Schrödinger's cat
A cat in a box is both alive and dead until someone looks. Really?
Erwin Schrödinger, 1935
The thought experiment nobody should do
In 1935 the Austrian physicist Erwin Schrödinger was annoyed. His colleagues were saying that, in quantum physics, a tiny particle doesn't have a definite state until it is measured. Before that, it is in a 'superposition' of all its possibilities at once.
To show how strange that idea became when you scaled it up, he imagined a cruel machine, purely in his head. Put a cat in a sealed box with a single radioactive atom, a detector and a device that releases poison if the atom decays. In one hour there's a 50–50 chance the atom has decayed.
According to the quantum rules, until you look, the atom is both decayed and not decayed. So the detector has both fired and not fired. So, Schrödinger said with a straight face, the cat must be both alive and dead at the same time.
He didn't believe that for a second. That was the point. Something, he argued, was missing from the theory.
The puzzle
Quantum rules that work perfectly for atoms seem to predict that a cat can be alive and dead at once.
See it for yourself
What's really going on
Most physicists today point to decoherence. A cat, a detector and even the air in the box are made of trillions of particles constantly bumping into each other. Those interactions spread the atom's quantum state into the environment almost instantly, so the box behaves like an ordinary box, with a cat that is either alive or dead. You just don't know which until you look.
But decoherence doesn't settle what is really happening. Some physicists think measurement truly collapses the possibilities into one. The many-worlds interpretation says both outcomes happen, in separate branches of reality. Superpositions are real and useful: quantum computers rely on them, using qubits that are 0 and 1 at once until measured.
Still argued about. Physicists agree on the maths and the predictions, but still argue about what the maths means.
Think about it
Does the cat itself count as an 'observer'? What about the detector?
More physics paradoxes
- Maxwell's demonA tiny demon opens a door and makes heat flow backwards. Free energy forever?
- The black hole information paradoxThrow a book into a black hole. When the hole evaporates, is the book's information gone forever?
- The double-slit experimentFire particles one at a time through two slits. They make a wave pattern. Watch them and the pattern vanishes.