Why don't we bury Schrodinger's Cat?

[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.

 

[PeterDonis]

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?

The answer to this is interpretation dependent. The point is that on any interpretation, there is no interference between the possible outcomes, so opening the box doesn't change anything about what's inside the box; all it does is enable interaction between what's inside and what's outside so the information about what's in the box can spread more widely. That is what the first description describes. But different QM interpretations will tell different stories about "what is actually happening" behind the scenes in order to make that description correct.

 

[PeterDonis]

How do you know "there was no change" if somehow the classical probability distribution "collapses" to one outcome?

Even on interpretations that make this claim, the collapse occurs when the cat decoheres inside the box, not when the box is opened. Opening the box still doesn't change anything about the cat.

 

[Lord Jestocost]

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."

That's fine so far, if you would leave out the half-sentence "and had been for some time".

 

[gmax137]

My understanding of QM is very dated, based on a handful of undergrad courses back in the 1970s. I don't recall the terms "coherence" and "decoherence," I'm not really sure what they mean. Is there a modern intro textbook that would help me catch up? I'd like to understand threads like this one better.

 

[PeterDonis]

My understanding of QM is very dated, based on a handful of undergrad courses back in the 1970s. I don't recall the terms "coherence" and "decoherence," I'm not really sure what they mean. Is there a modern intro textbook that would help me catch up? I'd like to understand threads like this one better.

I don't know if any textbooks have been updated to cover decoherence theory, but there is a good 2019 review article by Schlosshauer referenced here (as well as other PF threads):

https://www.physicsforums.com/threads/new-review-on-decoherence-by-schlosshauer.979635/