41

Before reading this answer (and to those who are downvoting), I am addressing if the cat is both alive and dead. I don't think the question is asking for a complete explanation of the Schrodinger's cat experiment, nor is it asking how this links to all of the deeper mysteries of quantum mechanics and how we should think of them. Therefore, while there is ...


36

This is is known as the Wigner's friend thought experiment. According to the many World's interpretation, the superpositions are not a problem. The whole universe ends up in a superposition where all experimental outcomes are realized, but such a superposition is entangled with the environment, from a macroscopic point of view it takes the form of a ...


28

In a bubble chamber experiment, film was the detecting medium, but film was taken automatically, by the thousands of frames. These bobbins of film went to the various laboratories involved in the experiment, and were scanned for interesting events which were measured and the cross sections for the interactions recorded. This is a clear example of an ...


26

First, a historical subtlety: Schrödinger has actually stolen the idea of the cat from Einstein. Second, both men – Einstein and Schrödinger – used the thought experiment to "explain" a point that was wrong. They thought it was absurd for quantum mechanics to say that the state $a|{\rm alive}\rangle+b|{\rm dead}\rangle$ was possible in Nature (it was ...


25

Basically the answer is yes, the cat is both dead and alive. People used to discuss this sort of thing in terms of the Copenhagen interpretation (CI) and the Many-Worlds interpretation (MWI), but those discussions tend not to be satisfying, because both CI and MWI are designed so that in almost all real-world measurements, they give the same predictions. A ...


21

This question strikes close to the heart of The measurement problem, which is the question of what (if anything) the process of measurement represents; and is all but synonymous with the question of how one ought to interpret quantum mechanics. As such, the answer to this question is (a) subject to debate; and (b) absent any substantial philosophical and/or ...


15

In practice, you can't actually get yourself into a quantum superposition of dead and alive. Exposing a quantum system to a thermal environment causes the state to lose coherence and collapse into one state or another. In the double slit experiment, you are the thermal environment that causes the collapse. "Observation" in quantum mechanics has nothing to ...


15

I feel like all the answers here are missing the point. The cat is not both alive and dead at the same time. That would be, as you put it, ludicrous. The truth is that the cat is in a superposition state of the states "alive" and "dead". The problem is that there is no way to make sense of this statement without studying the underlying mathematics. Humans ...


14

$\renewcommand{\ket}[1]{\left \lvert #1 \right \rangle}$ $\renewcommand{\bra}[1]{\left \langle #1 \right \rvert}$ We can see how decoherence really works, why it messes up superposition states, and why it's particularly prone to messing up states of large objects all through a very simple example $^{[a]}$. Single two-level system Suppose we have a quantum ...


13

What's wrong with my reasoning? Nothing! In fact you have more or less described decoherence. The idea is that any system inevitably interacts with its environment, and the more degrees of freedom the system has, i.e. the more complex it is, the faster it will interact with the rest of the universe and the superposed states will decohere.


11

All the paraphernalia of cat, box, etc are classical, i.e. obey classical mechanics and electrodynamics. They are a big fancy "detector" set up in order to illustrate the quantum mechanical probabilistic nature of a decaying nucleus. The observation is made by the "internal monitor" which interacts with a decay product of the nucleus, and triggers the ...


11

Let me rephrase what your teacher did. They first defined an operator $\hat{O}$ given by the matrix you have. They then noticed that the operator has two eigenvectors with eigenvalues $+1$ and $-1$, given by the two vectors that you have. Notice that these eigenvectors are orthonormal. They then interpreted the two eigenvectors as two states of a quantum ...


11

There seems to be some confusion here. The act of observation (which collapses the superposition to an eigenstate of $\hat O$) is profoundly different from operating on the state with $\hat O$ which doesn't collapse the state but, rather, returns a different superposition of eigenstates. For example: $$\hat O \Phi = \frac{1}{\sqrt{2}}\phi_{alive} - \frac{...


11

A theorem of von Neumann says that it doesn't make a bit of difference whether you model the cat (or anything else along the causal chain between closing the box and opening it to observe the cat) as capable of collapsing the wave function. You'll make exactly the same testable predictions no matter where along the way you place the collapse. (The argument ...


10

"Since it is uncertain on whether or not the cat could be dead or alive, it is concluded that it depends on the observer to make it either dead or alive." That would certainly be atrociously bad "reasoning", but that was not Schrodinger's argument at all. He introduced the cat thought-experiment as an attack on the idea that when a particle's wavefunction ...


10

I think you are confusing the observer effect and the uncertainty principle. Schrödinger's cat is about the former, not the latter. Also, a point could be made about the fact that, according to general relativity, energy does bend space-time.... although one could then make a counter point about the fact that the center of gravity could have changed? Anyway,...


10

Forgive the length. I find Schrödinger's cat is much easier to make sense of as a journey through QM, rather than just a few equations that someone says "solves your problems." Schrödinger's cat was definitely meant to be taken seriously, in that it was intended to be a serious challenge to naively applying the Copenhagen interpretation to macroscopic ...


10

Schrodinger's cat is an exercise in seeing how nonsensical the Copenhagen interpretation is, so answers that attempt to clarify it in terms of CI are not very helpful, in my opinion. As a framework for this answer, I'll repeat a point I make frequently: QM describes not the probabilistic evolution of a single deterministic state, but rather the ...


9

The visualization method you choose is directly and completely determined by the information you need to see regarding your state. For the states of a single bosonic mode, there are multiple different visualization methods, and they all have their pros and cons. In particular, there is a direct trade-off between the amount of information you can display on a ...


8

It is because of quantum statistical irreversibility, which is closely related to entropy, as the OP suspected. Qualitatively it is quite easy to understand this. From the laws of quantum mechanics on the microscopic level emerges a classical behaviour for macroscopic (i.e. many particle objects). Of course this is not sufficient though and does not give a ...


8

Following John Rennie's analogy, let the cat be a spin with up (alive) or down (dead) state. Note that you've expanded $|\#\rangle$ in terms of the orthonormal basis vectors: $$|\#\rangle = a|1\rangle + b|0\rangle$$ By definition of orthogonality (and the idea that "alive" and "dead" are orthogonal), $\langle 0|1\rangle = \langle 1|0\rangle = 0$. Now you'...


8

Yes, the superposition state of 'dead and alive' would then be projected onto the 'alive' state because in a way the person measures 'alive'. I assume the cat won't meow if it were dead. The quantum system (the cat) will be in contact with the outside world and as a result decoherence will erase all quantum behavior.


7

The thought experiment aimed at illustrating one concept, and questioning the validity of such concept: 1/ The concept: in quantum mechanics, before an observation is made (note the cat is in a closed box, and nobody can see what is going on in the box), a system is not in a defined state, but only has a certain probability to be in any state - here the ...


7

It depends of what interpretation of quantum mechanics you are using. By interpretation it is meant that the mathematical predictions of the quantum mechanics formalism are the same, but the philosophical meaning of each is what differs. In the copenhagen interpretation that you seem to describe, the wave function collapses when a conscious observer makes a ...


7

Remember Heisenberg's idea that you can't always measure position and velocity at the same time? So here's an electron, and there's stuff you are guaranteed not to know about it. You can know something about some combination of position and velocity, but that's like having one equation in two unknowns. You know something but you can't solve it like you ...


6

The expression in the picture contains kets only. Kets represent states of a system. In this case, the "alive" state is the first one and the "dead" state the second. The numerical factors are there for normalisation. It is assumed both states are equally likely, so they have the same numerical factor. If we call the expression in the picture $|\Psi\rangle$,...


6

You don't explicitly say so, but you're assuming the Copenhagen interpretation (CI) rather than the many-worlds interpretation (MWI). Your analysis is a perfectly good example of why the CI doesn't fundamentally make much sense. The CI treats measurement as a process that's different from other processes, even though measurement is a physical interaction ...


6

When you write: $$ \vert \text{#}\rangle=a\vert1\rangle+b\vert0\rangle $$ you are assuming there exists an alive operator and that this operator has eigenstates $|1\rangle$ and $|0\rangle$ that you can use as a basis for writing the wavefunction of the cat. Neither of these assumptions seem reasonable so as it stands the question makes no sense. However ...


5

This is an excellent question and stresses one of the weird features of quantum mechanics. Indeed, the scientist would in turn be in a superposition. And we could even measure this if we'd be able to maintain coherence of such large systems. Ultimately, your question is asking for the solution of the Measurement problem: Why don't we see any ...


5

In Schrodinger's cat experiment, the scientist is assumed to be an "classical observer" of the state of the cat, and thus all observations made by the scientist are assumed to be classical observations adhering to physics as we knew it before quantum mechanics came along. The thought experiment focuses on what sorts of statements about reality that ...


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