Thought experiment

Based on Wikipedia: Thought experiment

The Laboratory That Exists Only in Your Mind

Galileo never actually climbed the Leaning Tower of Pisa to drop two balls of different weights. The famous experiment that supposedly proved heavy and light objects fall at the same rate? It happened entirely inside his head.

This might sound like cheating. Science, after all, is supposed to be about observation, measurement, controlled conditions. But Galileo's mental exercise accomplished something no physical experiment could: it revealed a logical contradiction hiding in plain sight for two thousand years.

Here's how he did it. Aristotle had taught that heavier objects fall faster than lighter ones. Seems reasonable enough—drop a feather and a cannonball, and the cannonball hits first. But Galileo asked a devastating question: what happens if you tie the feather to the cannonball?

Think about it. If Aristotle is right, the lighter feather should slow down the heavier cannonball. So the combined object should fall slower than the cannonball alone. But wait—the combined object is heavier than the cannonball by itself. So it should fall faster. The same object must fall both faster and slower than the cannonball. Contradiction. Aristotle must be wrong.

No tower required. No measurements. Just pure reasoning about an imaginary scenario.

This is a thought experiment.

What Thought Experiments Actually Are

A thought experiment is an imaginary scenario designed to test an idea, expose a flaw, or illuminate a truth. It's a laboratory that exists only in your mind, where you can manipulate variables that would be impossible, impractical, or unethical to manipulate in the real world.

The German word for this is Gedankenexperiment—literally "thought experiment." A Danish physicist named Hans Christian Ørsted coined the term around 1812, though people had been conducting such experiments for millennia before anyone thought to name them. The ancient Greeks called a similar pattern of reasoning deiknymi, and it was their oldest form of mathematical proof, predating even Euclidean geometry.

What makes thought experiments different from mere daydreaming or speculation? Structure. A good thought experiment has clear premises, follows logical rules, and reaches a definite conclusion. It's not "what if dragons existed?"—it's "given everything we know about physics and biology, what would happen if X?"

The philosopher Ernst Mach, writing in the 1880s, used thought experiments as a teaching tool. He would ask his students to predict the outcome of an experiment in their minds before performing it physically. When reality differed from imagination, the gap itself became instructive. The student had to explain where their mental model went wrong.

The Cat That Changed Physics

In 1935, the Austrian physicist Erwin Schrödinger grew frustrated with the prevailing interpretation of quantum mechanics. The Copenhagen interpretation, championed by Niels Bohr and others, held that particles exist in a "superposition" of states until they're observed. An electron doesn't have a definite position until you measure it; until then, it's somehow in all possible positions at once.

Schrödinger thought this was absurd. To prove it, he invented a cat.

Imagine, he said, a cat sealed in a box with a radioactive atom, a Geiger counter, and a vial of poison. If the atom decays, the counter triggers, the vial breaks, and the cat dies. If the atom doesn't decay, the cat lives. According to quantum mechanics, the atom exists in a superposition of "decayed" and "not decayed" until observed. But this means the cat must also exist in a superposition of "alive" and "dead" until someone opens the box.

A cat that is simultaneously alive and dead.

Schrödinger intended this as a reductio ad absurdum—a proof by contradiction that the Copenhagen interpretation leads to ridiculous conclusions. Ironically, his thought experiment became one of the most famous illustrations of quantum weirdness, and many physicists today accept that the cat really is in a superposition until observed, however strange that seems.

Whether Schrödinger succeeded or failed depends on your perspective. But his cat achieved something remarkable: it took an abstract debate about subatomic particles and made it visceral, memorable, immediate. You can't easily picture an electron in superposition. You can picture a cat that's both alive and dead.

Maxwell's Demon and the Death of Perpetual Motion

In 1867, the Scottish physicist James Clerk Maxwell imagined a tiny demon.

Here's the setup. You have a box divided in two by a wall. Both halves contain air at the same temperature. Temperature, at the molecular level, is just the average speed of molecules—some are moving fast, some slow, but the average is the same on both sides.

Now Maxwell introduces his demon: a microscopic creature sitting at a tiny door in the dividing wall. When a fast-moving molecule approaches from the left, the demon opens the door and lets it through to the right. When a slow-moving molecule approaches from the right, the demon lets it through to the left. All other molecules are blocked.

Over time, the right side fills with fast molecules (hot) and the left side fills with slow molecules (cold). The demon has created a temperature difference without doing any work—violating the second law of thermodynamics, which says you can't decrease entropy without expending energy.

This thought experiment haunted physicists for over a century. It seemed to prove that the second law was merely statistical, not absolute. Maybe, with enough cleverness, you really could build a perpetual motion machine.

The resolution came in the 1960s and 1980s, when physicists realized that the demon must store information about each molecule it observes. Erasing that information—which the demon must eventually do when its memory fills up—requires energy and increases entropy. The books balance after all. The second law survives.

Maxwell's demon thus became a bridge between thermodynamics and information theory, revealing that information itself is physical, that knowing something has a cost measured in energy.

Einstein Chases a Light Beam

When Albert Einstein was sixteen years old, he asked himself a question: what would happen if you could run alongside a beam of light, matching its speed exactly?

According to the physics of his day, you should see the light frozen in place—a stationary electromagnetic wave, its peaks and troughs hanging motionless in space. But Maxwell's equations, which describe light, don't allow for stationary electromagnetic waves. They only describe waves that move at the speed of light.

This contradiction nagged at Einstein for ten years. Eventually, it led him to a radical conclusion: the speed of light must be the same for all observers, regardless of their motion. If you're chasing a light beam at nearly the speed of light, the beam still races away from you at the full speed of light. This isn't because the light sped up—it's because time and space themselves warp to preserve the constant speed of light.

This was the special theory of relativity, one of the most profound revisions to our understanding of the universe. And it began with a teenager imagining himself running very, very fast.

Einstein never conducted a physical experiment to prove special relativity. He couldn't—the speeds involved are impossible for any human or laboratory apparatus. But his thought experiment revealed a logical necessity, and subsequent physical experiments confirmed his predictions with extraordinary precision.

The Philosophical Toolkit

Philosophers have been running thought experiments even longer than physicists, and for similar reasons: to test ideas that can't be tested any other way.

Consider the trolley problem, perhaps the most famous thought experiment in ethics. A runaway trolley is barreling toward five people tied to the tracks. You can pull a lever to divert the trolley onto a side track, where only one person is tied. Do you pull the lever?

Most people say yes. Killing one to save five seems like simple math.

But now consider a variant: you're standing on a bridge above the tracks. A very large man is next to you. If you push him off the bridge, his body will stop the trolley, saving the five—but killing him. Do you push?

Most people say no. And yet the math is the same: one death to prevent five.

This divergence reveals something important about moral psychology. We don't simply calculate outcomes; we care about how those outcomes are achieved. There's a felt difference between redirecting a threat and using a person as a tool. The thought experiment doesn't tell us which intuition is correct, but it exposes the intuitions themselves, making them available for examination.

The Chinese Room

In 1980, the philosopher John Searle attacked artificial intelligence with a thought experiment.

Imagine yourself locked in a room. Through a slot in the door, people pass you cards with Chinese characters on them. You don't understand Chinese—the characters look like meaningless squiggles to you. But you have an enormous rulebook that tells you exactly which characters to write in response to any input. You follow the rules, write your response, and pass it back through the slot.

To the people outside, you appear to understand Chinese perfectly. Your responses are indistinguishable from those of a native speaker. But you don't understand a single word. You're just manipulating symbols according to rules.

Searle's conclusion: computers, no matter how sophisticated, don't actually understand anything. They're just manipulating symbols according to rules. What looks like intelligence is mere simulation.

This thought experiment sparked decades of debate. Critics argue that while you don't understand Chinese, the system as a whole—you plus the rulebook—might understand. Or that understanding might emerge from the right kind of symbol manipulation. The Chinese Room didn't settle the question of machine consciousness, but it gave the debate a concrete scenario to argue about.

The Veil of Ignorance

How would you design a society if you didn't know what position you'd occupy in it?

The philosopher John Rawls posed this question in his 1971 book A Theory of Justice. Imagine, he said, that you're behind a "veil of ignorance"—you're designing the rules of society, but you don't know whether you'll be rich or poor, healthy or sick, talented or ordinary, born into privilege or disadvantage.

From behind this veil, Rawls argued, rational people would choose principles that protect the worst-off members of society. After all, you might be one of them. You wouldn't design a system that crushes the poor if there's a chance you'll be poor yourself.

The veil of ignorance is a thought experiment designed to factor out self-interest from moral reasoning. It doesn't tell you what justice is, but it provides a method for thinking about it—a mental procedure that strips away the biases that come from knowing your own position.

The Limits of Imagination

Not all thought experiments work as intended. Some present scenarios that, on closer examination, turn out to be impossible or incoherent.

The philosopher Hilary Putnam asked us to imagine "Twin Earth"—a planet exactly like ours except that the clear, drinkable liquid in lakes and rivers isn't H₂O but some other chemical compound, XYZ, with all the same observable properties as water. Putnam used this to argue that the meaning of words depends on the external world, not just what's in our heads. When we say "water," we mean H₂O, even if we can't distinguish it from XYZ by taste or appearance.

But is Twin Earth even possible? Some philosophers argue that a liquid with all the observable properties of water must be water—that's just what those properties mean at the molecular level. If they're right, Twin Earth isn't logically possible, and conclusions drawn from it are suspect.

An even more extreme example: philosophical zombies. The philosopher David Chalmers asks us to imagine beings physically identical to humans in every way—same neurons firing in the same patterns—but with no conscious experience whatsoever. Nothing it's like to be them. Lights on, nobody home.

If zombies are conceivable, Chalmers argues, then consciousness must be something beyond the physical. But are zombies actually conceivable? Can you really imagine a being whose brain works exactly like yours but who has no inner experience? Some philosophers say no—that we only think we're imagining zombies because we're not thinking carefully enough about what physical identity really means.

This highlights a crucial limitation: thought experiments rely on our ability to imagine scenarios correctly. If our imagination smuggles in hidden assumptions or fails to grasp the full implications of a scenario, our conclusions may be worthless.

The EPR Paradox: When Thought Meets Reality

Sometimes thought experiments generate predictions that can be tested physically—and the results are surprising.

In 1935, Einstein collaborated with Boris Podolsky and Nathan Rosen on a thought experiment now known as the EPR paradox (from their initials). They imagined two particles that interact and then fly apart. According to quantum mechanics, measuring a property of one particle instantly determines the corresponding property of the other, no matter how far apart they are.

Einstein found this absurd. How could measuring a particle here instantaneously affect a particle light-years away? He called it "spooky action at a distance" and concluded that quantum mechanics must be incomplete—that there must be hidden variables we haven't discovered yet that determine the particles' properties in advance.

For decades, this remained a philosophical debate. Then, in 1964, the physicist John Bell devised a mathematical test—the Bell inequalities—that could distinguish between Einstein's hidden-variable theory and standard quantum mechanics. If you performed the right experiments, the numbers would tell you which picture was correct.

In the 1980s, the French physicist Alain Aspect performed those experiments. The results violated the Bell inequalities. Einstein was wrong. Quantum mechanics, with all its spooky action, was right.

Here we see thought experiments at their most powerful: not just as tools for clarifying concepts, but as generators of testable predictions. Einstein's thought experiment was proven wrong by a real experiment—but the real experiment would never have been designed without the thought experiment to motivate it.

The Practical Power of Imaginary Scenarios

Thought experiments aren't just for physicists and philosophers. They're embedded in everyday reasoning, though we rarely call them by that name.

When a lawyer asks "what if the defendant had called for help instead of fleeing?"—that's a thought experiment, exploring counterfactual scenarios to assign blame or establish liability.

When a business strategist asks "what if our main competitor went bankrupt tomorrow?"—that's a thought experiment, probing the resilience and dependencies of their business model.

When you ask yourself "what would I do if I lost my job next month?"—that's a thought experiment, rehearsing scenarios to prepare for uncertainty.

The technical term for this pattern of reasoning is "subjunctive" or "irrealis"—the grammar of things that aren't true but might be, could be, or would have been under different circumstances. It's one of the most powerful tools in the human cognitive toolkit, allowing us to learn from experiences we've never had and prepare for futures that may never arrive.

How to Run a Good Thought Experiment

A well-constructed thought experiment has several key features.

First, clear premises. You need to specify exactly what's true in your imaginary scenario and what's being held constant from reality. Ambiguity undermines the whole exercise.

Second, logical rigor. The conclusions must follow from the premises by valid reasoning. Thought experiments aren't about proving what you already believe; they're about discovering what must be true if your premises are true.

Third, isolation of variables. A good thought experiment changes only the thing you're testing. If you change multiple variables at once, you won't know which one caused the result.

Fourth, intuitive accessibility. The scenario should be vivid enough that people can actually imagine it and report their intuitions about it. Schrödinger's cat works because everyone can picture a cat in a box. An abstract scenario about quantum superposition wouldn't have the same impact.

Fifth, resistance to easy dismissal. The best thought experiments anticipate objections and close off escape routes. If there's an obvious way to dissolve the puzzle, the experiment hasn't done its job.

The Ancient Art of Mental Exploration

Long before anyone coined the term Gedankenexperiment, humans were running mental simulations to test their ideas about reality.

Plato's allegory of the cave is a thought experiment about the nature of knowledge. Imagine prisoners chained in a cave, able to see only shadows cast on a wall by objects passing before a fire behind them. They take the shadows for reality. One prisoner escapes, sees the sun, and realizes that what he thought was real was only a shadow of reality. Plato uses this to argue that the physical world we perceive is itself only a shadow of a higher realm of Forms.

The medieval Persian philosopher Avicenna, known in the Islamic world as Ibn Sina, devised the "Floating Man" thought experiment in the eleventh century. Imagine yourself suspended in empty space, isolated from all sensation—unable to see, hear, touch, smell, or taste anything. Would you still be aware of your own existence? Avicenna argued yes: even without any sensory input, you would know that you exist. This demonstrated, he claimed, the substantiality of the soul, independent of the body.

Thomas Hobbes and John Locke both imagined a "state of nature"—what human life would be like without government or social institutions—to investigate the origins and justification of political authority. Hobbes concluded that life in such a state would be "solitary, poor, nasty, brutish, and short," justifying even authoritarian government as an improvement. Locke reached different conclusions, imagining a state of nature governed by natural reason, from which more limited government could be derived.

These aren't scientific experiments. They don't make predictions that can be tested. But they do what science cannot: they illuminate concepts, clarify values, and reveal hidden assumptions in our thinking.

The Infinite Laboratory

Every time you ask "what if?"—and follow the question seriously, rigorously, to its logical conclusions—you're conducting a thought experiment.

You're joining a tradition that stretches back to the ancient Greeks and forward to the frontiers of physics. You're using the most powerful laboratory ever built: the human imagination, disciplined by logic, applied to questions that matter.

Galileo didn't need to climb the Tower of Pisa. Einstein didn't need to chase a light beam. Schrödinger didn't need to put a cat in a box. They just needed to think clearly about what would happen if they did.

The laboratory that exists only in your mind is open around the clock, requires no equipment, and can explore territories no physical experiment can reach. The admission price is just the willingness to take an idea seriously and follow it wherever it leads.