What is Schrodinger's cat? | New Scientist

Schrödinger’s cat — Sven Puth/EyeEm/Getty Images

Devised in 1935 by the Austrian physicist Erwin Schrödinger, this thought experiment was designed to shine a spotlight on the difficulty with interpreting quantum theory.

Quantum theory is very strange. It says that an object like a particle or an atom that adheres to quantum rules doesn’t have a reality that can be pinned down until it is measured. Until then, its properties, such as momentum, are encoded in a mathematical object known as a wave function that essentially says: if you make a measurement, here are a range of possible outcomes. The inevitable question that arose as the theory developed was: what, then, is the thing doing before that?

The most prominent answer in the 1930s came from the Copenhagen interpretation, developed in the Danish city by luminaries of quantum theory, Niels Bohr and Werner Heisenberg. This says that there really is no definitive reality before the measurement, and the object is in an undefined state known as a superposition.

Schrödinger’s thought experiment probed how this plays out when a quantum object is coupled to something more familiar. He imagined a box containing a radioactive atom, a vial of poison and a cat. Governed by quantum rules, the radioactive atom can either decay or not at any given moment. There’s no telling when the moment will come, but when it does decay, it breaks the vial, releases the poison and kills the cat.

If the Copenhagen interpretation is correct, then before any measurement has occurred, the atom, and so also the cat, are in a superposition of being decayed/dead and not decayed/alive. The absurdity of speaking of a simultaneously living and dead moggie was supposed to show that the Copenhagen interpretation must be lacking something.

The experiment played an important part in spurring other ways of thinking about quantum theory, including the many worlds interpretation, which says that the different possible realities of a quantum object crystallise into different parallel universes at the point of measurement.

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These days the thought experiment has taken on a kind of cult status. There are Schrödinger’s cat T-shirts, memes and hundreds of articles on the subject. In 2018, scientists published a more complicated variant of the thought experiment that appears to show that all existing interpretations of quantum theory are incomplete.