Thought experiment

A thought experiment ( or thought experiment) is a theoretical tool to support certain theories to refute, to illustrate or think ahead. It is a situation while mentally constructed which is real as impossible or very difficult to produce (for example, a trip with the speed of light ). Then you paint in the spirit of what consequences arise from this situation, if one applies the theory to the situation. An experiment is thus simulated in thought. However, a thought experiment is not an experiment in the real sense. The latter examines theories through empirical intuition from the outside, a thought experiment, however, is trapped within the theory, the empirical aspect is lacking.

Nevertheless, since some thought experiments that were not possible at the time when they were conceived today feasible in real experiments. For example, the empirical evidence was provided that clocks are dependent on the relative speed with which it moves, at different rates.

Other thought experiments have not proven to be feasible later than principle. For instance, today announced that Maxwell's demon in principle does not work, mainly of " quantum-mechanical reasons ." As this thought experiment was conceived, but was not yet known about quantum mechanics.

Even more complex are the relationships in the work of Albert Einstein and employees via the Einstein - Podolsky-Rosen paradox: Here the authors in 1935 due to a true thought experiment incorrectly quantum mechanics rejected as " in need of completion ": This error has but only with long delay and despite only correct and promising conclusions in the analysis as such emerged (see, inter alia, the real optical experiments of Alain Aspect ) after the so-called Bell's inequalities ( one in 1964 carried out mathematically rigorous theoretical work ), the philosophical foundations of EPR publication could be falsified.

Thought experiments are thus different from actual experiments and are generally of theoretical physics refer; but also in other disciplines, such as philosophy, they play an important role. Hans Christian Ørsted was the first the term thought experiment as a relation between mathematical and physical knowledge in Kant. In particular, the found of Albert Einstein special theory of relativity makes ample use of thought experiments. Einstein took the idea for it from its temporary teacher Ernst Mach, whose philosophical work back the notoriety of this term.

Popular thought experiments are particularly to examine whether a theory leads to paradoxical situations. Thus, the well-known example of Schrödinger's cat is simultaneously dead and alive with a probability quantum mechanically described, usually given as evidence that the affected theory in at least one respect is incomplete (for example, by failing to take into account violations of quantum coherence).

Known Thought Experiments

  • Chinese room - by John Searle in 1980 put forward as a refutation of the theory that a spiritual in the right way programmed computer states possessed
  • EPR paradox ( quantum mechanics) (conducted now experimental)
  • Elevator Experiment ( General Relativity ) - 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 must be the same inertial and gravitational mass. In the free-falling elevator, there is no gravity (practically shown by the drop tower experiments or by weightlessness in an aircraft in parabolic flight )
  • 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 demonstrations proportionum motuum localium contra omnes et Aristotilem philosophos that different heavy bodies fall at different speeds. The thought experiment is also found in the Discourses of Galileo Galilei and was often attributed to this.
  • Laplacian demon who could compute the past and future of the world according to classical physics
  • Light clock ( special theory of relativity ) - a moving clock (seen from outside) running slower
  • Maxwell's Demon - brings the second law of thermodynamics with information relating
  • Photons scales - Albert Einstein, Niels Bohr tried by the imperfection of the quantum theory to convince
  • Planiversum - describes life in a two-dimensional world
  • Schrödinger's Cat ( quantum mechanics) - raises interesting questions about the quantum mechanical measurement process to
  • Stevinsches thought experiment - Statement by the equilibrium conditions on slopes of Simon Stevin in 1600
  • Twin paradox ( special theory of relativity ) - in a fast spaceship time viewed from the outside passes more slowly, for the occupants of the distances are shortened

On the subject of " verifiability " thought experiments versus real experiments

Thought experiments belong to the respective theoretical discipline (such as theoretical physics or theoretical philosophy ), while real experiments belong to the respective experimental discipline. The difference seems to be obvious, but is subtle, as the example of the famous work of Albert Einstein to the EPR effect is clear. In this work (1935 ) Einstein has proposed not only said effect due to a thought experiment with two employees (the effect could now be realized ), but also due to the unusual properties of the effect of quantum mechanics rejected as " irrational and in need of completion ."

All mathematical conclusions of this work were correct, so Einstein then no logical error was detected, neither by thought experiments nor by real experiments. Until 1964 it was possible to the theoretical physicist John Bell to show (see Bell's inequality) that the validity of the targeted explicitly philosophical foundations of the EPR paper, adopting the so-called reality and locality of a physical theory is experimentally verifiable, through real experiments, not by thought experiments. Such real-world experiments have now been carried out several times (eg by Alain Aspect ) and always have the mentioned basic philosophical assumptions of Einstein's work falsified; ie, that quantum mechanics has proven in every case as not in need of completion. ( On the subject of " falsification of a theory ", see the philosophy of Karl Popper. )

In this one case, therefore, was Einstein wrong, but nevertheless with the mentioned EPR effect and its resultant quantum cryptography ( see also quantum entanglement ) for the practical applications of the so fiercely fought by him quantum mechanics leave something essential.

Thought experiments in philosophy

Thought experiments in philosophy usually have a key feature that distinguishes it from other illustrative means: you go from counterfactual circumstances from. In such a thought experiment is asked what would be the case if things were different than they actually do it. The degree of counterfactuality may be differentiated, but is basically always on a hypothetical situation. The form of such consideration can be represented as follows:

  • We usually assume and assert that the set S is true.
  • In our thought experiment we go now but believe that the world is quite different (or slightly different ) works. Then we no longer assume that S is true, but perhaps a different set of F, which is incompatible with S.
  • So we have no reason to hold S for absolutely true, but see S as a result of our specific (possibly variable ) situation.


To be distinguished from thought experiments in philosophy are parables, such as the cave of Plato. Primarily he is concerned with the clear presentation of his theory of knowledge, but Plato goes on to say how it would a person who, as in the cave captives reported that of the outside world.