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Why did Schrödinger invent his cat?
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Erwin Schrödinger invented his cat in 1935 to mock a reading of quantum theory he thought was absurd. It grew out of letters with Albert Einstein, who had suggested a keg of gunpowder. The same year Schrödinger coined the word "entanglement". The cat he meant as a criticism became the theory's most famous image.
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How did a complaint become a famous cat?
Card 1 · 1926
Erwin Schrödinger, an Austrian physicist, writes the wave equation at the heart of quantum theory. He hopes it describes real waves, without jumps.
Card 2 · The rival reading
Others, led by Niels Bohr and Max Born, read his waves as odds, with a jump when someone measures. Schrödinger dislikes it. "If we have to keep these damned quantum jumps," he's said to have complained, he regretted ever getting involved.
Card 3 · May 1935
Einstein, Boris Podolsky and Nathan Rosen publish their argument that quantum theory is incomplete (chapter 5). Einstein and Schrödinger begin swapping letters.
Card 4 · Gunpowder
Einstein writes about a keg of gunpowder that, by the theory's rules, ends up described as both exploded and not exploded. Clearly absurd, he says, for something so big.
Card 5 · The cat
Later that year Schrödinger publishes a long article on "the present situation in quantum mechanics". In one paragraph he swaps the keg for a cat, a radioactive atom and poison, and calls it a quite ridiculous case. In the same series he coins the word for linked quantum states: Verschränkung, entanglement.
Card 6 · The twist
For decades the cat sits quietly in the literature. Then it escapes into lectures, books, T-shirts and memes, usually as a picture of quantum theory rather than a criticism of it.
Card 7 · Today
Physicists build "cat states" in the lab (page 3a). The question Schrödinger raised, why we see one result, is still open (chapter 6).
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The world's most famous quantum image started as a complaint.
Schrödinger meant it as a reductio, and coined 'entanglement' the same year.
Next in the path · Chapter 4
What is interference, and why does everything depend on it?
Next in the path