Schrödingers cat, is it just mumbo-jumbo?

[rolnor]

TL;DR Summary

How about a second observer in the Schödering cat experiment?

If there is a person standing on the opposite side of the closed box with the poor cat containing a poisoning devise, and he can see the cat dying through a transparent window in the box. You stand on the other side of the box and you dont se the cat. What does this mean? Can you now say that the cat is both dead and alive? The person on the other side, that can see the cat will not experience this, he will just watch a cat die for no good reason?

 

[PeroK]

TL;DR Summary: How about a second observer in the Schödering cat experiment?

If there is a person standing on the opposite side of the closed box with the poor cat containing a poisoning devise, and he can see the cat dying through a transparent window in the box. You stand on the other side of the box and you dont se the cat. What does this mean? Can you now say that the cat is both dead and alive? The person on the other side, that can see the cat will not experience this, he will just watch a cat die for no good reason?

The box must be sealed so that it cannot interact with the outside environment.

You might want to read about Wigner's friend thought experiment.

However, you might want to take a more thorough approach to learning QM. Schrodinger's cat, although an important thought experiment, is something of a sideshow to the main subject.

 

[rolnor]

Thanx for your reply. But the cat will die at the same point in time regardless? It really will. Nothing special happens with the nucleid that sets of the poison just because you close the box? Its just stupid. If I recall Schroedinger himself thought it was stupid. Yes, I understand that I need to do some reading, but the math is heavy, its several years to get to your level and I am maybe not up to it. If the nucleid has a very short half-life, say 10 minutes, the chance that the cat is alive after one month is probably zero. If the nucleid has a half life of millions of years its a good chance the cat is alive after one month. That is if it did not need any food or water.

 

[PeroK]

Thanx for your reply. But the cat will die at the same point in time regardless? It really will. Nothing special happens with the nucleid that sets of the poison just because you close the box? Its just stupid. If I recall Schroedinger himself thought it was stupid. Yes, I understand that I need to do some reading, but the math is heavy, its several years to get to your level and I am maybe not up to it. If the nucleid has a very short half-life, say 10 minutes, the chance that the cat is alive after one month is probably zero. If the nucleid has a half life of millions of years its a good chance the cat is alive after one month. That is if it did not need any food or water.

That's not the point. Schrodinger correctly highlighted an unsolved problem with QM. We know that a cat behaves according to classical laws of physics. So, what happens if we link the fate of the cat to a radioactive atom that obeys the laws of QM?

The modern answer is that a process known as decoherence results in a large complex system like a cat reducing to the classical probabilities of either alive or dead.

Without decoherence, and taking basic QM at face value, Schrodinger's cat is an important thought experiment that needs an answer.

 

[Hill]

This mental exercise is not about cat. It is about an interpretation of quantum mechanics.

 

[DrChinese]

But the cat will die at the same point in time regardless?

The thought experiment is intended to highlight an important element of any quantum system: the superposition. It has nothing to do with a cat.

All quantum systems are in some sort of superposition, in which one or more observables are considered to be neither in one state or nor any other specifically. They are in "both" (if a two valued state) or "neither", depending on your personal preference for attempting to translate the actual state into words.

For example: if you know the X spin of an electron is UP, then the Y spin has no well-defined simultaneous value - it is in a superposition. Or you could consider it as both UP and DOWN, which is equivalent in the cat analogy to being both DEAD and ALIVE. Einstein thought that quantum observables must have a value at all times, in contradiction to the superposition principle.

 

[gentzen]

In 1935, nobody claimed that the cat was both dead and alive, not even Bohr. See for example the SEP article on the Copenhagen Interpretation, in which Jan Faye writes:

Fourth, although Bohr had spoken about “disturbing the phenomena by observation,” in some of his earliest papers on complementarity, he never had in mind the observer-induced collapse of the wave packet. Later he always talked about the interaction between the object and the measurement apparatus which was taken to be completely objective. Thus, Schrödinger’s Cat did not pose any riddle to Bohr. The cat would be dead or alive long before we open the box to find out. What Bohr claimed was, however, that the state of the object and the state of the instrument are dynamically inseparable during the interaction. Moreover, the atomic object does not posses any state separate from the one it manifests at the end of the interaction because the measuring instrument establishes the necessary conditions under which it makes sense to use the state concept.

On the other hand, latest in 1961 after the Wigner's Friend "conundrum" had been published, the claim that QM would predict the cat to be both alive and dead was well established. But of course, this claim was still as wrong and misguided as it would have been in 1935.

 

[Nugatory]

@rolnor, surely after the replies you received in your other thread you have figured out that starting with the pop-sci descriptions of QM is a waste of time.

If you were to refer to the paper in which Schrodinger introduced his cat thought experiment, you would see what Schrodinger really said. The idea of the dead/alive cat affected by a conscious observer is patently ridiculous and no one then or now really believed it - yet if we just applied the mathematical formalism as it was then understood (~1930) we would arrive at that conclusion. Thus, Schrodinger's point was that because his thought experiment seemed to predict that result something had to be bad wrong with the then current understanding of quantum mechanics.

It took another few decades to resolve this impasse. Try googling "quantum decoherence" or give David Lindley's vert layman-friendly book "Where does the weirdness go?" for an explanation.

 

[rolnor]

@rolnor, surely after the replies you received in your other thread you have figured out that starting with the pop-sci descriptions of QM is a waste of time.

If you were to refer to the paper in which Schrodinger introduced his cat thought experiment, you would see what Schrodinger really said. The idea of the dead/alive cat affected by a conscious observer is patently ridiculous and no one then or now really believed it - yet if we just applied the mathematical formalism as it was then understood (~1930) we would arrive at that conclusion. Thus, Schrodinger's point was that because his thought experiment seemed to predict that result something had to be bad wrong with the then current understanding of quantum mechanics.

It took another few decades to resolve this impasse. Try googling "quantum decoherence" or give David Lindley's vert layman-friendly book "Where does the weirdness go?" for an explanation.

Thanx. Its just that you could do the practical experiment and the cat will die at some point, with a radionucleid like flourine 18 with a halflife of 18minutes, the experiment will not need to run to long. You could have a camera in the box that monitors the death of the cat and takes a record of the time. After a a half year the cat will be dead for sure and you can check in the cameras memory when it died. Practical experiments are, as a matter of fact, more important than math. The cat will die at some point during that year, and it will be alive before, and dead after, not both. Is that not important? Should we let theories be more important than live experiments? If theories contradict experiments, the theories are wrong. Or the experiment wrongly designed. I understand about superposition and different states, but the discussion is also testable and I am not sure really what any result will tell you, the cat just dies at some point, its not more different than that it dies from starvation. That is also completely unpredictable when it exact happens. And also, the shortest possible time is planc time, so we can not measure anything exactly, can we? It will be plus minus one planc time even with a theoreticaly perfect clock? so when does the cat die? Exactly? (Its midnight here, I am tired, sorry I am babbling)

 

[PeterDonis]

sorry I am babbling

Yes, you are. You should stop doing that and take some time to consider the good responses you have been given, and read the references suggested.

The short answer to the question you appear to be asking, namely, does the cat die at a certain point, is yes, of course. But the short answer to the other question you appear to be asking, namely, does QM predict something different, some kind of superposition of "dead cat" and "alive cat", is no. QM predicts that the cat will die at some point, but that fact, and the reason why QM predicts that, were not well understood when Schrodinger wrote his paper. Now we do understand: the reason is decoherence.

 

[rolnor]

Great! So I was right then. Thanx! The schroedinger cat was mumbo jumbo and now when the theories has evolved its explained why. And popular science has not caught up with this new decoherence and is still babbling about the cat. I get it. And the decoherence is something about "leaking QM information to the mechanical world"? And the cat is the mechanical world? Am I in some part a little correct? i will read the book and the references tomorow.

 

[PeterDonis]

the decoherence is something about "leaking QM information to the mechanical world"?

I'm not sure what you mean by "the mechanical world". The Wikipedia page on decoherence gives a fairly good introduction and has good references:

https://en.wikipedia.org/wiki/Quantum_decoherence

 

[rolnor]

I read the wikip. page, do they not mention the mechanical world as being the non-QM world?

 

[PeterDonis]

I read the wikip. page, do they not mention the mechanical world as being the non-QM world?

Again, what is this "mechanical world" you speak of?

It's not a good idea to make up your own terminology.

 

[DaveC426913]

Again, what is this "mechanical world" you speak of?

At the risk of speaking for the OP, presumably he's talking about the macro/classical world, not ruled by QM.

 

[vanhees71]

I can only recommend to read Schrödingers paper (instead of the EPR paper and the even worse answer to it by Bohr), which is indeed about entanglement:

https://www.jstor.org/stable/986572

 

[vanhees71]

At the risk of speaking for the OP, presumably he's talking about the macro/classical world, not ruled by QM.

Fortunately the macro world is also governed by QT and not classical physics. Otherwise, we'd not exist, because there'd be no stable atoms, molecules, and condensed matter to begin with, let alone complicated stuff leading to life and human beings :-).

 

[Lord Jestocost]

In 1935, nobody claimed that the cat was both dead and alive, not even Bohr. See for example the SEP article on the Copenhagen Interpretation, in which Jan Faye writes:

On the other hand, latest in 1961 after the Wigner's Friend "conundrum" had been published, the claim that QM would predict the cat to be both alive and dead was well established. But of course, this claim was still as wrong and misguided as it would have been in 1935.

Carl Friedrich von Weizsäcker on Schrödinger’s cat in “The Structure of Physics”, section 9.3.2 (the book is a newly arranged and revised English version of "Aufbau der Physik" by Carl Friedrich von Weizsäcker)):9.3.2 Schrödinger's cat: The meaning of the wave function

Schrödinger had to admit, after the discussions described above, that a wave theory was not suitable for describing particle phenomena. For this reason he remained ever since of the opinion that quantum theory in its present form is not an adequate theory of reality, despite all its successes. He no longer participated in its development, and turned to Einsteinian-type of problems of a unified classical field theory.

In an article from 1935 (see Jammer 1974, pp. 215-218) he treats with irony the Copenhagen point of view by means a thought experiment. Let a living cat be locked up in a box and with it a deadly poison which can be released by a single radioactive atom inside the box. After one half-life of the atom the probability is $1/2$ for the cat being still alive, and $1/2$ for being dead. Schrödinger describes the $\psi$-function of the system at this time with the words: ‘The half-alive and the half-dead cat are smeared out over the entire box.’

The answer is trivial: the $\psi$-function is the list of all possible predictions. A probability $1/2$ for the two alternative possibilities (here: "living or dead") means that the two incompatible situations must now be considered equally possible at the instant of time meant by the prediction. There is no trace of a paradox.

Schrödinger's reason to consider the situation as paradoxical lay in his hope to interpret the $\psi$-function as an ‘objective’ wave field. In the implied deterministic description, he saw no reason to take seriously the difference between the present and the future. I have seen from a letter he once wrote to me (after the war) how foreign the idea was to him, as well as to many other physicists, that this difference was something to be taken seriously physically, and not merely 'subjectively'. …….

Schrödinger accomplished a heightening of the paradoxical impression by considering a living being as an example. The poor cat is treated here simply as a measurement instrument to illustrate the irreversibility of the measurement process by means of the striking, and to us humans, moving contrast between the states of life and death. We have argued above (Sect. 9.2e) that in quantum theory there is no inherent reason why it could not be applied to living beings. But we do not need this assumption for the discussion of Schrödinger's example. It suffices to remark that obviously there cannot be a clear description of a quantum theoretical thought experiment if on the one hand one uses living organisms as its integrating parts but on the other hand one does not take seriously the application of quantum theory, i.e. here simply the concept of probability, to the organisms.

 

[PeroK]

The quotation from Weizsaecker doesn't convince me, I'm sorry to say.

 

[gentzen]

The quotation from Weizsaecker doesn't convince me, I'm sorry to say.

So you believe that in 1935, people believed that the cat would be both dead and alive? Or you believe that in 1935, people believed that QM would predict the cat to be both dead and alive?

(That people believed such stuff shortly after Everett's thesis was published, but before H. D. Zeh established decoherence, is no surprise to me. OK, I have to admit that Einstein probably believed that QM would predict such strange things, in 1935.)

 

[PeroK]

So you believe that in 1935, people believed that the cat would be both dead and alive? Or you believe that in 1935, people believed that QM would predict the cat to be both dead and alive?

Weizsacker's language is perhaps a little too elaborate for me. What I thought he was saying is that there is no mystery to explain. That there is no paradox in the first place.

 

[vanhees71]

The quotation from Weizsaecker doesn't convince me, I'm sorry to say.

Well, yes. Pauli's and Fierzs's opinion about the "Baron" was very clear and precisely to the point. If there is one, who can confuse you even more than Heisenberg and Bohr, it's v. Weizsäcker ;-).

 

[Lord Jestocost]

Weizsacker's language is perhaps a little too elaborate for me. What I thought he was saying is that there is no mystery to explain. That there is no paradox in the first place.

Regarding Schrödinger's cat, Carl Friedrich von Weizsäcker clearly points out in his book “The Structure of Physics” why there is no paradox at all.

From chapter 9.2 “The semantic consistency of quantum theory”, section 9.2.2 ”Gaining information by means of measurement”:

“We summarize: $\psi$ is knowledge, and knowledge depends on the information collected by the knowing subject. Knowledge is of course not dreaming, not "merely subjective." It is knowledge of objective facts of the past which will turn out to be identical for anybody who has the necessary information; and it is a probability function for the future that holds for everybody who has the same information, and which can be checked empirically through measurement of relative frequencies in the manner described in the third chapter. All paradoxes occur only if one interprets $\psi$ itself in some other sense as an "objective fact," a fact going beyond that at a certain time a certain observer has a certain knowledge. Facts are past events which we in principle can know today.”

from chapter 9.3 “Paradoxes and alternatives”, section 9.3.1 “Preliminary Remarks”:

“The Copenhagen interpretation had its origins, both historically and factually, in Bohr's idea of correspondence. Classical physics is a given. It furnishes, in our terminology, the preconceptions for quantum theory. It is in this language that one describes measurements. The Copenhagen interpretation shows then the semantic consistency of quantum theory. It demonstrates precisely how one must modify the preconceptions, i.e., how one must restrict the range of applicability of classical physics so that no contradiction arises. The apparent paradoxes and alternatives which were discussed afterwards were without exception attempts to revoke, at least partially, the sacrifices one had to make in the preconceptions. These attempts were without success. This was an important "grieving process"; only then did it become quite clear how deep these sacrifices went. In this debate Einstein proved again his eminent intellectual standing. In his famous thought experiment with Podolsky and Rosen he precisely brought to light the central point of the sacrifice: the renunciation of the belief in the "objective reality" of physical objects. Therefore we devote an entire section to this grieving process, with Einstein's protest as its central part. But it remains a grieving process. None of the proposed returns to classical principles has succeeded.”

[Bold by LJ]

 

[PeroK]

I'm going to respond with Thomas Mann:

Um, aber, einen klaren Sachverhalt nicht kuenstlich zu verdunkeln ...

 

[rolnor]

I get the feeling here that I was right, the cat could die from starvation at any time, that would be the same thing, if you open the box the cat is dead or alive in just the same way. No mystery. The last two sentences states just that:

 

[PeterDonis]

the cat could die from starvation at any time, that would be the same thing

Only if you believe that the cat starving depends on a quantum event like the radioactive decay of a single atom.

When you take decoherence into account, the final result is the same in both cases (the cat is in the "dead" state), but the process involved is different, and Schrodinger's thought experiment was specifically about the quantum process.

 

[rolnor]

But it was also about what state the cat was in, dead and alive at the same time? Thats not a final result? And the cat could be dead or alive (or dead and alive) just as well from starvation?

 

[PeterDonis]

it was also about what state the cat was in, dead and alive at the same time?

It was about the fact that, if you just look at the unitary dynamics of QM, the cat ends up in a superposition of the "alive" and "dead" states, because it is entangled with the radioactive atom that is in a superposition of the "not decayed" and "decayed" states. "Dead and alive at the same time" is not a good description of what that means physically.

Thats not a final result?

In some interpretations (such as the MWI), the entangled state I described is a final result (or more precisely, the further entangled state that arises when the experimenter opens the box and looks at the cat and entangles his state with the cat/atom state).

And the cat could be dead or alive (or dead and alive) just as well from starvation?

Starvation is a classical process and does not involve any quantum indeterminacy; the dynamics of starvation do not involve a cat going into a superposition of "starved" and "not starved" states. So no, it's not the same as the case described above.

 

[vanhees71]

There are no classical and quantum processes, only quantum ones. The classicality of macroscopic systems is an emergend, approximate phenomenon. The cat has not only to be "coupled" to the decaying nucleus and the equipment coupled to it to kill it whenever the nucleus decays but also to "the environment", because you have to allow at least the cat to breath and eat, because other her death wouldn't be a sure indicator of the decay of the nucleus, because she could have died for other reasons. This, however, implies that the time evolution of the nucleus+murder machine+cat is no longer described by unitary time evolution but by some (non-Markovian!) master equation of this subsystem, which implies dissipation and decoherence, i.e., the cat is always well described by a quasi-classical macroscopic state and thus dead or alive and not in a quantum superposition thereof. Before looking, I can't know with certainty, whether I find a dead or alive cat, because whether the nucleus has decayed in the meantime or not is due to an inherently random process.

 

[kered rettop]

Just to point out that SC has nothing to do with whether someone can see the cat. It was once thought that observing - or seeing - the cat would somehow collapse its wave function. And yes, you can construct situations in which it is not entirely clear whether the cat has been observed. But SC is about the "quite ridiculous" situation where the cat is both alive and dead before it gets observed.

 

[PeroK]

Just to point out that SC has nothing to do with whether someone can see the cat. It was once thought that observing - or seeing - the cat would somehow collapse its wave function. And yes, you can construct situations in which it is not entirely clear whether the cat has been observed. But SC is about the "quite ridiculous" situation where the cat is both alive and dead before it gets observed.

There's nothing ridiculous about QM. SC may or may not be an unresolved problem. And, even if in general it is difficult to fully explain the transition from microsopic processes described by QM to the macroscopic world described by classical mechanics, that in itself is not a reason to question QM. It's only if that transition were to look impossible that something woudl have to give.

In the same way neither QM not GR is ridiculous, despite there being no obvious way to explain GR in terms of microscopic processes.

 

[kered rettop]

I did not say QM is ridiculous. I said that the cat being alive and dead was "quite ridiculous". The quotes indicate that the words are Schroedinger's, not mine. With the benefit of hindsight, I do not agree with him.

 

[vanhees71]

I think this paper by Schrödinger is very worthwhile reading. For some reason it's not famous like the EPR paper, which occured around the same time and discusses the same "issues" with QM. Maybe it's because Schrödinger is utmost clear, while EPR is nebulous, and there is the even worse answer by Bohr to it.

Of course, Schrödinger was right in saying that the cat "in superposition of states "alive" and "dead" is ridiculous. He couldn't know about decoherence and the full development of quantum many-body theory.

 

[kered rettop]

TL;DR Summary: How about a second observer in the Schödering cat experiment?

If there is a person standing on the opposite side of the closed box with the poor cat containing a poisoning devise, and he can see the cat dying through a transparent window in the box. You stand on the other side of the box and you dont se the cat. What does this mean? Can you now say that the cat is both dead and alive? The person on the other side, that can see the cat will not experience this, he will just watch a cat die for no good reason?

The state of the cat does not depend on whether someone knows about it. In fact observation doesn't really come into Schroedinger's Cat. The "quite ridiculous" aspect of the experiment was that the wave function of the cat would contain both the living and the dead cat before the box was opened or anyone looked.

 

[PeroK]

The state of the cat does not depend on whether someone knows about it. In fact observation doesn't really come into Schroedinger's Cat. The "quite ridiculous" aspect of the experiment was that the wave function of the cat would contain both the living and the dead cat before the box was opened or anyone looked.

If, owing to decoherence, the wavefunction superposition is experimentally indistinguishable from a simple classical either/or probability, then it's not ridiculous.

It's a non classical explanation. But that is inevitable given the quantum probabilistic nature of radioactive decay.

A classical explanation for decay would involve some sort of ageing process for a radioactive atom. Ironically, of course, probability would still be needed in terms of the initial state of the atom or the speed of ageing. QM puts probabilities at the heart of physics, where they belong(!), rather than trying to explain an essentially probabilistic universe by having layers of deterministic hidden variables ad infinitum.