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 is simultaneously alive and dead
. Yet, when one looks in the box, one sees the cat either
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.