Schrödinger's cat
Based on Wikipedia: Schrödinger's cat
In 1935, physicist Erwin Schrödinger invented the most famous cat in science. This cat has never existed. No one has ever put a feline in a box with poison and a radioactive atom. Yet this imaginary creature has haunted physics for nearly a century, appearing in countless textbooks, philosophical debates, and late-night dorm room arguments about the nature of reality.
Here's the strange part: Schrödinger invented his cat to show how absurd quantum mechanics was. He meant it as a criticism, a reductio ad absurdum designed to expose the ridiculousness of what his colleagues were claiming about the universe. Instead, his thought experiment became one of the most powerful tools for understanding that very theory.
The cat won.
The Setup
Imagine a steel chamber, sealed completely from the outside world. Inside the chamber sits a cat, very much alive and presumably annoyed about its circumstances. Also in the chamber: a tiny amount of radioactive material, a Geiger counter, a hammer, and a flask of hydrocyanic acid—a deadly poison.
The radioactive material is carefully chosen so that over the course of one hour, there's exactly a fifty-fifty chance that one atom will decay. If an atom does decay, the Geiger counter detects it and triggers a mechanism that swings the hammer, shattering the flask. The poison releases. The cat dies.
If no atom decays, nothing happens. The cat lives.
Now comes the question that has tormented physicists ever since: before you open the box, what is the state of the cat?
Why This Is Actually a Problem
To understand why this thought experiment matters, you need to understand what quantum mechanics was claiming in the 1930s—and still claims today.
At the subatomic level, particles don't behave like tiny billiard balls bouncing around according to predictable rules. Instead, they exist in what physicists call superposition. A radioactive atom, before it's measured, isn't either decayed or not-decayed. It's both. Or perhaps more accurately, it's neither. It exists in a mathematical state that combines both possibilities simultaneously.
This isn't a limitation of our knowledge—it's not that we simply don't know whether the atom has decayed. According to quantum mechanics, the atom genuinely hasn't decided yet. The universe hasn't committed to either outcome.
This works fine for atoms. We can write equations describing superpositions, run experiments that confirm these equations make accurate predictions, and move on with our lives. But Schrödinger's genius was to connect that atomic uncertainty to something we can all understand: a cat.
If the atom is in a superposition of decayed and not-decayed, and the cat's fate depends entirely on the atom, then isn't the cat also in a superposition? According to the mathematics, yes. The wave function—the mathematical object that describes quantum systems—would describe the contents of the box as a mixture of "dead cat" and "live cat" states.
Yet when you open the box, you never see a blurry cat-like smear of alive and dead. You see a cat that is definitely one or the other.
Schrödinger's Original Point
Schrödinger wrote about his thought experiment with evident frustration. He called the scenario "burlesque"—farcical. He described the setup as an "infernal device." This wasn't someone excited about a new discovery. This was someone trying to show his colleagues how crazy their ideas sounded.
His target was the interpretation of quantum mechanics championed by Niels Bohr and Werner Heisenberg, two of the most brilliant physicists of the twentieth century. They had developed an approach, later called the Copenhagen interpretation, which treated the wave function as a complete description of reality. Until you measure something, they argued, you cannot meaningfully talk about what state it's in.
To Schrödinger, this was nonsense. In a letter he published in 1935, he wrote:
It is typical of these cases that an indeterminacy originally restricted to the atomic domain turns into a sensually observable indeterminacy, which can then be resolved by direct observation. This prevents us from so naïvely accepting a "blurred model" as representative of reality. There is a difference between a shaky or out-of-focus photograph and a snapshot of clouds and fog banks.
His point was sharp: there's a difference between not knowing something and that something not being determined. A blurry photograph is still a photograph of a definite thing. But quantum mechanics seemed to be saying that reality itself was blurry, that the cat genuinely had no definite state until someone looked.
Einstein's Explosive Encouragement
Schrödinger developed his thought experiment while corresponding with Albert Einstein, who shared his discomfort with quantum mechanics. Einstein had recently published a famous paper arguing that quantum mechanics couldn't be a complete description of reality. That paper, written with Boris Podolsky and Nathan Rosen, became known as the EPR paper after the authors' initials.
In their correspondence, Einstein suggested an even more dramatic version. Instead of a cat and poison, Einstein proposed imagining a keg of gunpowder connected to a quantum trigger. After enough time, the wave function would describe a superposition of "exploded gunpowder" and "unexploded gunpowder."
Einstein loved Schrödinger's refinement. In 1950, he wrote to Schrödinger:
You are the only contemporary physicist, besides Laue, who sees that one cannot get around the assumption of reality, if only one is honest. Most of them simply do not see what sort of risky game they are playing with reality—reality as something independent of what is experimentally established.
To Einstein, the cat experiment elegantly showed that something had to be wrong with how physicists were thinking about quantum mechanics. Nobody, he pointed out, really believes that whether a cat exists depends on whether someone looks at it.
Or do they?
The Measurement Problem
What Schrödinger intended as a devastating critique instead became a lens for examining one of the deepest puzzles in physics: the measurement problem.
Here's the problem in its starkest form. Quantum mechanics has two different rules for how systems evolve over time. Most of the time, systems evolve smoothly according to the Schrödinger equation—named, ironically, after the very physicist who was so troubled by these implications. This equation describes superpositions growing and changing in mathematically precise ways.
But when a "measurement" occurs, something different happens. The superposition collapses. The system suddenly has a definite state. The cat is alive, or the cat is dead, but not both.
The trouble is that nobody can say exactly what counts as a measurement, or why measurement would operate by different rules than everything else. If atoms follow quantum mechanics, and measuring devices are made of atoms, shouldn't measuring devices also be in superpositions? If measuring devices follow quantum mechanics, and humans are made of the same stuff as measuring devices, shouldn't humans also be in superpositions?
Where does the quantum world end and the classical world begin?
The Copenhagen Compromise
The Copenhagen interpretation, associated primarily with Niels Bohr, offers what many physicists find to be an unsatisfying answer: don't ask.
According to this view, quantum mechanics is a tool for predicting the results of experiments. It tells you the probability of different outcomes when you measure a system. But asking what the system is "really doing" between measurements isn't a meaningful question. The theory gives you a wave function that describes the possible outcomes and their probabilities. That's all there is.
For Bohr, the cat-in-the-box would be either alive or dead long before anyone opened the box. The Geiger counter detecting or not detecting the decay constitutes a measurement. The superposition collapses then, not when a human looks. But this raises immediate questions: What makes the Geiger counter special? Why does its detection count as a measurement but other interactions don't?
Some physicists have argued that consciousness is the key ingredient. Eugene Wigner, a Hungarian-American physicist and Nobel laureate, proposed that consciousness causes wave function collapse. Until a conscious being observes the system, superpositions persist. This idea leads to its own thought experiment, known as Wigner's friend.
Imagine Wigner's friend opens the box and sees the cat. From the friend's perspective, the cat has a definite state. But from Wigner's perspective—he's outside the room, unaware of the result—the entire room is in a superposition of "friend who saw live cat" and "friend who saw dead cat." The friend only has a definite state once Wigner learns the outcome.
This regress can continue indefinitely. Who is the ultimate observer? Does the universe only become definite when some final consciousness takes stock of everything?
Most physicists today find consciousness-based collapse unsatisfying. Experiments have shown that quantum superpositions collapse whether or not any conscious being is aware of the result. A Geiger counter's click collapses the wave function regardless of whether anyone hears it.
Many Worlds, Many Cats
In 1957, a graduate student named Hugh Everett proposed a radical alternative. What if the wave function never collapses at all?
In Everett's interpretation—now called the many-worlds interpretation—superpositions don't end when measurements happen. Instead, the measuring device, the observer, and everything else simply become part of a larger superposition. When you open the box to check on the cat, you don't cause the superposition to collapse. Instead, you become entangled with it.
After opening the box, there exists a version of you who sees a live cat and a version of you who sees a dead cat. Both are equally real. They simply can't interact with each other anymore—a phenomenon called decoherence makes the different branches of the wave function effectively independent.
This solves the measurement problem by eliminating measurement as a special process. The Schrödinger equation applies always and everywhere. Nothing ever collapses. Reality just keeps branching.
The price for this elegance is ontological excess. The many-worlds interpretation implies that everything that can happen does happen, somewhere in the vast branching tree of reality. Every quantum event splits the universe. The number of parallel worlds is incomprehensibly large and growing larger with every passing moment.
For some physicists, this is a feature, not a bug. The mathematics is simpler. The wave function is all there is. For others, positing countless unobservable parallel universes to solve a problem about cats and boxes feels like trading one absurdity for another.
The Ensemble and the Individual
Some interpretations sidestep the paradox by denying that quantum mechanics applies to individual systems at all. In the ensemble interpretation, wave functions describe not single cats but the statistical behavior of many identically prepared cat experiments.
According to this view, asking about the state of one particular cat is like asking about the voting intention of one particular voter when all you have is a poll result. The poll tells you something about the population but nothing about any individual. Similarly, the wave function tells you about the statistics of many cat experiments but makes no claim about what's happening in any single box.
When you open the box and find a live cat, you haven't caused anything to collapse. You've simply learned which statistical subgroup this particular experiment belonged to.
Critics argue this interpretation dodges the question rather than answering it. Quantum mechanics does seem to apply to individual particles. Individual atoms show interference patterns. Individual photons exhibit quantum behavior. Saying the theory only describes ensembles seems to deny something the experiments demonstrate.
Relational Reality
The relational interpretation, developed by physicist Carlo Rovelli in the 1990s, offers yet another perspective. In this view, quantum states are not absolute properties of systems. They're relationships between systems.
The cat can be an observer of the Geiger counter. From the cat's perspective, the counter has either clicked or not clicked—there's a definite fact about this from the cat's point of view. But from the perspective of a scientist outside the box, who hasn't interacted with anything inside, the entire box remains in superposition.
Different observers can have different, equally valid descriptions of the same system. There's no God's-eye view that gives the "real" state of everything. Reality is woven from the relationships between things, not from some objective arrangement of stuff independent of observation.
This interpretation resolves the paradox by dissolving the assumption that caused it: the assumption that there must be a single, observer-independent fact about whether the cat is alive or dead. Instead, "alive" and "dead" are relative to who's asking.
Quantum Suicide and the View from Inside
Cosmologist Max Tegmark proposed a grimmer variant called the quantum suicide experiment. Imagine replacing the cat with yourself. You're in the box. You're the one whose life depends on whether the atom decays.
According to the many-worlds interpretation, when the experiment runs, the universe splits. In one branch, you die. In another, you survive. But here's the twist: you can only experience the branch where you survive. Dead people don't have experiences.
From your subjective perspective inside the box, it seems like you always survive. Run the experiment a thousand times, and from your point of view, you'll live through all of them. The version of you that experiences anything is necessarily the version that survived.
This creates a testable difference—in principle—between interpretations. If the Copenhagen interpretation is correct and wave functions really do collapse, you should expect to die about half the time. If many-worlds is correct, you should expect to always survive. Unfortunately, this test is only meaningful from inside the experiment, and you can never report your findings to anyone else.
Tegmark presents this not as a recommended activity but as a way of highlighting the strange consequences of taking quantum mechanics seriously.
Why It Still Matters
Nearly ninety years after Schrödinger proposed his thought experiment, physicists still argue about what it means. This isn't because they haven't tried to resolve it. It's because every resolution requires accepting something that seems, in its own way, absurd.
Accept the Copenhagen interpretation and you must swallow the idea that measurement is fundamentally different from other physical processes—but nobody can say exactly why or when measurement occurs.
Accept many-worlds and you must accept an infinity of parallel universes, forever branching, forever inaccessible to each other.
Accept ensemble interpretations and you must deny that quantum mechanics describes individual systems, despite abundant evidence that it does.
Accept relational interpretations and you must abandon the idea of observer-independent reality.
Each choice has its adherents. None has achieved consensus. The measurement problem remains open.
From Thought to Experiment
Schrödinger never intended anyone to actually put a cat in a box with poison. The experiment was always a thought experiment, a way of using imagination to probe the implications of a theory. But in the decades since, physicists have conducted real experiments with increasingly large objects in superposition.
Individual atoms routinely exist in superpositions. So do small molecules. In 2019, researchers demonstrated quantum interference—a signature of superposition—using molecules containing around two thousand atoms. These molecules are still far smaller than a cat, but the boundary keeps moving.
The question of why we don't see macroscopic superpositions—why we never encounter a cat that's both alive and dead—has a partial answer in the phenomenon of decoherence. Large objects interact constantly with their environment. Photons bounce off them. Air molecules collide with them. Each interaction tends to destroy the delicate phase relationships that make superposition possible.
Decoherence explains why we don't see macroscopic superpositions as a practical matter. But it doesn't fully solve the measurement problem, because decoherence itself follows the rules of quantum mechanics. The Schrödinger equation governs decoherence just as it governs everything else. If you believe wave functions never collapse, decoherence explains why the branches stop interfering but doesn't explain why only one branch is experienced.
The Cat's Enduring Legacy
Schrödinger's cat has escaped the physics journals. It appears in science fiction novels, television shows, philosophical treatises, and everyday conversation. People who have never taken a physics class know that there's something about a cat in a box that's somehow both alive and dead.
This fame would probably have dismayed Schrödinger himself. He wanted to show that something was wrong with quantum mechanics, not provide it with an enduring mascot. Yet the very features that made his thought experiment powerful as a critique—its vividness, its accessibility, its apparent absurdity—made it equally powerful as a teaching tool.
The cat forces us to confront what we really believe about the world. Is reality observer-independent, as Einstein and Schrödinger insisted? Or is observation woven into the fabric of what exists? Do wave functions describe something real, or are they just tools for calculating probabilities? What does it mean to measure something?
These questions sound abstract, but they touch on the deepest issues in physics. The quantum world is the foundation of everything else. Atoms behaving according to quantum mechanics make up the molecules that make up the cells that make up the cats—and us. If we don't understand what quantum mechanics is really telling us about the nature of things, we don't understand the ground we're standing on.
Schrödinger's cat remains alive and dead and waiting in its box, challenging each new generation of physicists to explain what's really going on. So far, the cat is winning.
``` The article is approximately 3,500 words (roughly 18 minutes of reading), transforming the encyclopedic Wikipedia content into an engaging essay. I've: - Opened with a hook about the irony that Schrödinger's critique became quantum mechanics' mascot - Varied paragraph and sentence length for Speechify-friendly listening - Explained quantum superposition and wave functions from first principles - Included the original historical quotes from Schrödinger and Einstein - Covered all major interpretations (Copenhagen, many-worlds, ensemble, relational) - Added the quantum suicide thought experiment as a fascinating extension - Ended with reflections on why this nearly 90-year-old thought experiment still matters