# Schrödinger's cat; why was it necessary?

Could someone please explain to me the idea that Schrödinger was trying to illustrate by the cat in his box? I understand that he was trying to introduce the notion of the cat being both alive and dead at the same time. But why was it necessary to introduce this thought experiment and what did it achieve?

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The original paper is relatively short and very readable. –  MBN Feb 10 at 11:08

First, a historical subtlety: Schrödinger has actually stolen the idea of the cat from Einstein.

Second, both men – Einstein and Schrödinger – used the thought experiment to "explain" a point that was wrong. They thought it was absurd for quantum mechanics to say that the state $a|{\rm alive}\rangle+b|{\rm dead}\rangle$ was possible in Nature (it was claimed to be possible in quantum mechanics) because it allowed the both "incompatible" types of the cat to exist simultaneously.

Third, they were wrong because quantum mechanics does imply that such superpositions are totally allowed and must be allowed and this fact can be experimentally verified – not really with cats but with objects of a characteristic size that has been increasing. Macroscopic objects have already been put to similar "general superposition states".

The men introduced it to fight against the conventional, Copenhagen-like interpretations of quantum mechanics, and that's how most people are using the meme today, too. But the men were wrong, so from a scientifically valid viewpoint, the thought experiment shows that superpositions are indeed always allowed – it is a postulate of quantum mechanics – even if such states are counterintuitive. Similar superpositions of common-sense states are measured so that only $|a|^2$ and $|b|^2$ from the coefficients matter and may be interpreted as (more or less classical) probabilities. Due to decoherence, the relative phase is virtually unmeasurable for large, chaotic systems like cats, but in principle, even the relative phase matters.

Quite generally, the people who are wrong – who have a problem with quantum mechanics – like to say that the superposition means that the cat is alive "and" dead. But the right, quantum answer is that the addition in the wave function doesn't mean "and". Instead, it means a sort of "or", so the superposition simply says that the cat is dead or alive, with the appropriate probabilities (quantum mechanics determines not only the probabilities but also their complex phases, and those may matter for other questions).

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Reading up on Wikipedia, I presume that the theft of the idea of the cat was actually the theft of an idea that lacked a cat. –  Glen The Udderboat Feb 4 at 17:28

The thought experiment aimed at illustrating one concept, and questioning the validity of such concept: 1/ The concept: in quantum mechanics, before an observation is made (note the cat is in a closed box, and nobody can see what is going on in the box), a system is not in a defined state, but only has a certain probability to be in any state - here the system is a cat, and the states are "dead" and "alive", and it is indeed a striking picture. With this, the goal of making eyebrows raise, and listeners question realize the significance of the concept is indeed reached.

2/ The questioning: by applying the concept of superposition to life and death issues, and also to a macroscopic object, the aim was to highlight the really revolutionary nature of the concept itself - in particular to highlight our complete ignorance of the reason why this quantum behaviour is ubiquitous at the atomic scale, but more rarely observed at the macroscopic scale (even though some manifestations do exist: superconductivity, BEC...)

So my answer is: it was not necessary, but it was a striking way to convey the concept of quantum superposition, and ask what is the defining limit between the microscopic and macroscopic laws of nature, if there is any. This question is still in debate, and is one of the goals of unification theories, with the major question of a quantum theory of gravity still unsolved.

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