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Taking the case of Schrodinger's cat, if the measurement of the cat is not yet done, then I don't know whether the cat is dead or alive. Epistemologically speaking, since I don't know about the condition of the cat, it is in superposition state of both dead and alive. But if my friend opens the box and measures it without my knowledge, then for me the cat is still in superposition state. If QM is epistemological in its complete sense then collapse of the wave-function is relative. For him the cat could be dead and for me it could have a finite probability of being alive. If my knowledge was the only factor that determines whether the state is in superposition or not (epistemology), then there would be a finite probability that I opened the box and found the cat alive and removed it from the box and go and meet him!. Einstein too said (in context of relativity) that reality is relative but here(QM) it takes entirely new meaning.

And what if I forgot that I have measured? There would be a finite probability that I measured it to be dead and then forgot and then again measured it only to find that it was alive!

Is QM completely epistemological? If yes then is the forgetting argument valid?

Both of them make no sense to me, but reality need not make sense to me.

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  • $\begingroup$ You are going to have to be more descriptive with your question. Its too hard to intemperate as its currently written. $\endgroup$ – user400188 Mar 30 at 3:49
  • $\begingroup$ In the title, you ask about reality. In the question body, you ask about quantum mechanics. Which do you mean? To be clear, quantum mechanics is definitely not a complete theory of reality, being why the distinction matters. $\endgroup$ – Nat Mar 30 at 6:52
  • $\begingroup$ Not a gist of what an answer to this particular question should look like, but, in general, reality is under no obligation to make sense to you (I mean to anyone, of course). ;-) $\endgroup$ – Feynmans Out for Grumpy Cat Mar 30 at 6:56
  • $\begingroup$ Also, in Relativity, it is extremely clear that reality is not relative--there is an objective reality (described in the language of geometric or covariant objects). The same can be said even in the context of Quantum Mechanics if the Many-Worlds formulation is true (which we cannot say with full certainty as of now because there are unresolved issues with obtaining the Born rule in a Many-Worlds set-up). $\endgroup$ – Feynmans Out for Grumpy Cat Mar 30 at 7:24
  • $\begingroup$ The standard label for this kind of thought experiment is Wigner's friend. $\endgroup$ – PM 2Ring Mar 30 at 7:40
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First off, we have to define "reality." It's a fuzzy word. You appear to be specifically using the Copenhagen Interpretation of Quantum Mechanics. It's a popular "reality," but not the only one.

The cat is not in a superposition of states because you do not know the condition of the cat. By the Copenhagen Interpretation (CI), the cat is in a superposition of states because it has not been observed. This is a small by crucial detail.

Thus, if you know the cat has not been observed, you know that it is in a superposition of states.

If your friend (assumed to be a classical entity) observes the cat, measuring it, the waveform collapses (according to CI). At this point the cat is alive or dead, but is not in a superposition of states.

At this point we can address an assumption you made. You stated "...for me the cat is still in superposition state." This is not true. For it to be true, it means you know that nobody observed it. But in this hypothetical situation, your friend has observed it, so you do not know that it is in a superposition of states. If you know your friend observed it, then you know the cat is either alive or dead. If you do not know whether your friend observed it, you do not know whether the cat is alive, dead, or in a superposition of states.

If you forget your measurement, the same rules apply. If you forgot the measurement, then the cat has still been observed -- it is either alive or dead, you just don't know which. If you forget whether a measurement has taken place, then you cannot know whether the cat is alive, dead, or in a superposition of states.

There are other interpretations with different rule. The Many Worlds Interpretation, in particular, considers a non-real solution, where the question "is the cat alive or dead" depends on the subject observing it, and the subject is "split" into multiple worlds to capture the probabilities associated with quantum mechanics.

That all being said, if you dig deep into the philosophy of science, you find that you cannot truly know anything about reality without making assumptions first. But that's another topic entirely.

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  • $\begingroup$ That is a good argument but what if I know that he didn't measure and he actually measured. That is what I mean by being completely based on epistemology. As per your answer, the wave function collapses not after me knowing but after somebody measuring. Then that is not completely epistemological . $\endgroup$ – Sai Baikampadi Mar 30 at 4:15
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    $\begingroup$ @SaiBaikampadi If you know he didn't measure, and he actually measured, then your knowledge is inconsistent with reality, and is not applicable. By the most traditional definition of "knowledge," knowledge has to be true. I'm not entirely confident I agree with your use of the term "epistemological," but for what you mean by the word the answer is simple: "No. No scientific theory has ever suggested reality is epistemological, the way you are using it." Scientific theories have constantly assumed we can learn knowledge about reality, as opposed to our knowledge shaping reality. $\endgroup$ – Cort Ammon Mar 30 at 5:12
  • $\begingroup$ "If you do not know whether your friend observed it, you do not know whether the cat is alive, dead, or in a superposition of states." Yes, if the focus is about "knowing" then this is correct and a fantastically clear description. But, it might be noticed that if you really haven't measured your friend (friendship?) then your friend is in a superposition of having measured the cat and of not having measured the cat with respect to you. And, in that case, it is not just that you don't know but it really is a superposition. $\endgroup$ – Feynmans Out for Grumpy Cat Mar 30 at 7:02
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    $\begingroup$ @DvijMankad For this, I assumed the Copenhagen Interpreation, which seemed as close to what the OP was looking for as I could manage. It is a theory for explaining how a classical being could consistently measure the results of a quantum system. I thus assumed that the friend is also a classical being (as they would want to be treated the same as I). If I were to treat the friend as an entity controlled by quantum laws, then I would have to be very precise in how I capture the idea of the friend measuring the cat, as I could no longer use the CI concept of collapsing a waveform to do it. $\endgroup$ – Cort Ammon Mar 30 at 16:21
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    $\begingroup$ Decoherence theories do a better job of capturing that nuance than the traditional CI interpretation. They capture the concept of measurement as an interaction between a particle and particles in decoherent states that are modeled statistically. They do a good job of capturing the nuances of having a friend "observe" the cat. This detail became a big deal a while back when someone constructed a thought experiment which, by naive approaches, suggested that a coin was heads or tails, depending on the observer. The trick was in mishandling the idea of a friend measuring. $\endgroup$ – Cort Ammon Mar 30 at 16:23
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Your particular example of the Schrodinger cat is unfortunate. In physics there are various frameworks where mathematical models describe "reality" .That means that the models are successful in describing and , important, predicting situations, as you are trying to do. The different frameworks can be shown mathematically to emerge from lower levels, i.e. classical mechanics which is the framework of you and your friend and the cat, emerges from quantum mechanics BUT in a mathematically complicated manner. Superposition of states can happen only in the framework of quantum mechanics by superposition of wavefunctions, in dimensions commensurate within the quantum uncertainty of $Δ(x)Δ(p)>h/{2π}$.

Paradoxes arise from mixing frameworks and the laws of frameworks. Quantum mechanics measures probabilities to be validated. Classical mechanics measures positions by rulers and energies by specially made instrumentation. The detectors used to measure the quantum mechanical probabilities obey the rules of classical theories, measuring energy and momenta (cat has different energy alive or dead, a classical object) etc and fitting distributions with quantum mechanical theory.

The cat in the Schrodinger box is a detector. A single probabilistic quantum mechanical decay of the nucleus will trigger the poison that will kill the cat. What is unknown is when the nucleus will decay , that is given by a quantum mechanical probability distribution, i.e. the wavefunction of the particle decaying complex conjugate square, as far as quantum theory goes. Thus the specific nucleus is in a state where it may or may not decay, with a given probability. If nobody detects it, its state will not be known. If the cat in the box detector detects it , it means there was a decay, i.e. a measurement in the probability distribution of the nucleus decay.

Thus you are getting a paradox because you ( and Schrodinger) are mixing two frameworks, the probabilistic of the underlying quantum mechanical decay, with the cat/poison as a detector.

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