Is the Cat dead or alive?

The title is a reference to a century old experiment that is known by the majority of the world. It refers to the famous (or infamous) ‘Schrodinger’s Cat’ experiment, thought of by one of the proponents of quantum mechanics, Erwin Schrodinger. It is a thought experiment that seeks to explain the concept of quantum superposition using a real life situation. Very simply put, it is an experiment where a cat, in a hypothetical situation, can be considered to be both dead and alive (which is quantum superposition) since its fate is dependent on a single event that may or may not occur.

Quantum Superposition

Before diving into the experiment itself, it is important to give a brief idea about quantum superposition. Quantum superposition is one of the most fundamental concepts of quantum mechanics. According to this principle, two or more quantum states can be “superposed” which, in other words, is the resultant of two or more quantum states is another valid quantum state and conversely, a single quantum state can represent a superposed form of two or more quantum states. It is similar to the superposition principle for electromagnetic waves illustrated by phenomena such as interference, diffraction and polarization.

Mathematically, quantum superposition and superposition principle for vectors (like force and field vectors) in classical physics are similar. This means that if we add two wave functions, we get another wave function. However, the physical interpretation is different. Superposition of two force vectors means that two forces add up to one larger force whereas the resultant of two wave functions means that the particle exists in both states at the same time. But, if someone were to observe the particle then it would be in only one quantum state. In other words, the resultant wave function “collapsed” into one of the original wave functions.

Schrodinger's Cat

Schrodinger’s cat experiment was set up with a hypothetical cat kept captive in a steel chamber with certain devices, which would be guarded against direct interference by the cat. The devices would be a Geiger counter (a device to measure radiation) in which a very small amount of radioactive substance is kept, so small that there is equal probability of it decaying and not decaying over the course of an hour; if the radioactive substance did decay then a small flask containing hydrocyanic acid would be broken by a hammer through a relay system connected to the Geiger counter.

The experimental setup

After explaining the setup, Schrodinger remarked in his letter to Einstein that if the setup is left to itself for an hour then the cat could either be dead or alive and thus the 𝛹-function, ie, the wave function of the system would describe the cat as both dead and alive. This means that the cat, considered as a quantum state, could be considered to be a quantum superposition of both its alive and dead state. Now if an observer were to look at the cat, then it would be either dead or alive and thus superposition would collapse (in more technical terms, the wave function would collapse).

Interpretations of Schrodinger's Cat

Schrodinger’s thought experiment poses an important question, ie, when does a quantum state stop being the superposition of different states and collapse into one single state? There is no single answer to this question but there are various interpretations of it.

The most famous interpretation is known as the Copenhagen interpretation according to which, the cat exists in both the “alive” and “dead” simultaneously until it is observed. As soon as it is observed, its wave function collapses into either the “dead” or the “alive” state. Another interpretation is known as the Many-Worlds interpretation (MWI). According to the MWI, there is no wave function collapse, but as soon as an observer opens the box with the cat, it splits into two realities, one with a dead cat/observer and another with a live cat/observer. According to the Bohr interpretation, a change in the Geiger counter can be considered to be a measurement in itself and an observer would be observing something that already happened and thus, Schrodinger’s cat would be either alive or dead long before the box is opened.

There are many other interpretations, the ones mentioned being the most prominent of them all. Schrodinger’s Cat proves how weird quantum mechanics really is and at the same time shows how interesting the study of quantum mechanics really is.