The Cat is a puerly thought experiment. There’s been several discussions of the subject :) See e.g. This: https://www.researchgate.net/post/What_do_you_think_about_this_example_against_Schodinger_cat_superposition#view=5a1353c096b7e430dd68d914

Lovely question that tackles some rather big issues. Thanks @Airat, yes that's a good thread, and this kind of topic often seems to generate a lot of interest on ResearchGate.

As posed the first and second parts of the problem seem interesting and qualitatively different. The first part of the question seems to be conceptually very similar to "Wigner's friend". So in that case we have the canonical Schroedinger's cat and scientist. The cat is in the superposition of alive and dead, then the scientist opens the box. She's a cat lover, so she's in a superposition of happy and sad. But she's also in a box with another scientist trying to work out if the scientist has collapsed - so the boxes have superpositions of 'dead/sad' and 'alive/happy'.

In deference to @Airat, these questions are no longer just thought experiments. As I have summarised above, the problem is practically untestable, but working on ever larger quantum systems does imply that it is not consciousness that determines wavefunction collapse, but interactions.

The biggest issue with Schroedinger's cat and related experiments is that so often we don't work to explain what the 'conjugate basis' is. So for example, if I do the same experiment as an EPR type experiment, I can have photons in horizontal/vertical polarisation, or in the diagonal basis. But to make the wavefunction 'weirdness' manifest, I really need to be able to define a measurement alive/dead, or |alive + dead> and |alive - dead>. I don't know what these concepts mean, and because of this, the difference between the Schroedinger Cat type experiment as usually posed can quickly become one of classical correlations, rather than quantum mechanical correlations.

A better way of doing this is to look at sending massive particles through interferometry systems (a la the Vienna group).

Returning to the question as posed. The conventional understanding is that putting blindfolds on the experimenter doesn't change the experiment in the least. The collapse of the wavefunction is purely mediated by the photons interacting with the blindfold (no consciousness required!)

In the second part of the question, again as posed, without some method of rotating bases, even if you apply a relativistic boost to A, I still think that the cat and B will be in a classically correlated state, but I could be wrong. I think that you need to work a bit harder on the question. You are getting near very interesting territory where wavefunction collapse and space-like separated regions may become important, and I'd like to know exactly what you're thinking of before trying to give a clearer answer.

I my opinion, the main piece of information is contained in the sentence "The box which has the cat is completely transparent”. This means that the box is not hermetically closed, preventing us - even in principle - to know what happens inside. A completely transparent wall, on the other hand, means interaction with light, which acts as a reservoir destroying coherence exponentially with the number of particles composing the “cat”. Another point is that the mere fact of covering the eyes has no relevance: what matters is if, in principle, there is some way of knowing what is happening. If there is, there will be no overlap. Knowing what mechanism is behind the impossibility to occur superposition can be tricky, but is just a matter of searching properly.

+Norton, after all this is a thought experiment, make the apparatus insensitive to the spectrum of seen light. e.g. what if the killing is achieved via radioactive decay?

Indeed, "what matters is if, in principle, there is some way of knowing what is happening. " I claim that in this setup the cat is watched by an observer until it boils... where did I go wrong?

Dear Asher, if I am understanding correctly, when you say what happens if one "make the apparatus insensitive to the spectrum of seen light. e.g. what if the killing is achieved via radioactive decay?" this seems to suggest merely to shift the frequency (visible to, say, microwave) used to interact with the cat, changing nothing.

Dear Norton, just for curtsey I will put myself (instead of Wigner) into the box... also, suppose that I know nothing about Schroedinger's experiment, as I am tricked to believe that this is for the advancement of medical practices. My point is that I know that I am alive until I don't -- at which point QM is really imaterial (wink, wink.) At no point am I in a superposition with my state of death! If as part of the setup I have my EKG/EEG recorded and 'transparent' to Teller (who does know about the scheme), then for both of us I am not in a superposition but pretty much alive until I am dead... right? If this is the case then, where is the need for a collapse?

+Andrew, I think that although you do not mention the concept 'decoherence', your answer suggests it to be the mechanism which ends the unitarity evolution. Whether consciousness is a decohering mechanism in the physical neurons or not, this is what Tegmark & Wheeler had to say about it: "...decoherence researchers generally agree that decoherence produces an effect that looks and smells like a collapse"

Dear Asher, I'll try to comment in another way, but I apologize if I'm being redundant or saying things you already know: Any macroscopic object, placed in an environment that has cooled even till absolute zero, loses coherence in a time that scales exponentially with the number of particles of that object. As you probably know, this is a well-known result in quantum optics, and was found in pioneering experiments by Haroche and his group in France for "mesoscopic cats". Some people do not believe (with solid arguments) that decoherence truly solve the Schrödinger's cat paradox, since decoherence applies to density matrix, while collapse models apply to state vectors. This is why the Continuous Spontaneous Localization (CSL) model, as in Ref. you provided, cannot be plainly discarded. Actually, if CSL model is correct, it will bring us new physics, since it modifies Schrödinger equation. But, for all practical purposes, superposition of macroscopic objects (10ˆ23 particles) cannot survive long enough for its overlap to be verified, let alone for enough time to separate them light years from another object. So, if we are talking about macroscopic objects (and this seems to be the case as you say "I will put myself (instead of Wigner) into the box”), there will be no superposition at all. This point really matters, since, as I am understanding, superposition of macroscopic objects (like cats, you, or Wigner’s friend) only survive for times of order 10^(-21) s, which, it seems to me, you are not taking into account. Summarizing my point of view, it seems to me that the decoherence model suffices to explain your gedankenexperiment, since decoherence model says that no superposition can be maintained time enough to generate paradoxes in real world.

+Norton, wouldn't that be nice if CSL/DP could be proven correct? but at the moment its legroom has shrunken even some more. Putting myself into the box is tantamount to implanting consciousness into a cat -- go figure how soon this would happen. If such a cat is in superposition then I would argue that it doesn't know what is the meaning of being alive -- which I think is a first hand experience. Regarding decoherence, the fact that 10^23 particles put the quantum back into the classical sounds like a patch. Albeit a nice one, and as you have mentioned yourself, on the density matrix level only, it does reek like collapse.

It seems to be that as long as even a single physicist still believes that the dangerous concept, "physical reality," is independent of observation we are going to have conceptual difficulties.

Dear Asher, I agree with your words "Regarding decoherence, the fact that 10^23 particles put the quantum back into the classical sounds like a patch.” And I agree just because if we accept that a real cat or a human observer could be put in superposition even for a moment like 10ˆ(-23)s, then we must accept cats dead and alive simultaneously, although for a brief moment, which should be, at least for our classical thinking, unacceptable.

On the other hand, the amount of particle is undoubtedly relevant and works nice for other facts such as, for example, the immateriality of the uncertainty principle or the vanishing undulatory behavior with the increasing number (mass) of particles.

Regarding your comment: "It seems to be that as long as even a single physicist still believes that the dangerous concept, "physical reality," is independent of observation we are going to have conceptual difficulties.” , it seems to me that, at least for macroscopic world, the “physical reality” should be independent of observation. I’m not claiming, like Bohr, the coexistence of two worlds, one classical and the other quantum, I’m just saying that no matter the mechanism responsible for the emergence of the “classical world”, classical world works like that: moon really is there even when nobody is looking.

Cheers.

I recommend watching Zanghi's talk about Bell and Bohmian mechanics -- https://www.youtube.com/watch?v=ZCLMb6HYyjs

Excellent recommendation. Thanks!

@Asher Klatchko, responding to your comment about decoherence - yes I agree that decoherence could certainly be responsible for collapse, and is the simplest canonical explanation. It should be clear that I do not think that consciousness plays any role in the situation: the conventional decoherence approach simply requires many interactions.

These days I am more taken by Many Worlds, and the idea that what we really care about is the question 'why do we not see interference and superposition in macroscopic objects'. Measurement collapse (often mediated by decoherence) is often suggested as the mechanism for this, and there is certainly predictive power in this approach. But philosophically we are still left with the unanswered question - what is a measurement? And we answer this in a circular fashion by saying that it is what generates decoherence.

At least with Many Worlds, we have no need to break superpositions or collapse anything, we merely entangle more and more particles and only observe superpositions when all of the quantum numbers of the rest of the universe are the same. This gives us the observation of a classical universe at macroscopic scales without any collapse or decoherence.

Of course, the more annoying thing is that we still don't have any way of testing this as decoherence satisfies all of the criteria to explain observations and experiments.

The mystery vanishes in the de Broglie-Bohm theory. The fate on the cat depends on the initial position of the particle inside the wave density only which deliver an radioactive decay or not . An observer plays no role. There is no measuring effect on the cat until you modify the experiment set up/wave function.

See for example the youtube videos:

https://www.youtube.com/watch?v=cpNSc-X6I8c

https://www.youtube.com/watch?v=lwFfpQMcj4Y