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Say I have Schrödinger's cat in a sealed box. It is neither alive nor dead.

Given that cats have a lifespan of ~20 years, if I wait a thousand years, I should be able to infer with 100% certainty that the cat is dead. But if I can say with 100% certainty that the cat is dead, then I have "collapsed the wavefunction" without performing a measurement. How does that make sense?

The only way this seems to make sense is if cats are actually $\hat{cats}$, and the "cats operator" turns $\frac{1}{\sqrt{2}}\left(|dead\rangle+|alive\rangle\right)$ into $|dead\rangle$ over a period of approximately 20 years - but that conclusion seems rather bizarre on an intuitive level.

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    $\begingroup$ Here is my opinion on this gedanken experiment physics.stackexchange.com/questions/382968/… $\endgroup$
    – anna v
    May 31, 2023 at 7:47
  • $\begingroup$ This is a good one. Obviously, cat analogy should not be taken literally, but it is interesting to reflect on whether a living thing can be in a superposition. $\endgroup$
    – Roger V.
    May 31, 2023 at 8:57
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    $\begingroup$ You're getting nonsense because you're applying quantum-theoretic formalism to macroscopic systems. This was the whole point of Schroedinger's thought experiment. $\endgroup$ May 31, 2023 at 9:56
  • $\begingroup$ Comments have been moved to chat; please do not continue the discussion here. Before posting a comment below this one, please review the purposes of comments. Comments that do not request clarification or suggest improvements usually belong as an answer, on Physics Meta, or in Physics Chat. Comments continuing discussion may be removed. $\endgroup$
    – Buzz
    Jun 1, 2023 at 21:55

12 Answers 12

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You have not collapsed the wave function. It will now be

$$\frac{1}{\sqrt{2}} \Big(| \mathrm{cat\ dead\ of\ old\ age}\rangle + | \mathrm{cat\ dead\ of\ cyanide\ poisoning}\rangle\Big).$$

(Of course, this assumes you left twenty years of food for the cat in the box.)

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    $\begingroup$ A third state: perhaps the cat has developed immunity to cyanide after so many experiments $\endgroup$
    – Juan Perez
    Jun 1, 2023 at 23:00
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    $\begingroup$ Quote by Robert A. Heinlein: “Never try to outstubborn a cat.” In the course of that 1000 years, it will occasionally wonder "What happened to that human I put outside the box". $\endgroup$ Jun 2, 2023 at 9:04
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    $\begingroup$ It makes no sense to write things such as $|\text{cat dead of ...}\rangle$. Dead cat is not a thing representable by a quantum state, it's closer to a macrostate in statistical physics, a class of microstates compatible with some condition. $\endgroup$ Jun 2, 2023 at 12:32
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    $\begingroup$ @JánLalinský: The point of my answer is that the cause of the cat's demise can be deduced from its corpse, so that the wave function does not collapse. In my opinion, this is a much more illuminating response than simply saying "a dead cat isn't in a pure quantum state, so you can't apply quantum mechanics." This might lead people to believe in the magical separation between the macroscopic realm and the quantum realm which you can find in certain popular explanations of quantum mechanics. $\endgroup$ Jun 2, 2023 at 12:39
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    $\begingroup$ @FlatterMann I will once again remind you that comments are for specific criticisms of the posts being commented upon, not for general discussion. If you think you have a better answer to this question than the ones being offered, then instead of commenting on every single answer, you should provide an answer of your own. $\endgroup$
    – ACuriousMind
    Jun 3, 2023 at 9:06
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You are missing the point of the experiment. When we do the real experiment with simpler substitutes for the cat, both dead and alive refer to states that are stable eigenfunctions of the Hamiltonian, and so should stay as they are no matter how much time you wait.

It is phrased as a cat just so that you get a gut feeling that it is weird.

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    $\begingroup$ That's because we are using unphysical Hamiltonians. They are a lot easier to deal with than the ones that are describing real systems... the downside is that they are making the wrong predictions for corner cases like Schroedinger's cat. $\endgroup$ May 31, 2023 at 7:04
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    $\begingroup$ ??? The actual point of the experiment is that it's a nonsense situation. It was his attempt at disproving the Copenhagen interpretation. $\endgroup$ Jun 2, 2023 at 1:23
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    $\begingroup$ @BlueRaja-DannyPflughoeft It definitely is the case that Schrödinger was trying to ridicule Copenhagen, but we are always seeing analogous stuff in experiments, so you must not think of it as nonsense. $\endgroup$ Jun 2, 2023 at 1:26
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    $\begingroup$ @jwenting that may be the point some people take out, but it was not Schroedinger's point. $\endgroup$ Jun 2, 2023 at 12:25
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    $\begingroup$ @jwenting - uh, no, observation altering something wasn't what Schroedinger had a problem with. At all. $\endgroup$
    – Davor
    Jun 2, 2023 at 16:18
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if I can say with 100% certainty that the cat is dead, then I have "collapsed the wavefunction"

I don't think so. "Collapsing the wave function" implies knowing the exact quantum state of every particle of matter inside the box. When we say that the cat either is "alive" or "dead" we're reducing all of that incomprehensible complexity to two macro states—two mutually exclusive subsets of the set of all possible states. Knowing whether the cat is alive or dead really is just a proxy for knowing the complete state of the contents of the box. It only gives you a single bit of information about the state, but what is represented by that single bit is enough to seed the philosophical discussion about what "collapse the wave function" really means.

After the cat's remains have been mouldering for 1000 years—when "alive" is the null set, and "dead" encompasses every possible state—then knowing that the cat must be dead tells you nothing at all about which of all of the possible states the box contents have reached.


See also, @J.G.'s comment on your question. Maybe what I've said here is just an expansion of J.G.'s more concise observation.

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    $\begingroup$ You can "collapse wave functions" without knowing the precise quantum state of every piece of matter in an experiment. In fact, you will never know the precise quantum state of every piece of matter in any experiment. $\endgroup$ Jun 2, 2023 at 12:58
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The states "dead" and "alive" are obviously a simplification of the actual state of the cat. The full state of the cat is much more complicated - and includes the state of every single cell in its body, and so on. This full state evolves over time (according to the Schrodinger equation, according to the Hamiltonian operator H).

So the state you called "alive" continues to evolve according to H, and after 1000 years, it will evolve into something that looks like a decayed cat body. Both "dead" and "alive" states will evolve over 1000 years to something that looks the same. You can call the resulting state "decayed" - you can't call it "dead" because it's not the same "dead" that you get immediately after the mechanism kills a cat (which looks nothing like a decayed cat skelaton).

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    $\begingroup$ One might also add that "decayed" cannot be a unique state; it must be a collective name for all states in a certain class. That's because "alive today" and "dead today" both evolve into "decayed", but unitary evolution is reversible. $\endgroup$
    – WillO
    May 31, 2023 at 19:40
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Being alive actually implies a whole lot of irreversible thermodynamic processes taking place. Same can be said about aging or dying. In this sense, the whole Schrödinger equation does not make sense (more precisely, it should not be taken literally), since the irreversible dynamics of cat dying excludes its possibility of being in a superposition of states.

Schrödinger likely was aware of that, since he was among the first to think seriously about the physics of life - see his book What is life?

Remark: But what did the cat die from?
Assuming that we can put a leaving creature into a superposition, and the cat is definitely dead after a thousand of years, the Schrödinger's question only changes from alive/dead to determining the cause of death: natural/poisoning.

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There can't be a continuous time evolution operator that takes $\frac{1}{\sqrt{2}}\left(|dead\rangle+|alive\rangle\right)$ to $|dead\rangle$ and also takes $|dead\rangle$ to $|dead\rangle$, because it wouldn't be unitary. (It wouldn't be invertible; also, by linearity it would have to take $|alive\rangle$ to $(\sqrt2{-}1)|dead\rangle$, which isn't properly normalized.)

Time evolution could take $|alive\rangle$ to $|dead'\rangle$ and $|dead\rangle$ to $|dead''\rangle$, where $|dead'\rangle$ and $|dead''\rangle$ are orthogonal states in the subspace of ways to skin a cat, which has dimension greater than 1. One of them could be identical to $|dead\rangle$, but "realistically" neither would be. There would still be a collapse when you broke the seal and exposed the remains to the environment, because the environment interacts with more than just the deadness property of the remains.

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In the Schrodinger's Cat thought experiment, the state of the cat is not really the interesting thing. Part of apparatus is a radioactive substance and Geiger counter. When the Geiger counter detects a decay of the substance, it triggers release of a poisonous gas that kills the cat.

While we know that the cat will eventually die due to natural aging, what we're really interested in is whether the substance has decayed, as this is a random quantum-mechanical process. The cat's aliveness is simply a proxy for that, intended to be more illustrative of the strange implications of QM. We could easily remove the cat from this process and talk about whether the box has any of the gas in it; if the gas is colored red, we could say that until we open the box, it contains a superposition of clear air and red gas.

For the purposes of this experiment, other causes of the cat dying or the gas being released are irrelevant.

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No. Schrödinger's experiment was a thought one, and was meant to show the paradox emerging from certain applications of quantum mechanics. It is not intended to get an experimental result, but rather illustrate a paradox of quantum superposition applied to real world objects. You are trying way too hard to be "smart", overthinking said thought experiment without understanding its purpose and conclusions, and getting something which makes no sense in the real world.

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All the smart and correct explanations have already been made, however, consider this:

The experiment has not changed at all, just the definitions did (slightly).

Now the question that cannot be answered until you open the box is no longer "did the cat die, or did the cat live", it is "did the cat die from the random poison trigger thing, or did the cat die from old age/starvation/whatever".

This can then be rewritten as two following questions instead, only answered by opening the box:

"did the random trigger thing happen (and the cat died from it)" OR "did the random trigger thing not happen (and the cat stayed alive)"

and

"did the random trigger thing happen (and the cat died from it)" OR "did the random trigger thing not happen (and the cat died by other means)".

The statements in parentheses are observations made by opening the box, while the statements outside are the information gained by making these observations.

If the observation part is removed, the questions become both the same (i.e., "did the random trigger thing happen?"), while the observed state of the cat is different by how the experiment is conducted, the thing that is uncertain until those observations are made is exactly the same, and therefore, the exact same "something that can be one or the other, but is unknown until it is observed" situation applies.

The cat is just an indicator of the state - if we change the way the indicator works, we are not changing the state in any way.

Therefore, the whole Schrödinger's cat experiment is essentially the same, although none of the possible outcomes are favourable for the cat.

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There are no physical laws stating that cats have a life time of (for example) 20, 50, .... years.

It follows that the observer can only know the state of the cat (alive or dead) is by opening the box.

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Yes, you can infer the cat is dead after 1,000 years, but you can't infer that the cat died of cyanide poisoning or old age, actually you can probably infer it didn't die of old age since it would starve to death within a week or two, but you can't infer if it was poisoned or starved to death. Strange that you say "how does that make sense?" since the entire point of Schrodeingers thought experiments was to demonstrate the absurdity of the Copenhagen interpretation, i.e. it doesn't make sense if the way its originally intended.

Whenever I hear or read "collapsed the wavefunction" my brain translates the phrase into "Abracadabra".

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No, because this misses the point. It's not important that the metaphor uses a cat. Getting into irrelevant hypotheticals, like how it would eat, or its natural lifespan, isn't a meaningful way to engage with the topic.

Now, if that's your idea of a fun party conversation, go for it! :D

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