I Why don't we bury Schrodinger's Cat?

[vanhees71]

In the paper we describe of course an open system, because we couple the particle to a heat bath, but the underlying equations are quantum time evolution. There's nothing "classical" in this approach! In other words, I don't like Copenhagen precisely because of the "Heisenberg cut" and (some flavors of it) assuming a "collapse of the quantum state" when measuring something on the system. There's no such thing, and it's also not necessary.

 

[PeterDonis]

wouldn't the cat just decohere itself?

Yes. In other words, even in the absence of external interactions, the cat still has a huge number of degrees of freedom, most of which cannot be individually tracked, so it will continually be decohering itself.

And if the cat was always in a state of decoherence, how then could it ever be entangled with anything?

Decoherence does not prevent entanglement. The cat can decohere itself but its individual atoms can still be entangled with each other. Similar remarks apply to entanglement with other things.

 

[Algr]

That's right. People are supposed to reject the idea of a decohered cat, just like Bushmonk did:

Am I using "decohered" wrong up there? PeterDonis seems to be using it in the opposite way. Maybe I mean "quantum superimposed" cat. To me decohered -> incoherent.

 

[PeterDonis]

To me decohered -> incoherent

Not really, no.

Basically, decoherence means that quantum interference becomes negligible. In the case of the cat, it means that quantum interference between "alive" and "dead" becomes negligible. Which in turn means that the entanglement between the cat and the apparatus inside the box, which either gets triggered by a quantum event and kills the cat, or doesn't get triggered and leaves the cat alive, will also show no interference between the cat being alive and dead.

However, that does not mean that there is no entanglement at all. The cat is still entangled with the apparatus, so their joint state will be the one I wrote down in post #26. What happens next is interpretation dependent, as has been discussed in previous posts.

 

[bob012345]

Decoherence does not prevent entanglement. The cat can decohere itself but its individual atoms can still be entangled with each other. Similar remarks apply to entanglement with other things.

But the scenario does talk of the cat being entangled with the atom which it can't as a whole. Suppose we replace the cat with a 1MT nuclear device. Are we to suppose the atom/device is in an entangled state till we open the box and see if it decayed (and triggered the device)? Somehow, I think we'd know if the atom decayed or not without opening the box.

 

[PeterDonis]

the scenario does talk of the cat being entangled with the atom which it can't as a whole

Sure it can. Yes, initially the effects of the atom being triggered will not interact with every single atom of the cat, but since all of the atoms of the cat interact with each other, any interaction with any part of the cat will quickly affect the state of all the atoms in the cat. That, in fact, is part of the process of decoherence--the effects of interactions that initially involve only a few degrees of freedom, spreading among a very large number of degrees of freedom that are not individually trackable.

Are we to suppose the atom/device is in an entangled state till we open the box and see if it decayed (and triggered the device)? Somehow, I think we'd know if the atom decayed or not without opening the box.

This is a quibble. The key point is not exactly what it takes for an external observer to tell whether the cat is alive or dead. The key point is that the state that the Schrodinger Equation predicts is the entangled state I have been describing, which is not a state in which the cat, by itself, is either alive or dead. It is entangled, and subsystems that are entangled do not have definite quantum states at all; only the overall joint system does. But we never observe such odd states with things like cats. That was Schrodinger's original point: either our observations are grossly misrepresenting "actual reality", or the Schrodinger Equation, by itself, cannot be a complete theory.

 

[bob012345]

That was Schrodinger's original point: either our observations are grossly misrepresenting "actual reality", or the Schrodinger Equation, by itself, cannot be a complete theory.

Thanks. Which of these two do you hold?

 

[Nugatory]

But the scenario does talk of the cat being entangled with the atom which it can't as a whole. Suppose we replace the cat with a 1MT nuclear device. Are we to suppose the atom/device is in an entangled state till we open the box and see if it decayed (and triggered the device)? Somehow, I think we'd know if the atom decayed or not without opening the box.

The discussion so far has considered the cat/atom system as isolated within the box - until the box is opened there is negligible interaction between the contents of the box and the outside. That's how Schrodinger could suggest (as a sort of straw man) a coherent superposition of dead and alive, and how decoherence can (more seriously) predict a cat that is dead or alive but we don't know which until we open the box.

"Negligible interaction between the contents of the box and the outside" doesn't apply when we replace the cat with a 1MT nuclear device. But that doesn't change the underlying principles, it just means that we need different conditions to achieve the necessary isolation. So let's do the experiment on the far side of the moon and we're back to a valid analogy with the cat and the box - naive 1920s vintage Copenhagen can't explain why we don;t have a coherent superposition of exploded and unexploded, decoherence tells us that either the bomb has exploded or it hasn't and we'll know which when we look.

 

[PeterDonis]

Which of these two do you hold?

I don't "hold" either of them. I think this is still an open area of inquiry.

If I had to guess how the inquiry will eventually turn out, I would guess that we will end up finding that the Schrodinger Equation, or more generally QM in its current form, is not a complete theory. But that's just a guess.

 

[bushmonk]

...every venue that (teaches) introductory physics goes through this same old thought experiment. The Great Courses quantum mechanics lectures describe sitting in three movie theater chairs at once. The lecturer then slogs through Schrodinger's cat. This metaphor just won't die.

This response captures something of my frustration. My experience is that the path towards a decent amateur understanding of QM is educationally inefficient. Much effort for little understanding. The path is obscured by debris, rabbit trails and sometimes rabbit holes.

It would not be a good idea to introduce classical mechanics by stating that according to classical mechanics, I, the teacher, have no choice about what I am about to say, and you, the student, are doomed to pass or fail your first test in the subject. It is all fixed by the prior state of the universe.

No. We start with measurements of time and position so that we an describe simple motions in mathematical terms. We take some ticker tape and try it. We leave determinism to the philosophers.

Likewise, I think it is not a good idea to say to the inquirer that, according to QM you can be in three theatre seats at once, that cats can be alive and dead, or that there are gazillions of copies of both you and the cat in gazillions of worlds. It may be true that there are people wrestling with why such statements are not true or are pressing forward with the idea that they are true, but that is not what we are going to talk about nor why QM is worth understanding.

No. Let's talk about an interferometer. Here is how it works. And so on. There is much to be learned.

Let's learn to balance a chequebook before we worry about Godel's Theorem.

The decoherence viewpoint makes it clear that, once the radioactive atom decays, the cat dies--it doesn't wait to die until you open the box.

Great. We agree on that. So we can bury that part of the cat in the box idea.

I gather that decoherence has opened up the black box of measurement but there are still smaller black boxes inside. I think this progress is great. Mysteries remain. But it doesn't mean that according to QM a cat is both dead and alive. Or that you can sit in three theatre seats at the same time. We just haven't agreed as to why it doesn't. The projection postulate is one idea. Many don't like it. OK. But we're all aiming at the same goal, namely eliminating things that don't happen.

No, it's a state. It doesn't have to be a microstate. A cat has a quantum state, but it's not a microstate.

I don't see why you object to microstate. As in thermodynamics, a microstate is the detailed, unknowable and rapidly changing specification of the microscopic constituents of a large system. "Alive cat" is a crude macroscopic description. For that reason, using a ket with "alive cat" in it is misleading, IMHO. As you said, a live cat is a subspace of Hilbert space.

I stated that the cat was a series of microstates discontinuously and randomly related to each other. Peter responded:

Not at all. If this were the case, cats, and objects generally, would not be describable to a very good approximation using classical physics.

This is a layman's description of the projection postulate in operation. I was using terminology from David Bohm's 1951 "Quantum Theory". The projection postulate is invoked to describe the observation that the quantum state abruptly changes, discards redundant possibilities in a random and discontinuous fashion and renormalizes.

I understand the MWI discards the projection postulate. There is controversy about whether they they have succeeded in explaining what it actually observed. So I don't want to get into it.

But whether they have or they have not, surely they must give an account of why, in this particular cat, an oxygen molecule entered a particular cell, when, by Shrodinger's equation, it could have continued along the bloodstream. Their answer is, if I understand it correctly, that a number of cats were spawned in other worlds to realize the other possibilities. Fine. But that is very different from an electron going through both slits without decoherence. No new electrons in new worlds need be spawned. As long as decoherence is not involved, there is no need to spawn new worlds. A cat is filled to the brim with decoherence. An electron going through a double slit is not. There's a difference, regardless of your interpretive preferences.

That is why I said that some of my statements would need translation into other interpretations.

 

[bob012345]

naive 1920s vintage Copenhagen can't explain why we don;t have a coherent superposition of exploded and unexploded, decoherence tells us that either the bomb has exploded or it hasn't and we'll know which when we look.

Decoherence sounds to me a lot like just trying to restore some semblance of common sense to a framework that has little if any.

 

[gentzen]

So let's do the experiment on the far side of the moon and we're back to a valid analogy with the cat and the box - naive 1920s vintage Copenhagen can't explain why we don't have a coherent superposition of exploded and unexploded

I think Mermin best captured naive 1920s vintage Copenhagen (Heisenberg style) in his In praise of measurement (based on his earlier Copenhagen Computation: How I Learned to Stop Worrying and Love Bohr). This Copenhagen interpretation has a clear model, and it is totally nonlocal and doesn't care. Worse, the model is closely related to the knowledge of some subjective observer. But it is a perfectly fine and clear mathematical model, as Mermin observes.

In his Physics and Beyond, Heisenberg "let's Einstein voice (in 1926)" this specific criticism:

On the other hand, the continuous element, which appears in interference experiments, must also be taken into account. Perhaps one must imagine the transitions from one stationary state to the next as so many fade-outs in a film. The change is not sudden—one picture gradually fades while the next comes into focus so that, for a time, both pictures become confused and one does not know which is which. Similarly, there may well be an intermediate state in which we cannot tell whether an atom is in the upper or the lower state."

"You are moving on very thin ice," Einstein warned me. "For you are suddenly speaking of what we know about nature and no longer about what nature really does. In science we ought to be concerned solely with what nature does. It might very well be that you and I know quite different things about nature. But who would be interested in that? Perhaps you and I alone. To everyone else it is a matter of complete indifference. In other words, if your theory is right, you will have to tell me sooner or later what the atom does when it passes from one stationary state to the next."

Einstein is quite correct that Heisenberg is talking about what we subjectively know—epistemology— and not about what is—ontology—what is going on in objective reality

So Einstein's objection to this non-local subjective observer centered mathematical model is that there must be some reality independent of the subjective observer, so this model is bad. And of course, its non-locality is also different from what Einstein would have expected. On the other hand, being centered on the subjective observer can be exploited to explain that non-locality away.

 

[Nugatory]

Decoherence sounds to me a lot like just trying to restore some semblance of common sense to a framework that has little if any.

Yet another word for the long list of words that have a specialized technical meaning misleadingly different from general use.... "coherence" can join "particle", "wave", "spin", "field", ....

 

[PeterDonis]

It doesn't mean that according to QM a cat is both dead and alive.

This is interpretation dependent; in the Many Worlds interpretation one can say the cat is both dead and alive; the dead and alive cats are in different "worlds" (branches of the wave function).

The only interpretation independent thing we can say is that we do not observe cats to be both dead and alive.

a microstate is the detailed, unknowable and rapidly changing specification of the microscopic constituents of a large system

This assumes that the microscopic constituents have well defined states. But if they are entangled, which in something like a cat they certainly will be, the microscopic constituents do not have well defined states. Only the overall system--the cat--does.

"Alive cat" is a crude macroscopic description.

Yes, agreed.

For that reason, using a ket with "alive cat" in it is misleading, IMHO.

Not in itself, no. Kets can be, and are, used to denote subspaces as well as individual states.

What is misleading, IMO, is to talk as though a ket representing a (huge) subspace of a cat's Hilbert space is just like a ket describing a single qubit, at least in any way that matters for a discussion of a scenario like Schrodinger's Cat. (For example, see multiple papers published on "Wigner's Friend" type experiments in which "Wigner's Friend" type scenarios involving qubits are claimed to tell us something meaningful about such scenarios involving humans.)

This is a layman's description of the projection postulate in operation. I was using terminology from David Bohm's 1951 "Quantum Theory". The projection postulate is invoked to describe the observation that the quantum state abruptly changes, discards redundant possibilities in a random and discontinuous fashion and renormalizes.

First, as you note, this is a layman's description, and we are aiming for something better here.

Second, the description is interpretation dependent; it basically assumes either a "physical collapse" interpretation (in which "collapse" is a real physical process) or a Bohmian interpretation (in which there are nonlocal hidden variables, the particle positions, that determine the single outcomes of measurements, and "collapse" is something that happens to the wave function in consequence of the particle positions). In an interpretation like the MWI, the description would simply be wrong, as you note:

I understand the MWI discards the projection postulate. There is controversy about whether they they have succeeded in explaining what it actually observed. So I don't want to get into it.

Fair enough, but then this is off topic:

whether they have or they have not, surely they must give an account of why, in this particular cat, an oxygen molecule entered a particular cell, when, by Shrodinger's equation, it could have continued along the bloodstream. Their answer is, if I understand it correctly, that a number of cats were spawned in other worlds to realize the other possibilities. Fine.

In terms of just QM independent of any interpretation, QM makes no pretense whatever of explaining why some particular oxygen molecule entered a particular cell. Nor can we make measurements that would test any such explanation. So the only real discussion we can have of such things is an interpretation dependent discussion.

But that is very different from an electron going through both slits without decoherence. No new electrons in new worlds need be spawned. As long as decoherence is not involved, there is no need to spawn new worlds. A cat is filled to the brim with decoherence. An electron going through a double slit is not.

And in cases where there is no decoherence, the MWI does not say that multiple worlds are spawned. (Note that the MWI was published a couple of decades at least before decoherence theory was developed, so the original MWI publications, and many pop science articles, do not correctly reflect our best current understanding in this regard.)

However, note that in the double slit experiment, when the electron hits the detector screen, decoherence does occur, and according to the MWI, multiple worlds are spawned. But the worlds are not "one world for each slit the electron could have gone through". The worlds are "one world for each point on the detector that the electron could have hit". In other words, the same overall interference pattern will be on the detector in each world, but the particular individual dots that make it up will be different in different worlds.

 

[vanhees71]

It

I think Mermin best captured naive 1920s vintage Copenhagen (Heisenberg style) in his In praise of measurement (based on his earlier Copenhagen Computation: How I Learned to Stop Worrying and Love Bohr). This Copenhagen interpretation has a clear model, and it is totally nonlocal and doesn't care. Worse, the model is closely related to the knowledge of some subjective observer. But it is a perfectly fine and clear mathematical model, as Mermin observes.

In his Physics and Beyond, Heisenberg "let's Einstein voice (in 1926)" this specific criticism:

So Einstein's objection to this non-local subjective observer centered mathematical model is that there must be some reality independent of the subjective observer, so this model is bad. And of course, its non-locality is also different from what Einstein would have expected. On the other hand, being centered on the subjective observer can be exploited to explain that non-locality away.

Einstein, of course, was right. There's nothing like a subjective element in QT. To the contrary according to QT the probabilistic nature is objective, i.e., in any state of a system this system's observables cannot all take determined values. Einstein rather objected to this indeterminism, which however, has been more and more confirmed by experiment, particularly all the stringent tests of the violation of Bell's inequalities, proving the properties predicted by entanglement.

Also the so-called non-locality is overcome with local relativistic QFT, which implements causality through the microcausality constraint on local observable operators as one of its defining principles.

 

[bhobba]

There isn't a clear description of how the quantum world transitions into the world we experience.

Research is ongoing:
https://www.sciencenews.org/blog/context/gell-mann-hartle-spin-quantum-narrative-about-reality

Thanks
Bill

 

[gentzen]

I think Mermin best captured naive 1920s vintage Copenhagen (Heisenberg style) in his ... This Copenhagen interpretation has a clear model, and it is totally nonlocal and doesn't care. Worse, the model is closely related to the knowledge of some subjective observer. But it is a perfectly fine and clear mathematical model, as Mermin observes.

So Einstein's objection to this non-local subjective observer centered mathematical model is that there must be some reality independent of the subjective observer, so this model is bad.

Einstein, of course, was right. There's nothing like a subjective element in QT. To the contrary according to QT the probabilistic nature is objective, i.e., in any state of a system ...

I would be careful to not confuse what Heisenberg lets Einstein say with Einstein's real words and objections. (Heisenberg clarifies both in the preface and in the way he writes how factually inaccurate his dialogs are.) Additionally, I am not sure whether you understood why I wrote that comment, which start with "... Mermin best captured naive 1920s vintage Copenhagen".
It is the mathematical model which is formulated from the perspective of some subjective observer. And it is only in this model that the cat has to wait for the observer. Both Heisenberg's model and Heisenberg himself simply stay silent (and agnostic) about there being some reality independent of the subjective observer.

And of course, its non-locality is also different from what Einstein would have expected. On the other hand, being centered on the subjective observer can be exploited to explain that non-locality away.

Also the so-called non-locality is overcome with local relativistic QFT, which implements causality through the microcausality constraint on local observable operators as one of its defining principles.

I am not sure that QFT provides a clear mathematical model in the same way as Heisenberg's model, or in the same way that the word "mathematical model" is typically used, for example in mathematical logic. QFT is a clear mathematical theory, no doubt. But often, there is a difference between a theory and a model of a theory. And of course, I noticed before that you don't understand why I am not sure about QFT in that respect.

 

[bob012345]

What is misleading, IMO, is to talk as though a ket representing a (huge) subspace of a cat's Hilbert space is just like a ket describing a single qubit, at least in any way that matters for a discussion of a scenario like Schrodinger's Cat.

I should think to actually write the Hamiltonian for a cat would involve Avogadro's number of nucleus terms and something like Avogadro's number factorial of interaction terms.

 

[PeterDonis]

I should think to actually write the Hamiltonian for a cat would involve Avogadro's number of nucleus terms and something like Avogadro's number factorial of interaction terms.

Yes, the Hamiltonian is a whole separate issue. However, for purposes of this discussion, it isn't really necessary to go into detail about the Hamiltonian. The assumption the Schrodinger's Cat scenario makes about the cat's Hamiltonian in isolation is simple: that Hamiltonian preserves the cat's alive/dead state (i.e., if it is alive, it stays alive, and if it is dead, it stays dead--in Hilbert space terms, the cat's internal Hamiltonian keeps the cat's state in the same subspace, alive or dead). So the only possibility for a transition of the cat from alive to dead comes from external interactions--which in this case means the effects of the cyanide released if the radioactive decay triggers it.

 

[bob012345]

Yes, the Hamiltonian is a whole separate issue. However, for purposes of this discussion, it isn't really necessary to go into detail about the Hamiltonian. The assumption the Schrodinger's Cat scenario makes about the cat's Hamiltonian in isolation is simple: that Hamiltonian preserves the cat's alive/dead state (i.e., if it is alive, it stays alive, and if it is dead, it stays dead--in Hilbert space terms, the cat's internal Hamiltonian keeps the cat's state in the same subspace, alive or dead). So the only possibility for a transition of the cat from alive to dead comes from external interactions--which in this case means the effects of the cyanide released if the radioactive decay triggers it.

One has to start the experiment by putting the cat, poison and trigger in the box so the state is completely specified when the box is closed. What then causes the state to become entangled?

 

[bhobba]

I would be careful to not confuse what Heisenberg lets Einstein say with Einstein's real words and objections.

What Einstein thought about QM is often misunderstood. The best article I have found is from Scientific American:
https://www.scientificamerican.com/article/what-einstein-really-thought-about-quantum-mechanics/

Einstein thought it was a valid theory and held to the Ensemble Interpretation.

His issue was the Copenhagen claim it was a complete theory.

He had no issues with its probabilistic aspect - after all, he worked on the foundations of Statistical Mechanics.

The issue was the claim such a theory is complete. He is not the only one, even today, some believe it is incomplete. I think Peter does, for example. Me - I will wait to see what future research reveals, eg the paper by Gell-Mann.

Thanks
Bill

 

[mattt]

One has to start the experiment by putting the cat, poison and trigger in the box so the state is completely specified when the box is closed. What then causes the state to become entangled?

Dynamical evolution by Schrodinger equation.

 

[Lord Jestocost]

What Einstein thought about QM is often misunderstood.......

To my mind, Paul Dirac exactly understood Einstein's confusion. Paul Dirac put it in a nutshell in his article “The Evolution of the Physicist’s Picture of Nature” (Scientific American Vol. 208, No. 5 (May 1963)):

“That is how quantum mechanics was discovered. It led to a drastic change in the physicist’s picture of the world, perhaps the biggest that has yet taken place. This change comes from our having to give up the deterministic picture we had always taken for granted. We are led to a theory that does not predict with certainty what is going to happen in the future but gives us information only about the probability of occurrence of various events. This giving up of determinacy has been a very controversial subject, and some people do not like it at all. Einstein in particular never liked it.

Although Einstein was one of the great contributors to the development of quantum mechanics, he still was always rather hostile to the form that quantum mechanics evolved into during his lifetime and that it still retains."

 

[Lord Jestocost]

One has to start the experiment by putting the cat, poison and trigger in the box so the state is completely specified when the box is closed. What then causes the state to become entangled?

Those, who couple in some confused way the decay of an individual radioactive isotope to the future fate of a cat in a completely isolated box, do not have to be surprised that one is then forced to exclusively apply a probabilistic approach for the prediction of the cat’s future. What else can one expect before the isolated box is opened at a certain time. Before performing an observation, you can only denote the probabilities that the cat will be either alive and bloody furious or dead. That's it!

 

[gentzen]

What Einstein thought about QM is often misunderstood. The best article I have found is from Scientific American:
https://www.scientificamerican.com/article/what-einstein-really-thought-about-quantum-mechanics/

Einstein thought it was a valid theory and held to the Ensemble Interpretation.

His issue was the Copenhagen claim it was a complete theory.

Yes, but he was the one who gave the meaning what „complete“ meant in that context, and Scientific American is simply wrong when they write:

Einstein also thought it took a lot of chutzpah for Copenhagenists to claim that quantum mechanics was complete, a final theory never to be superseded.

Neither Einstein nor his opponents interpreted „complete“ in the sense of „a final theory never to be superseeded“.

 

[vanhees71]

What Einstein thought about QM is often misunderstood. The best article I have found is from Scientific American:
https://www.scientificamerican.com/article/what-einstein-really-thought-about-quantum-mechanics/

Einstein thought it was a valid theory and held to the Ensemble Interpretation.

His issue was the Copenhagen claim it was a complete theory.

He had no issues with its probabilistic aspect - after all, he worked on the foundations of Statistical Mechanics.

The issue was the claim such a theory is complete. He is not the only one, even today, some believe it is incomplete. I think Peter does, for example. Me - I will wait to see what future research reveals, eg the paper by Gell-Mann.

Thanks
Bill

I'd also say, there's no complete theory of everything yet, but for other reasons. It's not the indeterministic element in the "quantum-theoretical worldview" but the lack of a satisfying description of quantum gravity that's lacking.

For me the validity of the predictions of QT in contradistinction to what Bell calls "local realistic theories" is a very strong hint that the intrinsic irreducible randomness concerning the values of observables is a feature of Nature.

To put it differently: There are not the slightest hints to "hidden variables" which may determine, e.g., when an unstable nucleus decays or leads to a causal explanation. It's simply random, when the decay occurs. For $\alpha$ decay it's explained by Gamov's application of the tunnel effect, for $\beta$ and $\gamma$ decay it's the coupling to electroweak fields, etc.

 

[PeroK]

To put it differently: There are not the slightest hints to "hidden variables" which may determine, e.g., when an unstable nucleus decays or leads to a causal explanation. It's simply random, when the decay occurs.

It seems to me that inherent randomness is in many ways the simplest way for nature to operate. For example, if an unstable particle is created in high energy collision, then the hidden variable that determines its precise lifetime would have to come from somewhere. And that variable itself would have to be randomly distributed. So, you need yet more fundamental processes to set these hidden variables. And you still have a probabilistic distribution for their values.

It seems to me that even before the tests of Bell's inequality, hidden variables only moved the problem of randomness one level deeper. As the variable values themselves must have been randomly allocated somehow- by yet another layer of hidden variables?

This is partly why Bohr, Heisenberg and Born etc. were convinced to allow QM to be fundamentally probabilistic. Because it's simpler than moving the randomness down through one or more layers of hidden variables, whose own randomness still required an explanation.

 

[vanhees71]

The difference between the randomness of QT and HV variables is that in the HV theory all observables always have determined values, but they are unknown, because the values of the HV cannot be known for some "fundamental reason". Thus you have to describe them probabilistically, but the probabilities are just due to our ignorance of the HVs, as in classical statistical physics.

Bell's great achievement was to show that the so introduced "classical probabilities" of any HV theory that also includes "locality" (which however means rather "separability", i.e., the statistical independence of properties at far-distant places), contradict the "quantum probabilities" predicted by QT in terms of the violation of Bell's inequalities, which must hold in the "local realistic" HV theories, i.e., no matter how the precise HV model looks like, it must contradict QT, and that made QT testable against such local realistic HV theories.

The fact that all experiments with high significance and accuracy are in accordance with the predictions of QT and contradict the Bell inequalities, i.e., the predictions of the local realistic HV theories, is for me a convincing argument to believe that there are no such HVs determining the values of all observables of a system but that these values are "really" indetermined if the system is not prepared in a state, where some given set of (compatible) observables is determined.

 

[bob012345]

Dynamical evolution by Schrodinger equation.

So, in a closed box the state becomes entangled but suppose the box is left open?

 

[PeterDonis]

One has to start the experiment by putting the cat, poison and trigger in the box so the state is completely specified when the box is closed.

Yes.

What then causes the state to become entangled?

The Schrodinger Equation acting on the initial state when the box was closed. We can assume that that initial state will be a product state of the trigger/poison and cat subsystems. But the Hamiltonian of the overall system inside the box includes an interaction between the trigger/poison subsystem and the cat subsystem (since the poison kills the cat if the trigger activates), which will produce an entangled state under unitary evolution.

 

[PeterDonis]

Those, who couple in some confused way the decay of an individual radioactive isotope to the future fate of a cat in a completely isolated box

What is "confused" about the coupling? It's obvious: if the radioactive atom decays, it triggers the release of poison that kills the cat. In quantum state language, the poison causes the cat to transition from the "alive" subspace of its Hilbert space to the "dead" subspace, irreversibly. Which means that, as I said in post #80, there must be an interaction term in the Hamiltonian that couples the trigger (atom)/poison and cat subsystems. There nothing "confused" here at all.

 

[vanhees71]

Of course, in the real world the cat is never in an entangled state, because it's always "coupled to the environment". At least you have to let it breathe and having enough oxygen, if you don't want to die just by suffocating before the nucleus hasn't decay, because then the cat wouldn't be a reasonable measurement device for registering this decay to begin with. Through this coupling to the environment, which is in or close to thermal equilibrium, the cat's state is very efficiently decohered, and the entire paradox is gone. As for any macroscopic system under usual everyday-life circumstance you almost always can describe this state by classical physics, and it's always in a usual macroscopic "cat state" as we know it from everyday live.

 

[ergospherical]

And if not, why isn’t Schrodinger’s cat dead and buried.

It could be dead and buried, but it could also be buried alive.

 

[PeterDonis]

Of course, in the real world the cat is never in an entangled state, because it's always "coupled to the environment"

Coupling to the environment doesn't mean the cat is not in an entangled state. It just means the entanglement includes the environment as well as the cat.

Through this coupling to the environment, which is in or close to thermal equilibrium, the cat's state is very efficiently decohered

This is true, but it is also true that even a hypothetical "self-contained cat" that had its own internal energy source and 100% recycled everything would also be very efficiently decohered, just by its own internal interactions, because of its huge number of degrees of freedom, most of which cannot be individually tracked.

In the Schrodinger's Cat scenario, the air the cat breathes, etc., are assumed to be isolated with the cat inside the box, so just substitute "cat/air/etc." for "cat" in everything I've posted and it still holds true.

the entire paradox is gone

No, it isn't. It is just clarified. As I said way back in post #2 of this thread.

 

[Lord Jestocost]

This is partly why Bohr, Heisenberg and Born etc. were convinced to allow QM to be fundamentally probabilistic.

Maybe, because they accepted that chance could be a fundamental principle of nature.

 

[bob012345]

This about sums up my feelings about the subject....

 

[Lord Jestocost]

It could be dead and buried, but it could also be buried alive.

Isn't this called "in a state of suspended animation" (in German "scheintot")?

 

[bhobba]

Maybe, because they accepted that chance could be a fundamental principle of nature.

I like to think Gleason may have had something to do with it.

Thanks
Bill

 

[pholmes]

If I say: "The cat is either alive or dead, and I don't know which, and I'll find when I open the box. No change to the cat happens when I open the box. The only change is that I (myself) now know in what condition the cat is, and had been for some time."

But then a Quantum Physics Scientist says: "Oh no, that's a totally inadequate description. That cat is in a State of being both alive and dead at the same time, until you open the box. At that moment, the State of the cat changes."

Is that saying the same thing?
What is more-correct, more true to actual reality, with that 2nd description?

 

[PeterDonis]

If I say: "The cat is either alive or dead, and I don't know which, and I'll find when I open the box. No change to the cat happens when I open the box. The only change is that I (myself) now know in what condition the cat is, and had been for some time."

But then a Quantum Physics Scientist says: "Oh no, that's a totally inadequate description. That cat is in a State of being both alive and dead at the same time, until you open the box. At that moment, the State of the cat changes."

Is that saying the same thing?

No. And the words you are putting in the Quantum Physics Scientist's mouth are not something any quantum physicist today would say, not since we gained a good understanding of decoherence theory. Any quantum physicist today would say that your first description is correct because the decoherence time of the cat is so short.

 

[bob012345]

No. And the words you are putting in the Quantum Physics Scientist's mouth are not something any quantum physicist today would say, not since we gained a good understanding of decoherence theory. Any quantum physicist today would say that your first description is correct because the decoherence time of the cat is so short.

It seems to me that no cat has ever been nor ever could be in a coherent state. Cats are inherently incoherent! :)

 

[PeterDonis]

It seems to me that no cat has ever been nor ever could be in a coherent state. Cats are inherently incoherent! :)

Joking aside, there is nothing in decoherence theory that prevents the possibility in principle of preparing a cat in a state that, at the instant it was prepared, would be "coherent". It just wouldn't stay coherent for more than a miniscule amount of time.

 

[bob012345]

Joking aside, there is nothing in decoherence theory that prevents the possibility in principle of preparing a cat in a state that, at the instant it was prepared, would be "coherent". It just wouldn't stay coherent for more than a miniscule amount of time.

By in principle don't you really mean impossible to achieve in this universe?

 

[PeterDonis]

By in principle don't you really mean impossible to achieve in this universe?

No; "impossible" would mean impossible in principle. "Not possible with our current or foreseeable future technology" would be better.

 

[bob012345]

No; "impossible" would mean impossible in principle. "Not possible with our current or foreseeable future technology" would be better.

Ok, that's what I really meant. Thanks.

 

[haushofer]

No. And the words you are putting in the Quantum Physics Scientist's mouth are not something any quantum physicist today would say, not since we gained a good understanding of decoherence theory. Any quantum physicist today would say that your first description is correct because the decoherence time of the cat is so short.

Maybe I'm misunderstanding, but how can the first description be right if decoherence doesn't produce a definite outcome but 'merely' a classical probability distribution without interference between the two outcomes?

 

[bhobba]

If I say: "The cat is either alive or dead, and I don't know which, and I'll find when I open the box. No change to the cat happens when I open the box. The only change is that I (myself) now know in what condition the cat is, and had been for some time."

Due to decoherence, it is in a mixed state, either dead or alive, not a superposition, regardless of whether the box lid is opened.

The issue is it is not a pure mixed state. The difference is technical, but there is no way to distinguish between a mixed state from decoherence and a pure one, yet they are different. Some say it solves the measurement problem for all practical purposes. IMHO, the issue shifts to while we can't tell the difference, there is one. That is if you consider it an issue and not just an update in knowledge.

Thanks
Bill

 

[vanhees71]

Maybe I'm misunderstanding, but how can the first description be right if decoherence doesn't produce a definite outcome but 'merely' a classical probability distribution without interference between the two outcomes?

For macroscopic systems this probability distribution is very sharply peaked for the relevant macroscopic observables, i.e., their (quantum+ thermal) fluctuions are very small compared to their expectation values.

 

[haushofer]

For macroscopic systems this probability distribution is very sharply peaked for the relevant macroscopic observables, i.e., their (quantum+ thermal) fluctuions are very small compared to their expectation values.

But the first description says:

"If I say: "The cat is either alive or dead, and I don't know which, and I'll find when I open the box. No change to the cat happens when I open the box. The only change is that I (myself) now know in what condition the cat is, and had been for some time."

How do you know "there was no change" if somehow the classical probability distribution "collapses" to one outcome? Isn't this exactly why decoherence doesn't solve the measurement problem completely? Isn't this then Einstein's complaint of lack of realism in QM (the catvwas dead; QM just couldn't tell you, only the classical probability distribution)?

 

[PeroK]

But the first description says:

"If I say: "The cat is either alive or dead, and I don't know which, and I'll find when I open the box. No change to the cat happens when I open the box. The only change is that I (myself) now know in what condition the cat is, and had been for some time."

Yes, exactly.

How do you know "there was no change" if somehow the classical probability distribution "collapses" to one outcome? Isn't this exactly why decoherence doesn't solve the measurement problem completely?

That is true for a coherent quatum system. Just the isolated radioactive isotope, for example. It's not true for a cat. Decoherence may or may not explain that.

sn't this then Einstein's complaint of lack of realism in QM (the catvwas dead; QM just couldn't tell you, only the classical probability distribution)?

Not really. The cat acts like a macroscopic measuring device and established an outcome for the radioactive isotope.

The measurement problem is at what point a system becomes complex enough to constitute a measurement. Decoherence may or may not explain that too.