What is Schrödinger’s Cat?

G Darley

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What is Schrödinger’s Cat? A simple explanation
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The concepts we face when contemplating the peculiar science of quantum mechanics can undoubtedly be challenging to comprehend, even when applied to infinitesimally small particles. However, when extended to everyday objects they might seem impossible. This is exactly what Schrödinger’s famous thought experiment seeks to highlight. 

Thought experiments are hypothetical scenarios which are constructed for the purpose of deliberating their outcomes and consequences. This one was initially proposed in 1935 by Erwin Schrödinger to underline what he saw as fundamental issues with the Copenhagen interpretation of quantum mechanics, particularly how wavefunction was dependent on an external observer. To emphasise this, he constructed a system where a large-scale object could become superposed because of its connection to a quantum particle. 

The Experiment

A cat is placed in a sealed box along with a radioactive atom, a Geiger counter, and a vial of poison which is set to be released should the Geiger counter be triggered. The radioactive atom could decay at any moment which would set off the Geiger counter and kill the cat. 

However, crucially the atom is of a sufficiently small scale that it obeys the laws of quantum mechanics. Therefore, as per the Copenhagen interpretation, it is in a superposition of states being simultaneously decayed and not decayed up until the point it is observed. 

So, by extension, within this sealed box, the cat is both alive and dead until someone opens it, at which point the wave function for this system would collapse and the cat would assume one of the two possible states. 


Of course, in reality, it is fairly evident that this would not occur – a cat cannot be both alive and dead at the same time. To this day, it remains unknown exactly when quantum superpositions collapse, and thus the problem highlighted by this thought experiment remains unsolved. However, there have been various interpretations that propose theoretical answers to this dilemma. For example, in the many worlds interpretation, the cat is both alive and dead however, the outcomes are decoherent and therefore do not interact with each other. If the observer opens the box and finds a dead cat they are simply in the universe where the cat died and there is another equally real universe where the cat survived. 

In the relational interpretation, the human observer, the Geiger counter, and the cat can all be considered observers, however, they can all have a different account of events because they have different levels of information. For instance, at the point the poison is released, the cat will know whether it has been poisoned but the human will not, and therefore for the cat the wave function has apparently collapsed, while for the human this does not occur until they open the box. 

Finally, according to objective collapse theories, the wave function collapses spontaneously when certain conditions (which we do not yet understand) are met. This might be after a certain time, or at a certain mass or temperature, but either way, the cat’s fate would be decided long before it is observed. While this might appear to be logical, in the current laws of quantum mechanics, superpositions don’t collapse due to the passage of time so it would require an adjustment of our current understanding. 


The absurdity of this experiment has resulted in its strong presence throughout popular culture, transcending the field of physics, and as an easily digestible illustration of the behaviour of atoms, it has become a valuable tool for education. Schrödinger’s cat is unquestionably one of the cornerstones of quantum mechanics, being the subject of much theoretical discussion surrounding the nature of superposition, and the rules by which it is governed. 

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