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The premise behind Schrödinger's cat as I understand it is there is a radioisotope whose quantum randomness is tied to the alive or dead status of a cat in a box. The idea is that the cat is both dead and alive until you look, because the state of alive or dead is entangled to a quantum superposition that is both states at the same time.

However, I would argue that the cat is not both dead and alive and is either dead or alive even before you look. If a quantum superposition is only determined (collapses into a definite position) once an observer observes it, then wouldn't the Geiger counter connected to the radioisotope and hammer and bottle of poison have already 'looked' and thus 'found out,' and thus caused the superposition to collapse, even before a human observer opened the box?

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    $\begingroup$ I keep saying this, but one more time won't hurt : it's not a very well thought out thought experiment and it's best not to dive deep into it. It was intended to illustrate some concepts that were not very well communicated at the time, but it tends to confuse more than clear up. Clever bloke, Schrodinger, but perhaps not so good with cats. :-) $\endgroup$ Commented Feb 22, 2023 at 22:16
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    $\begingroup$ Indeed, this has been the mainstream view for decades: the observation/collapse/decoherence is at the level of the Geiger counter, and there never have been or could be dead/alive cat interference effects. If you are still talking about this, you have been duped. $\endgroup$ Commented Feb 22, 2023 at 22:38
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    $\begingroup$ FWIW, Schroedinger himself thought that the idea of the cat being in a superposition of dead and alive states was ridiculous. en.m.wikipedia.org/wiki/Schr%C3%B6dinger%27s_cat He described the thought experiment as a way of illustrating his dissatisfaction with the Copenhagen interpretation. But, believers in the Copenhagen interpretation will tell you that anything you could possibly do to show that the cat is not in a superposition of states would count as an "observation." And, of course, you can only observe it to either be definitely alive or definitely dead. $\endgroup$ Commented Feb 23, 2023 at 0:26
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    $\begingroup$ P.S., No. The Copenhagen interpretation does not allow the experimenter to say that the Geiger counter is an "observer." The Geiger counter is inside the box. The experimenter is outside the box. Until the experimenter opens the box, they cannot know anything about what has happened inside the box apart from whatever they may know of the composite wave function that represents every possible thing that could have happened. The heart of the controversy--the idea that separates the true believers from the skeptics--is that Copenhagen says, whatever the experimenter cannot know is not real. $\endgroup$ Commented Feb 23, 2023 at 1:00
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    $\begingroup$ No cat can be simultaneously dead and alive, and nothing in quantum mechanics or any of its interpretations suggests otherwise. "Dead" is, by definition, the same thing as "not alive". The state of Schriodinger's cat is neither dead nor alive, let alone both, and when people say otherwise, I can't help wondering whether they don't know what the English word "dead" means or whether they've failed to realize that doing physics is not a license to spout obvious nonsense. $\endgroup$
    – WillO
    Commented Feb 23, 2023 at 3:41

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It is a matter of interpretations of quantum mechanics and the details have not been definitely closed.

One key point that you are missing here is that the box is isolated completely from the outside world.

In that case, either the whole contents of the box are in a superposition until it is open, or the Geiger counter or some other variables are enough to select one of the two outcomes.

  • Copenhagen interpretation claims that while not observing the system, it follows Schrödinger's equation, and when measuring everything collapses to a given outcome. Schrödinger's thought experiments puts Copenhagen advocates in a thought spot because it is not clear what constitutes a measurement (this is an open problem in general) so some will say that yes the cat is isolated from the outside world thus in a superposition of dead plus alive, and some will say that the Geiger counter is a large enough device. But if you choose the latter you have to define what is large.

  • Many-world interpreters only believe that Schrödinger's equation is the only thing that exists (there is no collapse). They say that while the box is isolated the cat is indeed in a superposition. But once you open it, it entangles with the rest of the universe, splitting into two worlds with the two different outcomes (now it is you and the cat that are in a superposition of having observed the cat dead and having observed the cat as alive).

  • Bohmians believe in hidden variables. They say that quantum mechanics is incomplete and that there are variables that determine the outcome. In that way the cat was never in a superposition of alive plus dead but one of the two. However this path is in conflict with Bell's theorem and such variables have to be very weird in a different way (nonlocal).

There are many more, however let me offer one modern perspective of this problem that partially solves it. That view is called decoherence. If you have two entangled particles but you only observe one, it behaves as if it is just a particle with a definite state (mixed state). The same happens if we look into a subsystem instead of the entire box, the calculations show that the quantumness of the radioisotope gets lost the more it gets entangled to other particles. So looking at the particle alone, we can say definitely that it looks like a system with a definite state due to the large number of particles in the measuring device interacting with the radioisotope. And as there are no perfect boxes in this universe, we can always zoom out more and more (particle and Geiger, particle+Geiger+air, box, box+lab, and so on) and see that the cat was never in a superposition because decoherence is even stronger the more and more particles in the subsystem. However the warning here is the same as in the above problems, what would happen if the box was truly completely isolated? And even if it is not, what happens when you look at the entire system/universe? Physicists have not agreed on a definite answer.

Additional warning: note that aside from the impracticality of the experiment, it is not a good one. There is no measurement you can make (at least not in that specific configuration) that can tell you if the cat was in a superposition or not in the first place. It is indistinguishable from an experiment where we replace the Geiger counter and radioisotope with a classical coin throw. You should maybe take a look at Bell tests and entanglement problems that show much more decisively why quantum mechanics breaks classical intuition.

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The idea is that the cat is both dead and alive until you look, because the state of alive or dead is entangled to a quantum superposition that is both states at the same time.

That's a possible idea, but it's not necessarily true, and it is not the idea that Schrödinger wanted to communicate.

His aim was to show that extrapolating the quantum-theoretic description of state (allowing entanglement) to macroscopic systems makes that description not viable as description of the real state of the system, which macroscopic systems are assumed to have at all times; it is dubious and even ridiculous.

This is because the description allows macroscopic systems in contact with quantum systems (such as decaying nucleus) to be in strange states which are (by the very theory that predicts them) impossible to observe (because these quantum states can't be observed, observation destroys them in favor of the "sane" states), and macroscopic systems are observed all the time and such "insane" states indeed were never observed. So the very idea of applying the formalism which produces such states easily and all the time is suspect.

It is one thing to allow the assumption the electron has no definite position, and entirely another to admit that macroscopic machine has no definite state when nobody's looking. It does not matter if it is cat or Geiger counter. The trouble is with the positivist attitude that things don't exist when we're not looking. Schrödinger did not like that, and many people still don't. It is suspect to believe that things exist that can't be measured (superpositions of macroscopic systems), and that things that can be measured (he usual macroscopic states) don't exist most of the time when we're not measuring.

There is a pragmatic way out that preserves usefulness of the quantum theory - use quantum theory as generalized statistical physics, the so called "statistical interpretation". It predicts probabilities of results of measurements, with good track record. It does not necessarily say much about "what is really going on" or "what is the real state of the system" when no measurement is being made. Quantum state is a concept we use, rather than 1-to-1 image of the real state of the system. It's like probability distribution in statistical physics; a tool, a map, not the studied thing, not the territory.

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An observer or looker is sometimes defined as something that keeps a record of an event or outcome. In this respect the Geiger counter by causing the bottle of poison to be irreversibly broken has recorded an event and so from the point of view of the Geiger counter the cat can be dead .But outside the box an observer still does not know if the cat is dead or alive because no information passes to this observer and they cannot record any event that may or may not have happened.

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It is not possible, even in principle, to isolate the content of the box. The matter inside consists of charged, massive particles (electrons and nuclei), so there will be electric, magnetic and gravitational fields "leaking" outside. If you measure the gravitational field outside the box you could detect if the cat is still moving for example. There is no way to shield this field.

In conclusion, we can learn nothing from this experiment since it cannot be done.

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    $\begingroup$ It is a thought construction to point out some undesirable properties of the quantum-theoretic description. Nobody ever suggested the experiment should be done, no strange states of the cat can be observed even according to QT. That is the point Schroedinger makes. Complicating the construction with gravity field misses the point. $\endgroup$ Commented Feb 23, 2023 at 13:27
  • $\begingroup$ @JánLalinský, Yes, Scrhodinger ridiculed Bohr's interpretation of QM. Still, this thought experiment is supposed to reveal some deep insights about QM and even now, there are a lot of papers published on this issue. The problem is that the experiment cannot be done, not for practical reasons but for theoretical ones. It's not just the gravitational field, but the electric field as well. An electron in the box will still produce an electric field outside which can be measured so, the box cannot isolate the electron. $\endgroup$
    – Andrei
    Commented Feb 24, 2023 at 6:14
  • $\begingroup$ @JánLalinský, In other words, the experiment, as described, contradicts the laws of physics. Pondering about it is just like trying to optimize a helicopter by discussing how Indian gurus levitate. $\endgroup$
    – Andrei
    Commented Feb 24, 2023 at 6:21
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    $\begingroup$ > "this thought experiment is supposed to reveal some deep insights" That's how it is sometimes presented, but I do not think that is what Schroedinger had in mind. $\endgroup$ Commented Feb 24, 2023 at 16:58
  • $\begingroup$ How does it contradict laws of physics? It is an argument for why QT description is not appropriate for macroscopic bodies. $\endgroup$ Commented Feb 24, 2023 at 16:59
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In quantum mechanics, when information about an observable is copied out of a system, interference between different values of that observable is suppressed:

https://arxiv.org/abs/quant-ph/0703160

As a result, versions of that system with different values of that observable evolve independently of one another. Information about the cat in the Schrodinger's Cat (SC) experiment is copied into the box by the cat exerting pressure on the walls of the box and from there to the surface the box is resting on, the information is copied into thermal photons and air currents and so on. This all takes place on timescales much slower than the timescale on which you would open the box:

https://arxiv.org/abs/1111.2189

https://arxiv.org/abs/quant-ph/0306072

So well before you open the box there will be a fact of the matter about what you're going to see when you open the box.

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