Schrödinger's cat: a cat, a flask of poison, and a
radioactive source are placed in a sealed box. If an internal monitor (e.g.
Geiger counter) detects radioactivity (i.e. a single atom decaying), the flask is shattered, releasing the poison, which kills the cat. The Copenhagen interpretation of quantum mechanics implies that after a while, the cat is
simultaneously 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.