacuum interferometer, coherence can persist long enough to measure.

Markus Arndt's group at the University of Vienna has pushed this boundary systematically. In 2011, they demonstrated quantum interference — a signature of superposition — in molecules containing 430 atoms, specifically phthalocyanine derivatives with a mass of around 614 atomic mass units. In 2019, the same group's work with oligo-tetraphenylporphyrins and functionalized oligomers established superposition in objects of approximately 2,000 atomic mass units, containing around 2,000 atoms. These are not elementary particles. Under an electron microscope they look like small but real molecular structures.

Source: Eibenberger, S., Gerlich, S., Arndt, M., Mayor, M., & Tüxen, J. (2013). Matter-wave interference of particles selected from a molecular library with masses exceeding 10,000 amu. Physical Chemistry Chemical Physics, 15(35), 14696–14700.

Source: Fein, Y. Y., Geyer, P., Zwick, P., Kiałka, F., Pedalino, S., Mayor, M., Gerlich, S., & Arndt, M. (2019). Quantum superposition of molecules beyond 25 kDa. Nature Physics, 15, 1242–1245.

Roger Penrose has argued that the question of where superposition ends is not merely an engineering challenge to be solved with better vacuum systems. In his gravitational collapse hypothesis, published in General Relativity and Gravitation in 1996, Penrose proposes that spacetime curvature introduces a fundamental instability into superpositions involving mass distributions above a threshold he calculates at roughly 10^-8 grams — around 10^14 atomic mass units, still far above what has been tested experimentally. Below that threshold, quantum mechanics operates cleanly. Above it, gravity collapses the superposition without any observer needed.

Source: Penrose, R. (1996). On gravity's role in quantum state reduction. General Relativity and Gravitation, 28(5), 581–600.

The MAQRO satellite mission proposal, developed by Rainer Kaltenbaek and colleagues, would test these ideas in space. Low-Earth orbit eliminates thermal vibrations and gravity gradients that limit ground-based experiments, potentially allowing superposition tests on nanoparticles of 10^8 to 10^10 atomic mass units — approaching the regime where Penrose-type collapse would be detectable if it exists. The proposal has been submitted to ESA as a medium-class mission concept but has not yet been selected for flight.

The 2019 Wigner's Friend experimental test by Proietti and colleagues at Heriot-Watt University operationalized a scenario closely related to the measurement problem. Using photon pairs and two independent "observers" (detectors in separated optical circuits), they demonstrated results inconsistent with the assumption that observers always agree on objective measurement outcomes. This does not resolve the measurement problem — it sharpens it. Even in a world where we can build experiments to probe the quantum-classical boundary, the interpretation of what those experiments reveal remains genuinely contested.

Source: Proietti, M. et al. (2019). Experimental test of local observer-independence. Science Advances, 5(9), eaaw9832.

The gap between Schrödinger's 2,000-atom molecule and a real macroscopic cat — which contains roughly 10^27 atoms — is enormous, and no physicist expects to close it in the foreseeable future. But the direction of travel matters: the boundary between quantum and classical is not fixed. It is wherever our ability to suppress decoherence ends. That it is a practical limit rather than a fundamental one is itself a significant result, and one that Schrödinger in 1935 would have found genuinely surprising.

FAQs

Q: Is Schrödinger's cat a real experiment? A: No. It is a thought experiment — a hypothetical scenario designed to expose a philosophical problem, not something that could be or was physically performed. Schrödinger was asking: if quantum mechanics applies to atoms, and atoms are connected to macroscopic objects, then shouldn't quantum mechanics produce absurd results at the macroscopic scale? The cat was his illustration of this absurdity.

Q: Was Schrödinger arguing that the cat is actually both alive and dead? A: No. Schrödinger was arguing the opposite — that the prediction of a "simultaneously alive and dead" cat was absurd and therefore evidence that quantum mechanics was incomplete or misinterpreted. The popular version of the thought experiment, which celebrates the cat's dual state as a profound quantum fact, has the punchline backwards.

Q: What does "wave function collapse" mean? A: Before measurement, quantum mechanics describes a system with a wave function — a mathematical object that encodes the probability of each possible outcome. Upon measurement, the system "collapses" to one of those outcomes. The wave function for the atom might assign 50% probability to decayed and 50% to undecayed; upon measurement, it becomes 100% one or the other. The collapse happens instantaneously, which is what bothered Einstein and Schrödinger — it seemed to imply something nonlocal.

Q: Do quantum effects apply to cats in any real sense? A: In principle, quantum mechanics describes all matter, including cats. In practice, decoherence ensures that quantum superpositions in macroscopic objects collapse on timescales far faster than any possible measurement — roughly 10^-23 seconds for an object the size of a cat. So while a cat is "quantum" in the sense that its atoms obey quantum mechanics, it is "classical" in the sense that quantum coherence across macroscopic states doesn't persist long enough to matter.

Q: What is entanglement, and why did Schrödinger name it in this paper? A: Quantum entanglement occurs when two or more particles are correlated in such a way that the state of each cannot be described independently of the others, even when they are separated by large distances. Schrödinger coined the term Verschränkung (entanglement) in the same 1935 paper as the cat thought experiment because both concepts arose from his critique of the completeness of quantum mechanics.

Q: Which interpretation of quantum mechanics is correct? A: No consensus exists. Copenhagen remains the default "shut up and calculate" approach used in most physics curricula. Many-worlds has strong advocates among quantum information researchers. Decoherence theory is widely accepted as explaining the classical appearance of the macroscopic world, but it does not fully resolve the measurement problem. The debate is ongoing and is one of the deepest open questions in the foundations of physics.

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Read next:

• Erwin Schrödinger: The Physicist Who Hated His Own Cat
• The Double-Slit Experiment
• Werner Heisenberg and the Uncertainty Principle