Thought experiment

A thought experiment (Ger. Gedankenexperiment) is (typically) an experiment that can't, or shouldn't, actually be performed, but that still leads to interesting results or understanding just by its consideration. Sometimes the German term Gedankenexperiment is used in English.

Famous scientific thought experiments

 * Schrödinger's cat is perhaps one of the more famous thought experiments where a cat is said to be both dead and alive in a demonstration of quantum superposition. Often misunderstood. It was conceived to question the Copenhagen ("superposition") interpretation of quantum mechanics, by showing dramatic macroscopic consequence of a quantum effect under that interpretation. It was not meant to assert that macroscopic superposition is a necessary consequence of quantum theory.


 * Quantum suicide (and also quantum immortality) is a variant on the cat-in-a-box experiment that tests the implications of the "many worlds" and Copenhagen interpretation of quantum mechanics, which are only interpretations and currently have no practical differences.


 * Maxwell's demon is a famous thought experiment questioning the second law of thermodynamics.


 * Newcomb's Problem is a thought experiment popular with those working on decision theory. In the experiment you have two boxes and an intelligent being who can predict your move perfectly and alters the content of the boxes based on that prediction; leading to a paradox of what decision strategy gives you the most profitable result.


 * The Twins Paradox of special relativity. Concerning the predicted differential ageing of twins reunited after one has been on a rocket trip. Often misunderstood. The paradox is NOT simply that the reunited twins are no longer the same age (that is just a non-intuitive consequence of Relativity theory). The paradox is meant to be that since velocity is relative, either twin may regard himself as the stationary one, so when they are reunited each twin must paradoxically find the other twin older than himself. There is no paradox because the twins are NOT both free to regard themselves as permanently stationary &mdash; the rocket twin "knows" he has undergone acceleration. The simple rules of Special Relativity do not apply in accelerated frames of reference.


 * The Grandfather Paradox relating to time travel postulates what happens if you go back in time and kill your grandfather, thus preventing you from being born and preventing you from going back in time and killing him, which would leave him alive so that you could go back and kill him, preventing you from existing in order to go back in time and kill him, so he'd survive and you'd live, allowing you to go back in time and kill your grandfather… just smile and nod.


 * Newton's Cannonball, used to demonstrate how orbits work. Best demonstrated with a Flash game.

Less famous scientific thought experiments

 * The Buttered cat theory treats the problem of whether a falling cat with a slice of buttered toast attached to its back can ever hit the ground. Cats always land on their feet and toast butter-side down, creating a paradox.

Famous philosophical thought experiments

 * AI-box experiment, an experiment devised by Eliezer Yudkowsky to show that a suitably advanced artificial intelligence can convince, or perhaps even trick or coerce, people into "releasing" it — that is, allowing it access to infrastructure, manufacturing capabilities, the Internet and so on.
 * The Chinese room, an experiment challenging the validity of the Turing test. Here, the experiment is intended to demonstrate that a machine that passes the Turing test isn't necessarily conscious.


 * The China brain experiment, where the population of China (as it has a large population and can be visualised, in more recent years this could perhaps be called the "India Brain") simulates the neurons within a working brain to demonstrate the emergent nature of consciousness.


 * The Experience Machine, a thought experiment by Robert Nozick which attempts to refute hedonism. The thought goes that if you could plug yourself into a machine which gives you intensely pleasurable experiences, surely you would do so. The fact that most of us wouldn't want to sit passively by a machine for our happiness proves that feeling pleasure is not the only thing that matters.


 * The Trolley Problem, a thought experiment first put forward by Philippa Foot, which distinguishes between "killing" and "letting die". The thought goes that you're standing by a set of tracks as a runaway trolley flies by. If it continues on its course, the five people tied to the tracks will be killed. If it is diverted onto a second set of tracks, the one person tied to the tracks will die. You are in front of the diverting lever and must choose &mdash; do nothing, and allow five people to die, or pull the lever, and intentionally kill one person (many variations have been formulated, such as The fat man or The loop variant).


 * Mary in the Gray Room, on which a girl called Mary, who lived her entire life in a black-and-white room learning everything possible about optics and the science of colors, from how light is reflected and absorbed in the cornea to what colors are supposed to represent, leaves her room and actually experiences color for the first time. The discussion is what would be Mary's reaction: would she be surprised or not?


 * Russell's Teapot, an analogy proposed by Bertrand Russell, where the existence of a teapot orbiting the sun somewhere between Earth and Mars is proposed, and those who disagree with this idea are invited to "disprove" it (or at least try).


 * Brain in a vat: if we really can't know anything, our entire reality can be fake.


 * Ship of Theseus: Imagine that a ship which fought a great battle, while captained by the hero Theseus, is stored in a harbor as a museum piece. As the years go by some of its rotten parts are replaced by new ones until, eventually, all of the parts from the original ship are replaced. If someone were to recover the original parts and rebuild the ship at another location, which of the ships should be considered the true ship of Theseus?  Is there a point at which the structure at the first location stopped being the ship of Theseus?