The buttered cat paradox — the question of whether a cat will always land on its feet even if it has strapped on its back a piece of toast, which always lands buttered side down — may only be a humorous thought experiment, but the fact that cats right themselves during a fall to land on their feet is not. It posed a genuine puzzle to generations of scientists because it seems to defy the laws of nature, specifically angular momentum conservation. If a cat — approximated by a cylinder — starts its fall without a rotational component it cannot pick up angular momentum during its descent. And yet, it does.

In the late 19th century, a series of photographs of a falling cat1 provided some clues for a better understanding of the physiology involved in a cat’s righting reflex. It also ruled out one explanation of the physics: that air resistance could supply the necessary angular momentum. But it wasn’t until 1969 that a simple mathematical model was published2, which could explain how cats violate angular momentum conservation, or rather how they don’t. The solution? Modelling the cat not as one rigid cylinder but allowing it to bend, or using two cylinders in the model.

Feline flexibility continues to inspire semi-serious discussions, for example around the question of whether cats are liquids. Part of the definition of a liquid is that it takes on the shape of the container holding it. Countless memes on the Internet show that cats do the same. This photographic evidence may not match the scientific rigour of the tumbling cat series, but it was enough to get one physicist thinking. Marc-Antoine Fardin used the idea of liquid cats to illustrate a range of rheological principles3, winning him the 2017 Ig Nobel Prize in Physics.

However, the most famous cat in physics is undoubtedly Schrödinger’s cat. The thought experiment Erwin Schrödinger originally conceived as a criticism of the Copenhagen interpretation of quantum mechanics has developed a life (and death) of its own. What started as a challenge to quantum mechanics has become its popular symbol. The dead-and-alive cat lends its name to popular science books, appears in a range of science fiction stories and serves as the all-around mascot of quantum physics. People who know nothing about quantum mechanics still know the cat.

Schrödinger may have only used the cat in his thought experiment to emphasize a seeming absurdity, but the superposition it is meant to illustrate is real. For example, coherent states of an optical mode can have different phases, and it is possible to create a superposition of two such states with opposite phase. This is called a cat state or, if the mean photon number of the mode is low, a kitten state. Unlike the thought experiment that gave them their name, cat states are not a mere curiosity. They have applications in quantum information, where cat codes are an encoding scheme for quantum error correction.

There is another famous cat with ambiguity issues: the Cheshire Cat from Alice in Wonderland, which can vanish, leaving only its grin behind. Although that sounds as absurd as a cat that is simultaneously dead and alive, it comes as no surprise that quantum physicists have made it real too, in their own way. Under certain conditions, a quantum system can behave as if it is in a different place from its physical property. For example, a specific measurement protocol reveals that a neutron may take a different path through a silicon interferometer than its magnetic moment4. These quantum Cheshire cats can even exchange their grins5.

Not content with being the passive objects of (thought) experiments, some cats have joined the ranks of researchers. Possibly the only, but certainly the most well-known, feline physicist is F. D. C. Willard — Chester to his friends. He co-authored his first paper with owner Jack Hetherington in 1975 because Hetherington had used the plural ‘we’ throughout the article and didn’t want to retype it. Chester now has a Google Scholar profile, which records 113 citations across his four publications.

Although Nature Reviews Physics does not endorse feline authorship, we love to see pictures of Felis catus in our pages. You may not have noticed, but we have managed to sneak in quite a few cover cats over the years. How many can you count?