The gedankenexperiment (thought experiment) has proved to be very useful in quantum theory. Physicists often conduct thought experiments prior to an actual experiment or when a particular physical experiment is impossible to conduct. (It was Einstein’s gedankenexperiment of chasing a light beam which resulted in Special Relativity theory.)
The most famous gedankenexperiment was published by Erwin Schrödinger in the mid-1930’s. To explain wave function collapse in relation to large objects, he imagined putting a live cat into a steel chamber, along with a very small amount of a radioactive material tied to a Geiger counter, which was rigged to a vial of poison. If even a single atom of the radioactive material decayed during the test period, a relay mechanism would trip a hammer, which would, in turn, break the vial of poison and kill the cat. Then Schrödinger imagined sealing the steel chamber shut.
Because the steel chamber is sealed, an observer cannot know whether or not an atom of the radioactive material has decayed, and consequently, cannot know whether the vial has been broken, the poison released, and the cat killed. According to quantum theory, since we cannot know, the cat is both alive and dead for as long as the chamber is sealed, in a superposition of states. It is only when we open the chamber and observe the condition of the cat (thereby collapsing the wave function), that the superposition is lost and the cat becomes either alive or dead.
This paradox demonstrates that observation itself affects an outcome, as an outcome, as such, does not exist until it is observed.
Schrödinger’s Cat (Wikipedia)
Schrödinger’s Cat (Physics Department, Trinity College)