# Tag Info

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 ...

19

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 ...

10

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.

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

"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 ...

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

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 ...

4

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 $|\Psi\rangle$,...

3

This is a thought-provoking question, but... the concept of observer is only important if one takes the Copenhagen interpretation. So let us stick to that for the purpose of answering the OP. The cat cannot be an observer of itself. In the logic of the Copenhagen set-up, there is a quantum mechanical object being observed, a classical measurement ...

3

In Schrodinger’s cat experiment there is a mixture of a macroscopic object, the cat, and a quantum mechanical object, the radioactive atom, which if decayed it would activate a hammer to break the flask, with the poisonous gas in it, and therefore killing the cat. This description is very straight forward and not very hard to understand: the cat in the box ...

3

You're right that this experiment isn't really a realistic one. In fact, it's difficult to imagine how we could even tell whether a given box were such a perfectly isolating box. However, if we put aside the (somewhat important) question of empirically determining whether we actually obtained a cat in a superposition of life and death, we can try to imagine ...

3

You really ought to watch the talk by Charles Bennett at http://pirsa.org/11050052/. At first, he analyzed the Mach-Zehnder interferometer and concluded the which-way information of the photon did not exist. Then, he went on to quantum Darwinism where redundant copies of the information are stored in the environment. This multipartite entanglement with the ...

2

Your question effectively reduces to that of where you place the Heisenberg cut in an idealized model of the experiment. That's not different from the issues that are already probed by the original thought experiment and the Wigner's friend variant. It's implicit in the quantum formalism that (unless one introduces POVMs) one models experiments using ...

2

The experiment is intended to highlight the problem of quantum superposition applied to macroscopic objects. It's not inconceivable that a radioactive nucleus can be in a superposition of states. When you interact the nucleus with the detector, hammer and glass of poison, the wavefunction that describes these items become entangled with the wavefunction of ...

2

I have always disliked this thought experiment because, even though it was proposed as an amplifier of quantum mechanical effects, it is really nothing more than a game on probability, and one can get random probabilities by many classical means. Toss a coin, heads cat alive tails cat dead. The concept of both alive and dead is ridiculous in the ...

2

The equation (5.36) in the (newest version) of the paper is: $$ρ_S (x, x′ , t) = ρ_S (x, x′ , 0) e^{ −γt\left(\frac{x−x'}{\lambda_T}\right)^2} \tag{1}$$ where $\rho_S$ should be the off-diagonal term in the density operator, $\gamma$ the relaxation coefficient and $\lambda_T=\frac{\hbar}{\sqrt{2Mk_BT}}$ is the thermal de Broglie wavelength of the system. ...

2

Ket(s) and Bra(s) represent state of a system. The simplest interpretation is to see kets and states as (complex) vectors. In your case, the state could be described by the vector : $$\vec S = \frac{1}{\sqrt{2}} \vec{alive} + \frac{1}{\sqrt{2}} \vec{dead}$$ Here, $\vec{alive}$ and $\vec{dead}$ are a basis for the states, so they are orthogonal vectors : $\... 2 I think you misunderstand the purpose of Schroedinger's Cat as a thought experiment. The idea was to show the ridiculousness and weakness of the Copenhagen interpretation of quantum mechanics, not to present a realistic scenario. 2 That the clock functions implies that the wavefunction of a perfectly isolated clock would evolve almost deterministically. That's why Schrodinger chose a different setup involving a radioactive decay that can happen with a probability of 1/2. 2 If you really want to understand how QM formalism relates to the Schroedinger's cat and the double slit experiment, you should study a QM textbook. To answer your question in a simple way: The double slit experiment is indeed consistent with the Copenhagen interpretation, in the sense that wave mechanics predicts the outcome of the experiment correctly. ... 2 No they cannot. Any living system is an incredibly complicated, interacting quantum system. The countionous interaction (noise, heat bath, etc.) between particles makes it impossible for a coherent state to exist. However this is only true for the whole organism, on the level of single proteins, or other biomolecules quantum coherence may play an important ... 2 Following Lubos' comments, it is clear that your problem is equivalent to two quantum computers exchanging signals (or equivalently, a single one with a halting state). If your question is: Will the simulated cat be "alive" (that is, enjoying his simulated life), or "dead" (that is, the computer has halted and the cat no longer experiencing a simulation) ... 1 You don't really get rid of the superposition. What happens is that the system gets entangled with the environment. This makes the system behave as if it were described by classical physics, but in reality it is still a quantum mechanical system and the superpositions are still there albeit hidden due to the entanglement with the environment. If the cat is ... 1 Quantum mechanics says there is a superposition. There is no reason to doubt QM on this point. The quantum processes of entanglement and decoherence make it impossible for us to be aware of the superposition. We will only be aware of one alternative, which fits well with our experience. 1 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. In this thought experiment the cat and the poison mechanism are used as a detector of the quantum mechanical state that will release the poison according to quantum mechanical probabilities. In a similar ... 1 Schrödinger's cat is a thought experiment, sometimes described as a paradox. The scenario presents a cat that may be both alive and dead, depending on an earlier random event. Although the original "experiment" was imaginary, similar principles have been researched and used in practical applications. The thought experiment is also often featured in ... 1 All the thought experiment is saying (and its not flawless obviously its just an analogy and analogies are not perfect) is that the cat is in both states of alive and dead because you haven't made the measurement yet. Schrodinger was giving a simplified version of whats going on and demonstrated the weirdness with this thought experiment. All that is ... 1 Decoherence happens because in a macroscopic system you are not able to create a small isolated system. In practice you are deal with statistical mixture and not pure state. There's a good description on Wikipedia. 1 There are two states (in QM with ket's$\left|\phantom{H}\right>$we mark possible states) of the Schrödinger's cat. 1st state means (the one on the left) the cat is alive and 2nd state (the one on the right) means it is dead. We could write this down like this:$$\underbrace{\frac{1}{\sqrt{2}}}_{\llap{\text{amplitude for first state}}} \,\,\left|\text{... 1 The easy part is the numerical coefficients which are there so the state is normalized, which means$\langle \psi | \psi \rangle=1\$. The easier part is what's written in the kets, one of them illustrates the state of a dead cat and the other a living one. The point is, due to the canonical form of quantum mechanics, before doing a measurement the cat's ...

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