Schrödinger's cat: a cat, a flask of poison, and a radioactive source are placed in a sealed box. If an internal monitor detects radioactivity (i.e. a single atom decaying), the flask is shattered, releasing the poison that kills the cat. The Copenhagen interpretation of quantum mechanics implies that after a while, the cat issimultaneously alive and dead. Yet, when one looks in the box, one sees the cat either alive or dead, not both alive and dead. This poses the question of when exactly quantum superposition ends and reality collapses into one possibility or the other. “[It] prevents us from so naively accepting as valid a "blurred model" for representing reality,” Schrödinger wrote. “In itself, this would not embody anything unclear or contradictory.”
Schrödinger’s Cat has been used to illustrate the differences between emerging theories in quantum mechanics, by testing how they would approach the experiment.
For example, the ‘many worlds interpretation’, developed in the 1950s, would argue that when the box is opened, the observer and dead-and-alive cat split into two realities, in one of which the observer sees a dead cat and the other an alive one.
The experiment has also been widely influential in popular culture, having been referenced in TV shows such as Futurama, Doctor Who and The Big Bang Theory as well as appearing in the works of Douglas Adams and Terry Pratchett.