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What is Schrödinger's Cat? © Getty Images

What is Schrödinger's Cat?

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Can a cat be dead and alive at the same time? If it helps explain quantum physics, then why not!

The Schrödinger's cat thought experiment demonstrates the strange nature of quantum superposition. A cat is placed in a box with a vial of poison, which will be automatically smashed if a radioactive particle decays. Radioactive decay is one of the probability-driven aspects of quantum physics. We can't say when a given particle will decay, we only know the probability of it decaying in a certain period. After some time has elapsed, an unobserved particle will be in a superposition of decayed and not decayed states. All that exists before measurement (someone looking inside the box) is the probabilities. But since the life of the cat depends on the state of the particle, does this mean the kitty is simultaneously dead and alive?


In reality, we could never witness the cat being both alive and dead - as soon as we look in the box, the cat will be in just one state. And the practicalities of the experiment don't even allow for this. For the detector to be able to release the poison it would have to interact with the particle, forcing it to be either decayed or not decayed.

Despite its limitations, Schrödinger's cat gives us a feel for the weirdness of superposition, and while such a test wouldn't be possible with a complex organism like a cat, proposed experiments with a tardigrade could bring an aspect of the Schrödinger's cat experiment closer to reality.


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Brian is a writer of popular science books, with a background in experimental physics. The topics he writes on range from infinity to how to build a time machine. He has also written regular columns, features and reviews for numerous magazines and newspapers, and given lectures at the Royal Institution in London, Oxford and Cambridge Universities, and Cheltenham Festival of Science.


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