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What is it like to be Schrödinger's cat?

In 1935 physicist Erwin Schrödinger devised a thought experiment that still makes people's heads spin (up and down, simultaneously). 

By Staff / August 12, 2013

A pair of kittens are observed sitting in a bag during the World Cat Exhibition in Dortmund, Germany, in April.

Frank Augstein/AP


Monday marks the 126th anniversary of the birth of the Austrian-Irish scientist Erwin Schrödinger.

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Schrödinger's work spanned the breadth of physics and even spilled over into biology, but he is best known for Schrödinger's cat, a thought experiment that criticized the Copenhagen interpretation of quantum mechanics. 

Developed in the 1920s by the Danish physicist Niels Bohr, with input from Werner Heisenberg, Max Born, and other physicists, the Copenhagen interpretation is an attempt to make sense of some very weird phenomena observed in the subatomic realm. 

Subatomic particles, it seems, are not really at all like anything we're familiar with. Electrons, for instance, sometimes behave like waves, destructively interfering with one another. And the exact position and momentum of an electron cannot be specified at any given instant.

According to the Copenhagen interpretation, that's because electrons and all other subatomic particles actually exist in multiple places at once, and can have multiple, contradictory properties at the same time. It is only when a particle's properties are measured, said Bohr, that the "wavefunction" of possibilities collapses into a specific state. 

The Copenhagen interpretation is the most widely accepted interpretation of quantum mechanics. It can be used to predict – with astonishing accuracy – the behavior of the most fundamental constituents of matter.

But Schrödinger wasn't a fan. Little things like electrons, he thought, ought to follow the same rules as big things like billiard balls, planets, galaxies, physicists, and cats. 

To express his displeasure at Bohr's quantum capriciousness, in 1935 Schrödinger devised a thought experiment by which the seemingly indeterminate state of a relatively little thing – in this case, the product of radioactive decay – can influence the seemingly understandable state of a relatively big thing, in this case a potentially unfortunate cat. 

Imagine, wrote Schrödinger, a cat sealed in a box for one hour with a diabolical device consisting of a tiny radioactive substance, a Geiger counter, a hammer, and a flask of hydrocyanic acid. When the Geiger counter detects a high-speed electron flung from the radioactive material, it triggers the hammer, which smashes the flask, releasing the hydrogen cyanide and quickly causing all nine of the cat's lives to succumb to hypoxia. 

But there's only a 50 percent chance that, in the space of sixty minutes, the substance will decay and trigger the feline-killing mechanism. There's a 50 percent chance that it won't.


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