Go further · Chapter 3 · 3a · Real-world case
What does Schrödinger's cat really mean?
The short answer (tap to open)
Schrödinger's cat is a 1935 thought experiment, not a claim that cats are half dead. Erwin Schrödinger used it to show that applying quantum rules to everything leads to an absurd result, so something must be missing. Today decoherence explains why you never see a blurred cat. Why you see one result is still debated.
What's the set-up?
A cat sits in a sealed box with a tiny amount of radioactive material, a detector, a hammer and a vial of poison. In an hour there's a 50% chance one atom decays. If it does, the detector trips the hammer, the vial breaks and the cat dies. Nobody could or should do this. It's a thought experiment.
What did Schrödinger say happens?
Quantum rules describe the atom as a mix of "decayed" and "not decayed". The atom links to the detector, the detector to the hammer, the hammer to the vial, the vial to the cat. Apply the same rule to every link and after an hour the maths describes a mix of "dead cat" and "living cat".
Schrödinger called this a "quite ridiculous" case. His point was that quantum theory can't simply stop at small things, and that it doesn't say where or how a single result appears. That's the measurement problem of chapter 6.
Why do we never see a blurred cat?
Chapter 3 gives the first part of the answer: decoherence. A real cat is warm, breathing and bombarded by air molecules and light. Its surroundings record whether it's alive far faster than anything can measure. Any blend washes out almost instantly, so the box holds what looks like either a live cat or a dead one long before anyone opens it.
What decoherence doesn't settle is which of the two you find, and why. Copenhagen, Many-Worlds, pilot waves, collapse models and the rest (chapter 6 and page 6b) each answer that differently.
An atom, vacuum near absolute zero: Long enough to see and use.
Illustrative. The point is the trend, not the numbers.
Each size, in words
- An atom. Vacuum near absolute zero: Long enough to see and use. Room air: Far too short to see.
- A molecule. Vacuum near absolute zero: Long enough to see and use. Room air: Gone faster than any clock can measure.
- A virus. Vacuum near absolute zero: Brief, but still measurable in the best labs. Room air: Gone faster than any clock can measure.
- A dust grain. Vacuum near absolute zero: Far too short to see. Room air: Gone faster than any clock can measure.
- A cat. Vacuum near absolute zero: Gone faster than any clock can measure. Room air: Gone faster than any clock can measure.
Illustrative. The point is the trend, not the numbers.
Has anyone made a real "cat"?
Physicists use "cat state" for any large-ish object put into a blend of two clearly different states. Serge Haroche's team built cat states of light in the 1990s. In 2023 a team at ETH Zurich put a tiny crystal, about 16 millionths of a gram, into a blend of two vibrations at once. Big, by quantum standards. Nowhere near a cat.
Check
Two quick ones
Collect
Your aha card
The cat is a warning, not a fact.
Decoherence hides the blend; which result you see is the open question.