Curiosity both did and did not kill the cat

Can a cat appear to be simultaneously dead and alive? Through our observations of everyday phenomena controlled by the laws of classical mechanics and human intuition, the question seems nothing far from absurd. It depicts the strangeness of quantum mechanics perfectly. This conundrum comes from the famous thought experiment proposed by Erwin Schrödinger: an Austrian physicist with an apparent dislike for felines. Published in 1935, Schrödinger’s paper was an important contribution to our understanding of quantum mechanics.

Aged 56, Erwin Schrödinger’s Nobel prize photograph Image: Nobel Foundation

Imagine a live cat being placed into a box, along with a radioactive source – a Geiger counter –  and a sealed flask containing lethal poison. The flask is only shattered if the Geiger counter detects radioactive decay, at which the cat is killed by the escaping poison. Once the lid is closed, the fate of the cat becomes unknown and it is not until the lid is removed that an observation can be made concerning the cat’s condition. In other words, there is no way of knowing if the cat is alive or dead without physically checking.

In the quantum world, the cat is said to be in a superposition of states. This means that it is neither alive nor dead, but a mixture of both. Until observed, this state is maintained. Some theorists suggest that the very act of observing the cat “collapses the superposition”, making the cat either alive or dead.

The question arises as to why innocent cats have been subjected to such cruelty, and how the situation could possibly offer insight into quantum mechanics. To demonstrate the phenomenon on a quantum scale, a physical system like an electron orbiting a hydrogen ion is used. In the Copenhagen interpretation of quantum mechanics, it is theorised that the electron exists in a cloud of probability rather than as a single particle that is constantly orbiting the nucleus of the atom. The electron’s position is only determined once it is measured. This measurement is said to force the electron into a single one of its many iterations, thus collapsing its state of superposition.

This is comparable to the question of the cat being alive or dead before the box is opened. However, as in any area of scientific study, disagreements have arisen. Schrödinger proposed his thought experiment to purposefully demonstrate the invalidity of the Copenhagen interpretation by questioning what should be considered a valid observer and what is sufficient enough to cause a collapse of the superposition of states. Does the Geiger counter, or even the cat itself, qualify as an observer capable of causing the collapse of the state?

Nonetheless, as a paradox suggested to serve as an example of the irrationality of another physicist’s interpretation, the consequences of Schrödinger’s contemplations have had astonishing profits to the field of quantum theory. By allowing scientists to use this concept as an example for them to explain how their theories differ it has thus enabled the progression of the field in areas such as the advancement of quantum computing, and the investigation into the characteristics of quantum entanglement.

A graphical representation of the contents of Schrödinger’s box Image: Doug Hatfield

As well as having scientific value, the simplicity of Schrödinger’s idea has helped to popularise quantum physics. The thought experiment has even become prominent in mainstream culture. English author Douglas Adams depicts the use of clairvoyance by the protagonist of Dirk Gently’s Holistic Detective Agency to see inside Schrödinger’s box. Amusingly the cat was neither alive nor dead but had simply grown tired of being subjected to the experiment and wandered off.

Returning to the initial question of whether a cat can appear to be both dead and alive at a single point in time results in us being able to conclude that everyone can hold their own interpretation of this famous experiment. The beauty of Schrödinger’s otherwise simplistic proposal lies in the fact that it is purely theoretical and consequently provokes critical thinkers, physicists and non-physicists alike, to think twice.

Disclaimer*: No cats were harmed in the writing of this article.