Schrödinger's famous cat was meant as a joke at quantum theory's expense.
Everyone knows the cat in the box that is both alive and dead until you look. Few know that Erwin Schrödinger invented it in 1935 to mock that idea, not to promote it. His absurd example stuck, and it still frames physics' deepest unsolved puzzle: when does 'both' become 'one'?
In 1935 Albert Einstein and two colleagues published the EPR paper, arguing that quantum mechanics was incomplete. Schrödinger discussed it with Einstein by letter, and Einstein offered an example of a keg of gunpowder that, by quantum rules, would end up in a blend of exploded and unexploded states. Schrödinger sharpened this into his cat.
His set-up: a cat is sealed in a steel chamber with a tiny amount of radioactive material, so little that in an hour one atom might decay, or with equal odds none will. If one does, a Geiger counter triggers a hammer that shatters a flask of poison. Quantum mechanics describes the undecayed atom as a superposition of 'decayed' and 'not decayed' until measured. Link the cat to the atom, and the maths seems to say the cat is equally alive and dead. Schrödinger called it a 'quite ridiculous case'. His point was that an interpretation leading there, associated with Niels Bohr and Werner Heisenberg, could not be describing reality.
The cat stayed because it captures the measurement problem so vividly. Quantum theory predicts a spread of possible outcomes, but we only ever see one. Where does the change happen? One common answer is that the Geiger counter itself counts as a measurement, so the cat's fate is settled long before anyone opens the box. Hugh Everett's 1957 many-worlds interpretation says both outcomes happen, and the observer splits along with the cat. Others once suggested that a conscious observer is what triggers the collapse.
No one has put a cat in a superposition, and the experiment was never meant to be done. But physicists have created 'cat states' with photons, a trapped beryllium ion and superconducting loops where electrons flow both ways around at once. Each pushes the boundary between the quantum and everyday worlds a little further.
Source: Wikipedia — Schrödinger's cat · Text summarised from Wikipedia (CC BY-SA 4.0)
