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Happy Birthday to Schrödinger’s Cat

In the 80 years since Erwin Schrödinger first outlined a quantum mechanics thought experiment involving a cat, the feline’s popularity has only grown

(ANDRZEJ WOJCICKI/Science Photo Library/Corbis)

Inside a box, there is a cat. It may be dead or it may be alive, because also in that small space is a vial of poisonous gas that has a 50/50 chance of releasing its destruction upon kitty. But no one can know the feline’s fate until they look. This is the basic story of Schrödinger’s Cat, a thought experiment physicist Erwin Schrödinger first proposed 80 years ago today. 

The uncertainty of the cat’s state - alive or dead - means that it exists in a superposition of simultaneously dead and alive. Or at least it would if it were a particle. Schrödinger originally wrote about the cat to make it clear exactly how weird quantum physics can get. In the thought experiment, the cat’s fate is tied to the fate of a radioactive sample, explains Paul Halpern at NOVA. Since decay is random, there is no way to predict when it will happen for an individual atom, "collapsing" - as quantum mechanics puts it - the multiple possibilities into a single outcome​.

Halpern points out that nearly everyone has heard about Schrödinger’s cat, at least as the punch line of a joke. In this age of internet cat videos, that isn’t surprising, but Schrödinger himself would probably be bemused at the popularity of his cat. That’s because from 1935, when the physicist wrote about the idea, until his death in 1961, the cat scarcely showed up in research literature. Even the original appearance of the cat was brief. Halpern writes:

Schrödinger’s paper is a spacious mansion of an article, in which the kitty conundrum occupies just a windowsill-sized space. Yet, like any haughty housecat, Schrödinger’s cat has ended up dominating the whole manor.

So how did the cat make the leap from an aside to main event? The first time the feline ventured beyond discussions among physicists appears to be when philosopher Hilary Putnam​ wrote a 1965 book about quantum mechanics and then discussed the oddness of Schrödinger’s cat during in a review of his book for Scientific American, Halpern reports. From the pages of that popular magazine, science fiction writers took the cat into other realms.

Now, many experiments and non-physics papers reference the idea. But some physicists have moved on. They discuss a superposition of different states using framework called decoherence, where the "collapse" that introduces all the confusion about dead and alive cats isn’t necessary. But without a furry mascot, that idea is unlikely to unseat Schrödinger’s cat in the general consciousness. In pop culture at least, the kitty is very much alive.


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