A thought experiment proposed by Erwin Schrödinger to demonstrate the seemingly absurd character of reality at the level of Quantum Mechanics. The thought experiment demonstrates quantum indeterminacy.

In Schrödinger's thought experiment, a cat is placed into a steel box with a small sample of radioactive material, a vial of cyanide, and a detector for detecting whether the radioactive sample has decayed and emitted a particle. If a single atom of the sample decays, the detector will detect it, and break the vial of cyanide. This will kill the cat.

Since an outside observer of the box has no way of knowing if the radio-active material has decided to decay, and release a particle, there is no way to know if the cat is dead or alive. Because there is no way to tell if the cat is dead or alive until it is observed, in essence, the cat is both dead and alive at the same time.

Schrödinger's cat was just an explanatory device to illustrate the concept of what really goes on at the quantum level. At the quantum level, where there is no way to tell about the state of a particle, reality actually is in both states at the same time. The particle is not in one state or the other, but is in all possible states simultaneously. This is called superposition.


Note: Also spelled: Schrodinger's Cat, or Schroedinger's Cat when the alphabet does not fully support the native spelling.