A thought experiment (also thought experiment or thought game ) is a mental aid to support, refute, illustrate or think further certain theories . A situation is conceptually constructed that cannot or can only be created with great difficulty in real terms (for example a journey at almost the speed of light ). Then one imagines the consequences of this situation if one applies the theory to the situation. An experiment is simulated in your mind . A thought experiment, however, is not an experiment in the strict sense. The latter illuminates theories through empirical observation from the outside , but a thought experiment is trapped within the theory, the empirical aspect is missing.
Thought experiment versus real experiment
Nevertheless, some thought experiments that were not feasible at the time they were conceived are now understandable in real experiments. For example, empirical evidence was provided that clocks move at different speeds depending on the relative speed with which they are moved.
Other thought experiments later turned out to be not feasible in principle. For example, it is now known that Maxwell's demon does not work in principle, mainly for "quantum mechanical reasons". When this thought experiment was devised, nothing was known about quantum mechanics either .
The interrelationships in Albert Einstein's and co-workers' work on the Einstein-Podolsky-Rosen Paradox (EPR) are more complex : In 1935, based on an accurate thought experiment, the authors wrongly rejected quantum mechanics as “in need of supplementation”: This mistake has been made only with great delay and despite exclusively correct and promising conclusions in the analysis as such (see also the real optical experiments of Alain Aspect ), after the so-called Bell's inequalities (a mathematically rigorous theoretical work carried out in 1964) the philosophical Basics of EPR publication could be falsified .
Thought experiments differ fundamentally from real experiments and can often be assigned to theoretical physics ; but also in other disciplines, e.g. B. in philosophy , they play an important role. Hans Christian Ørsted was the first to introduce the term thought experiment as a relationship between mathematical and physical knowledge in Kant . Especially that of Albert Einstein found special theory of relativity makes ample use of thought experiments. Einstein took the idea for this from his temporary teacher Ernst Mach , on whose philosophical work the popularity of this term goes back.
Thought experiments are particularly popular to test whether a theory leads to paradoxical situations. For example, the well-known example of Schrödinger's cat , who is dead and alive at the same time with a quantum mechanically described probability , is usually given as evidence that the theory in question is incomplete in at least one respect (for example, by ignoring violations of quantum mechanical coherence) .
Thought experiments belong to the respective theoretical discipline (e.g. theoretical physics), while real experiments belong to the respective experimental discipline. The difference seems obvious, but it is subtle, as is clear from the example of Albert Einstein's famous work on the EPR effect . In this work (1935) Einstein and two employees not only suggested the said effect based on a thought experiment (the effect has since been realized), but also rejected quantum mechanics as "unreasonable and in need of supplementation" due to the unusual properties of the effect.
All the mathematical conclusions of this work were correct, so that Einstein could not prove a logical error at the time, neither through thought experiments nor through real experiments. It was not until 1964 that the theoretical physicist John Bell succeeded in showing (see Bell's inequality ) that the validity of the explicitly mentioned philosophical foundations of EPR work, the assumption of the so-called reality and locality of a physical theory, can be verified experimentally, namely through real ones Experiments, not thought experiments. Such real experiments have meanwhile been carried out several times (e.g. by Alain Aspect ) and have always falsified the aforementioned basic philosophical assumptions of Einstein's work; d. In other words, quantum mechanics has proven to be in no need of supplementation in any case . (On the subject of "falsifying a theory" see Karl Popper's philosophy .)
In this one case Einstein was wrong, but with the mentioned EPR effect and the quantum cryptography based on it (see also quantum entanglement ) for the practical applications of the quantum mechanics that he fought so fiercely, he left something very important.
Well-known thought experiments
- Braitenberg vehicles - cybernetic robotic vehicles devised by Valentino Braitenberg that seem to show the behavior of living beings
- EPR paradox ( quantum mechanics ) (now carried out experimentally)
- Elevator experiment ( general relativity theory ) - in a closed elevator it is not possible to decide whether the acceleration of a test mass is caused by the elevator motor or an external gravitational field, so inertial and heavy mass must be the same. In the free-falling elevator no weight there (practically drop tower experiments or by gravity in an aircraft in parabolic flight shown)
- Flatland - A Romance in many Dimensions . Edwin A. Abbott . Journey through basic geometric shapes 1884
- Free fall - Giovanni Battista Benedetti refuted in 1554 in his work Demonstratio proportionum motuum localium contra Aristotilem et omnes philosophos that bodies of different weights fall at different speeds. The thought experiment can also be found in the Discorsi of Galileo Galilei and was often ascribed to him.
- Laplace demon who, according to classical physics , could calculate the past and future of the world
- Light clock ( special theory of relativity ) - a moving clock (observed from outside) runs slower
- Maxwell's demon - brings the second law of thermodynamics with information relating
- Photon balance - Albert Einstein tried to convince Niels Bohr of the imperfection of quantum theory
- Planiverse - describes life in a two-dimensional world
- Schrödinger's cat (quantum mechanics) - raises interesting questions about the quantum mechanical measurement process
- Stevin's thought experiment - explanation of the equilibrium relationships on inclined planes by Simon Stevin around 1600
- The twin paradox ( special theory of relativity ) - in a fast spaceship, time passes more slowly when viewed from the outside, the distances are shortened for the occupants
Thought experiments in philosophy usually have one essential feature that distinguishes them from other illustrative means: They start from counterfactual circumstances. Such a thought experiment asks what would be the case if things were different from what they actually do. The degree of counterfactivity can vary, but basically there is always a hypothetical situation. The form of such a consideration can be represented as follows:
- We usually assume and claim that the sentence S is true.
- In our thought experiment we now assume that the world works very differently (or a little differently). Then we no longer assume that S is true, but perhaps another sentence F that is incompatible with S.
- So we have no reason to consider S to be absolutely true, but to see S as the result of our specific, (possibly changeable) situation.
- Trolley problem - Philippa Foot
- Gettier Problem - Edmund Gettier
- Chinese Room - John Searle
- Veil of Ignorance - John Rawls
- Brain in the tank - Hilary Putnam
- Mill example - Gottfried Wilhelm Leibniz
- Twin Earth - Hilary Putnam
- Marsh Man - Donald Davidson
- Mary - Frank Cameron Jackson
- Utility Monster - Robert Nozick
- Theseus' ship
- Spaceman Reproduction - Mary Anne Warren
A distinction must be made between thought experiments from philosophy and parables, which are intended to clarify an abstract state of affairs with a clear situation. In the allegory of the cave , Plato is concerned with the graphic representation of his epistemology ; However, the parable also contains elements of a thought experiment insofar as Plato continues to explain how a person would fare if he reported about the outside world to someone who was bound in the cave.
- Parable , the view from an unfamiliar perspective, has something in common with the thought experiment
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- Brooks, DHM "The Method of Thought Experiment" in: Metaphilosophy, vol. 25, No. 1 pp. 71-83.
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- Behmel, A. Thought experiments in the philosophy of mind, Stuttgart 2001.
- Macho, Th. U. A. WUNSCHEL (Ed.) Science & Fiction. Thought experiments in science, philosophy and literature. Frankfurt / M. 2004.
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- Cohnitz, D. Thought Experiments in Philosophy, Paderborn 2006.
- RA Sorensen: Thought Experiments , Oxford University Press 1990.
- Michel, JG "Arguing with Thought Experiments: A Case Study in the Philosophy of Mind." In The Search for the Spirit , JG Michel & G. Münster (eds.), Pp. 81–120, Münster 2013.
- Special Issue: Ernst Mach and the thought experiment around 1900 , introduced by Bernhard Kleeberg, in: Reports on the History of Science , March 2015, Volume 38, Issue 1 , pp. 1–101.
- Entry in Edward N. Zalta (Ed.): Stanford Encyclopedia of Philosophy .
- JHY Fehige, Uni Mainz "The thought experiment - an independent knowledge method?" - On the epistemological discussion since Ernst Mach (PDF file; 295 kB)
- Ulrich Kühne: "Thought experiment and explanation" (1997) - essay on the philosophical significance of thought experiments in the natural sciences
- Compilation by Joachim Eberhardt
- Ulrich Gähde: Thought experiments in epistemology and physics . In: Julian Nida-Rümelin (Ed.): Rationality, realism, revision . Walter de Gruyter, 2000, ISBN 3110163934 , p. 457.