In Act One of Copenhagen, the characters make a joking reference to “Schrödinger’s cat.” This cat is not Schrödinger’s pet kitty, but the subject of a famous thought experiment that has become the basis for many a physics joke over the years.
In 1935, Austrian physicist Erwin Schrödinger used this paradox to object to what he viewed as a problem with the Copenhagen interpretation of quantum mechanics. In this idea, an imaginary cat locked in a box with a vial of poison (or, as in the video below, gunpowder) may be both alive and dead at the same time. But when a person looks into the box, the cat will be either alive or dead, not both.
Watch this YouTube video by MinutePhysics for a fun explanation: