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I've heard the Schrödinger's cat "paradox" (although there's nothing particularly paradoxical about it, just counterintuitive), but I've never been clear on whether or not it's meant to be taken with a grain of salt.

I've always assumed it was just a metaphor for the behavior of particles at the quantum scale, not a description of what would actually happen if you put a cat in a box. The idea that a conscious, human observation of the contents of the box - rather than a more mechanical interaction such as the dead cat hitting the bottom of the box or the live cat breathing the air in the box - would have any bearing on the state of the system quite honestly seems insane to me. I see no reason that the gravitational effects of the massive particles in the cat, the interaction of the cat with the box, the air, or other parts of the cat itself wouldn't be sufficient to collapse the wave-function. Why does someone need to see it? Is nature really supposed to treat conscious observers differently from any other object?

So, my question is this: Do quantum physicists literally believe the implications of this thought experiment, taken at face value, or is it an impossible but conceptually illustrative explanation of what happens when observing single particles? This paradox has been explained to me many times by different sources, and not once have a heard a disclaimer at the end along the lines of: "but, of course, this is completely ridiculous and only really happens with very small objects, not complex systems like cats and boxes." Is this disclaimer implied, or do people really believe this? (in which case my philosophical understanding of quantum physics needs serious revision to understand how this cat predicament could actually be the case.) Thanks in advance!

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Try to remember that Schroedinger put this forward as a counterexample of I think the Copenhagen Interpretation. The reason it seems so ill-formed is that Schroedinger didn't really spend a lot of time on it... –  Nic Jan 24 '13 at 17:08

2 Answers 2

There was a lot of disclaimer that was implied. For one, a "box" in physics is much better at containing things than the cardboard boxes you see containing cats on the internet. The box here is understood to not allow any information to pass through its walls except where and when we specify. That is, there is no environmental contamination to decohere things.

Further, there needs to be some reason why that cat's state might change.1 If you put a live cat in a box, even a perfect one, for a little while, nothing will change over that time, classically or quantum mechanically. Without some other agent, there is no significant mixing of the $\lvert \text{live} \rangle$ and $\lvert \text{dead} \rangle$ states as the wavefunction evolves over a short time. That's why the thought experiment is often phrased with a poison gas apparatus linked to a radioactive substance tuned in such a way as to be triggered with 50% probability over the course of the experiment. You need to throw in some of that quantum randomness, as well as a mechanism for spreading it to the whole contained system, for this sort of thing.


1 No cats were harmed in the making of this post.

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1 - but it's possible a large number of alternate universes were created –  Martin Beckett Jan 24 '13 at 19:44

See Is Schrödinger’s cat misleading? And what would happen if Planck constant is bigger?. This isn't an exact duplicate, but it explains why we do believe the cat can be in a superposition of states, but the superposition only lasts a very very short time before it decoheres.

The decoherence has nothing to do with consciousness, simply the number of degrees of freedom of the environment. The article by Luboš Motl that I linked in the above answer explains this in some detail and I strongly recommend it as it's fascinating reading.

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