Quantum-classical correspondence?

[xylai]

I don't know why scientists investigate the quantum-classical correspondence?

I think it can not help us solve any problem in quantum mechanics.

Any comment is welcome!

Thank you!

 

[Dmitry67]

We don't have what is called bird's view: we see only a shadow on the wall, trying to guess a shape of 3D object.

That is why Quantum Decoherence is learned: to be sure that our vision of the quantum world (obtained purely from the observation of the classical things: indicators, cameras, voltmeters etc) is valid.

And yes, it is not supposed to give anything new regarding the High Energy, for example.

 

[xylai]

We don't have what is called bird's view: we see only a shadow on the wall, trying to guess a shape of 3D object.

That is why Quantum Decoherence is learned: to be sure that our vision of the quantum world (obtained purely from the observation of the classical things: indicators, cameras, voltmeters etc) is valid.

And yes, it is not supposed to give anything new regarding the High Energy, for example.

Thank you for your reply!
But I am still confused!

 

[ZapperZ]

I don't know why scientists investigate the quantum-classical correspondence?

I think it can not help us solve any problem in quantum mechanics.

Any comment is welcomed!

Thank you!

How would you know it doesn't?

The fact that the classical world and the quantum world are SO different, BEGS the question on the nature of the transition between those two. And considering that our devices are getting smaller and smaller, at some point, we WILL bump against this mesoscopic scale where both of them will either mix, or fluctuate, between each other. How is such information not useful?

Zz.

 

[Fra]

> I think it can not help us solve any problem in quantum mechanics.

What "problems" are you thinking about?

I think some of the problems of understanding this correspondence become even more acute when one tries to ponder how a coherent framework that incorporates both GR and QM might look like, because one seems to run into self-observations and regress, and then it becomes difficuly ot insist on a static line between classical and quantum. Any boundary seems to be ambigous or subjective.

There seems to be different roads here. Either you picture some external birds view, representing some kind of quantum reality, or "gods view", which you can picture as an abstraction of the laws of physics, and from this picture the inside views are "computed/prediced".

Or you think that makes no sense, and instead turn this around, and ask how an apparently stable objective reality, can emerge as seen from an inside observer.

So either the gods view, is use to derive the frogs view. Or the frogs view somehow must explain the "coincidences" that all frogs tend to be synchronized to consistently describe different projections of some "effectively objective abstraction quantum reality".

I subscrive to the latter.

One version of the former, quite common, is to have a realist view of these symmetries. And thus, these symmetries neatly escapes scientific questioning. They are considered objective ontologies of reality, just for us to discover, thus their status as observable or not is a non-issue. In this view, it's like the scientists take the place of God.

The latter idea, instead must explain how the symmetries, as subjective after all, in the classical limit, are likely to produce objectivitiy. I think of this as evolving and emergent symmetries. In this view, the scientist proving space is no different than an atom probing it's environment. We are about as lost and ignorant. The problem for this approach, is to argue how come, given this denial of objective universal law, very stable laws, are nevertheless observed.

Somehow both approaches address a similar problem, but from different starting points.

/Fredrik

 

[Halcyon-on]

The more important problematics in modern physics are more or less indirectly related with QM.

A correct interpretation of QM in terms of classical dynamics could allow, for instance, the solution of quantize gravity. More practically, it could be useful in the the renormalization procedure of Feynman diagrams avoiding cancellation between infinities, virtual particles or stuff like that. (How far can we go calculating more and more loops order in diagrams?)

An example of how powerful it could be is given by the AdS/QCD correspondence which allows to calculate non-perturbative QCD using classical configurations of the fields in an AdS metric.

 

[xylai]

> I think it can not help us solve any problem in quantum mechanics.

What "problems" are you thinking about?

I think some of the problems of understanding this correspondence become even more acute when one tries to ponder how a coherent framework that incorporates both GR and QM might look like, because one seems to run into self-observations and regress, and then it becomes difficuly ot insist on a static line between classical and quantum. Any boundary seems to be ambigous or subjective.

There seems to be different roads here. Either you picture some external birds view, representing some kind of quantum reality, or "gods view", which you can picture as an abstraction of the laws of physics, and from this picture the inside views are "computed/prediced".

Or you think that makes no sense, and instead turn this around, and ask how an apparently stable objective reality, can emerge as seen from an inside observer.

So either the gods view, is use to derive the frogs view. Or the frogs view somehow must explain the "coincidences" that all frogs tend to be synchronized to consistently describe different projections of some "effectively objective abstraction quantum reality".

I subscrive to the latter.

One version of the former, quite common, is to have a realist view of these symmetries. And thus, these symmetries neatly escapes scientific questioning. They are considered objective ontologies of reality, just for us to discover, thus their status as observable or not is a non-issue. In this view, it's like the scientists take the place of God.

The latter idea, instead must explain how the symmetries, as subjective after all, in the classical limit, are likely to produce objectivitiy. I think of this as evolving and emergent symmetries. In this view, the scientist proving space is no different than an atom probing it's environment. We are about as lost and ignorant. The problem for this approach, is to argue how come, given this denial of objective universal law, very stable laws, are nevertheless observed.

Somehow both approaches address a similar problem, but from different starting points.

/Fredrik

Looking at the same problem with the different views.

But I think the quantum-classical correspondence can be valid only in a small and special scale.

 

[naturale]

So, the quantum-classical correspondence that you have in mind is a partial correspondence. Now think to a complete correspondence, think to have a classical theory that originate QM.

 

[xylai]

So, the quantum-classical correspondence that you have in mind is a partial correspondence. Now think to a complete correspondence, think to have a classical theory that originate QM.

Yes, we can think classical theory orginates QM.
In my opinion, this subjects to quantum to cassical transition.

Maybe I am confused between "quantum-classical correspondence" and "quantum to cassical transition".

Any comments are welcome!

 

[Fra]

But I think the quantum-classical correspondence can be valid only in a small and special scale.

It seems you have specific proposals in mind, that are at variation with some "standard views" of the correspondence such as quantum expectation values obeying classical laws. If that's your point, I agree, but that is a disucssion of its ow.

I interpreted your original post as you objecting not to specific views of the correspondence, but on the correspondence beeing researched at all.

/Fredrik

 

[Coldcall]

Should there be a quantum-classical correspondence in the first place? I know quantum physics keeps trying to find interpretations which create a causal and logical transition from the microscopic to the macroscopic, but that strategy appears to have failed.

Once again the heart of this problem lay in the measurement problem: its like all roads lead to Rome. Had any of the interpretations actually provided a genuine foundational solution we would not still have a classical-quantum paradox..as we do.

Interesting how the majority of threads on this quantum physics forum directly or indirectly allude to measurement problem phenomena.

 

[Fra]

Once again the heart of this problem lay in the measurement problem

I agree there.

/Fredrik

 

[Dmitry67]

Should there be a quantum-classical correspondence in the first place? I know quantum physics keeps trying to find interpretations which create a causal and logical transition from the microscopic to the macroscopic, but that strategy appears to have failed.

Once again the heart of this problem lay in the measurement problem: its like all roads lead to Rome. Had any of the interpretations actually provided a genuine foundational solution we would not still have a classical-quantum paradox..as we do.

Interesting how the majority of threads on this quantum physics forum directly or indirectly allude to measurement problem phenomena.

The measurement problem had been solved about 10 years ago.
http://en.wikipedia.org/wiki/Quantum_decoherence

 

[ZapperZ]

The measurement problem had been solved about 10 years ago.
http://en.wikipedia.org/wiki/Quantum_decoherence

Don't be so sure. One could also arrive at the classical scenario not by invoking decoherence, but simply due to coarse-grained measurement[1].

Zz.

[1] J. Kofler and C. Brukner, Phys. Rev. Lett. v.99, p.180403 (2007).

 

[Dmitry67]

Is it available for free? I did not find any free sources. And I don't want to pay for reading what I am not agree with :)
Or could you explain the ganaral idea?

 

[Coldcall]

The measurement problem had been solved about 10 years ago.
http://en.wikipedia.org/wiki/Quantum_decoherence

Perhaps you did not read the whole of the wiki article you referred to:

"However, decoherence by itself may not give a complete solution of the measurement problem, since all components of the wave function still exist in a global superposition, which is explicitly acknowledged in the many-worlds interpretation. All decoherence explains, in this view, is why these coherences are no longer available for inspection by local observers. To present a solution to the measurement problem in most interpretations of quantum mechanics, decoherence must be supplied with some nontrivial interpretational considerations (as for example Wojciech Zurek tends to do in his Existential interpretation). However, according to Everett and DeWitt the many-worlds interpretation can be derived from the formalism alone, in which case no extra interpretational layer is required."

So even Zurek, one of the founders of decoherence interpretation admits its not a foundational solution as such.

 

[Dmitry67]

Yes, I read it many times and I, as Many Worlds fan, really like the part you quoted.
So step 1 is to accept the Decoherence
After that the only logical possibility is to accept Many Worlds.

 

[Coldcall]

I think this paper a fair assesment on the question whether decoherence solves the measurement problem - and its free :-)

http://arxiv.org/abs/quant-ph/0312059

 

[Coldcall]

Yes, I read it many times and I, as Many Worlds fan, really like the part you quoted.
So step 1 is to accept the Decoherence
After that the only logical possibility is to accept Many Worlds.

I accept both decoherence and many-worlds as interpretations; neither solves the "measurement problem".

 

[Dmitry67]

Hm, could you explain why?

 

[Fra]

My main objection to the various bird views, is that they fail to address the real question. The logic of decoherence is fairly clear, and it's partly sound. But it is not a complete answer.

As I see it, the interesting questions can not even be formulate relative to a birds view.

The whole concept of a unique wavefunction of the universe, is an abused IMO. Then you might as well toss out measurement theory and find something else.

It seems Einstein has said

"The dynamics and the postulate of collapse are flatly in contradiction with one another ... the postulate of collapse seems to be right about what happens when we make measurements, and the dynamics seems to be bizarrely wrong about what happens when we make measurements, and yet the dynamics seems to be right about what happens whenever we aren't making measurements."

Then is seems Einstein never really understood the point with a measurement theory, as an ambition to implement in a deeper sense part of a scientific method into our theory.

As we know the unitary evolution, applies IN BETWEEN measurements. The UTILITY of prediction exists only in between measurements. You throw dice when you don't know, as a guide. But when you get real information, it makes no sense to compare your single dice stroke with the real input. The real input collapses the dice every time, by making it obsolete.

The main confusion seems to be that it is difficult to understand how such a subjectivity of observers, can still produce the effectively objective world we macroscopically observe. But given that the situation is somewhat similary in relativity, that the subjectivity of observer frames, are in fact stored by various observer-symmetries, I find it somewhat paradoxal that Einstein, of all people, showed such resistance to - what I think this is - in fact just extending the class of observers.

To, me the obvious resolution would be that, well observers are interacting aren't they? Yes they are. And that's exactly where this "apparent" inconsistency comes in. The observers deviating description of each other and their common environment, is the origin of certain interactions.

If we introduce gods of birds views, we loose the origin of interactions. Instead we end up with a new question we can not answer - why does the laws of physics look like this? ALSO, we have the more important question, these scheme fails to address: to describe the physical process by which a real frog observer, acquires information about these laws, from interactions.

/Fredrik

 

[Dmitry67]

1 As I see it, the interesting questions can not even be formulate relative to a birds view.
2 The main confusion seems to be that it is difficult to understand how such a subjectivity of observers, can still produce the effectively objective world we macroscopically observe.
3 Instead we end up with a new question we can not answer - why does the laws of physics look like this?
4 ALSO, we have the more important question, these scheme fails to address: to describe the physical process by which a real frog observer, acquires information about these laws, from interactions.

/Fredrik

1 Why not? Frog's view can be derived from the Bird's view.
2 Quantum Decoherence is calculated based on some arbitrary basis (or the Decomposition of the Universe into systems, how Ilja calls it). For different decomposition you get different iews of the different frogs. This is exactly what you call a 'subjectivity'
3 As you might remember, Max Tegmax had provided a very good and complete answer. I don't know any other theory which can explain it - without MUH.
4 This physical process is called an experment. That is the same physicists were doing before and today - trying to understand the shape of 3D object looking at the shadow from the different angles.

 

[Dmitry67]

As we know the unitary evolution, applies IN BETWEEN measurements.

Well, this all sound very CI-like. Magical role of measurements, special role of observers, nature, obeying the laws not always, but only IN BETWEEN the measurements :)

 

[Coldcall]

Hm, could you explain why?

Could i explain what?

The paper i referred you to is a good assesment because it takes many views into account, most of them from decoherence proponents such as Joos and Zurek, who clearly don't think its a foundational solution to the measurement problem.

 

[Fra]

I tried to convey the points before but failed...

1 Why not? Frog's view can be derived from the Bird's view.
2 Quantum Decoherence is calculated based on some arbitrary basis (or the Decomposition of the Universe into systems, how Ilja calls it). For different decomposition you get different iews of the different frogs. This is exactly what you call a 'subjectivity'

Your idea and "derivation from the birds view" is not totally flawed IMO. It does in fact have a place and a point even for me. But it's only half the story.

My objection is on the status of this birds view.

I object to that you think of this "derivation" itself, in a realist sense. IE. the CHOICE of derivation itself, escapes proper scientific questioning. Your "birds view" seem to not be subject to questioning. It simply is. Just like the laws pf physics simply are.

In the context of the questioning I miss. The problem is not that if we have this or those laws, then we can derive this or that. The problem is inseparable from the problem of first INFERRING the laws from observations=measurements=interactions.

To me, that makes not sense. However, I also think in terms of a kind of birds view and frogs view. But in my view, the birds view is not really a observer independnet birds view, the only justifiable bird views, are in fact just frog views of other frog-systems. Ie. one frog, watching 100 frogs, sort of gets frogs-birds view of them.

In this sense, there is no objective universal realist birds view. Like I have a feeling you think there is.

I am suggesting that there is not way to ESTABLISH wether the frogs-birds view, is a "real" birds view. Moreover do I argue, that it does not matter for interactions.

3 As you might remember, Max Tegmax had provided a very good and complete answer.

In total Tegemark's way of reasoning doesn't appeal to me. In my eyes I can't see that he gave any complete answers to any good questions.

I don't know any other theory which can explain it - without MUH.

To why does the laws of physics look like this? As I see it, the most promising alternative is the evolutionary idea of evolving law and evolving observers. In a sense, the evolving law is encoded in the evolving observers. So evolving observer and evolving law go hand in hand. And the distinction between law, and initial conditions are totally wiped out in this view. Because both are questioned by the same system. Both constitute information. It's just that "law", is a compressed/condensed and evolved form of information.

But I don't thikn it's necessarily exactly Smolins black hole bounce, there might be othre ways for laws to evolve. But indeed no one has executed this yet. Why is that, after all these year is a good question. That can't be my responsibility.

4 This physical process is called an experment. That is the same physicists were doing before and today - trying to understand the shape of 3D object looking at the shadow from the different angles.

Yes. But if you think again about that. What is, at the fundamental level, the difference between "experiment" and "physical interaction"? I'm sure you must agree that human knowledge of physical law HAS evolved and continuous to do so. So scientists best "birds view" is still de facto evolving.

The point is that the attempt to make a distincion between the image and the real thing, fails everytime you try it. Because any such attempt is itself a process.

I think the same applies to physical law at non-human level.

The problem of infering an object from it's shadow, is that it does in fact depend on a MODEL. Or a logic. There are clearly an infinite number of possible ways for a shadow to appear. So the inference relies on a background structure. What I am saying is that this background structure, that DOES exists even to me, OTOH is not fixed, it is ALSO evolving. And it's not evolving as per some fixed meta laws, it's a self-referential self-organisation, and this infinite regress that I think you think is bad, is simlpy the drive for time in my view.

It's the reason why we have a dynamical world. The analysis suggest IMO that this infinite regress, described as a process, are actually goverend by inertial like concepts. So the infinite regress is not a madly chaotic or spining uncontrolled process, it's constrained relative to it's prior state only.

What I want, is to understand and characterize this process better. This is why realist view of physical laws, simply won't do. They violate the very founding principle. Physical laws, are a result of evolved inference, and the mysterious stability of law, is due to a kind of inertia, which in turn is a result of the self-preserving nature of observers. And non-preseving observers, are for fairly clear reasons rarely observed by other observers :)

/Fredrik

 

[Fra]

Well, this all sound very CI-like. Magical role of measurements, special role of observers, nature, obeying the laws not always, but only IN BETWEEN the measurements :)

The role of the observer in this context, is not more magic, than the role of matter and energy is to spacetime. In GR they evolve _together_. That said, I'm not prepare to raise GR-way as the solution, but this simple analogy may hint an ever deeper lesson.

The relational idea, is that spacetime is somehow the relations between material objects. But it's nevertheless that case that those relations can't be justified if you actually REMOVE the objects.

To solve the measurement problem by REMOVING the observer completely, is throwing the baby out with the water.

A solution of the measurement problem can as well be the concept of evolving observers.

/Fredrik

 

[Dmitry67]

Fra, I hope I understand your objection and the concept of a frog-bird view.

I will try to provide an example. Virtual 'beings' in a computer game have an illusion that their world is real, so they can try to make their own vision of the Birds view of their world. That 'birds view' will be a map of all levels of the game, but it won't have any relation to our world.

In fact, you can construct such bird-level laws which can block any possibility to discover the real correct birds laws by the local frogs, or lead the local frogs into 'discovering' the illusionary set of laws.

But such bird-level laws are EVIL. So I can reply with just one Einsteins quote:

God is subtle but he is not malicious

 

[Fra]

Fra, I hope I understand your objection and the concept of a frog-bird view.

Maybe we may have an mutual misunderstanding here, in despite of a birds view I can nothing but guess and act upon that - in that spirit I'll comment below :)

In fact, you can construct such bird-level laws which can block any possibility to discover the real correct birds laws by the local frogs, or lead the local frogs into 'discovering' the illusionary set of laws.

Like your infinite regress objection - this may actually happen in nature. Again, this is not a problem, it's a possible to exploit to understand nature.

Why is it that some system would respond to an electric field, and some don't? could it be, that some systems simply don't SEE the electric field? But why is it that apparently ALL systems respond to gravity?

This can be reflected upon in a general structural way. Maybe, the explanation has to do with the internal structure of charged particles? What is charge?

So maybe you can say that the neutral particles lives an illusion of there beeing no electric field. But that's part of my point.

This is the exploit. You seem to think it's a "problem". Same with infinite regress, which is bad bacause it makes Your axiomatisation a PITA. But how about of the infinite regress is simply the arrow of time? Again this is good, not a "problem.

I have a feeling I can't convince you at this point. About the EVIL view, I don't see it as EVIL at all. At least not more EVIL than what's called for :)

/Fredrik

 

[Count Iblis]

As we know the unitary evolution, applies IN BETWEEN measurements

If it doesn't apply after a measurement is made, then it doesn't apply in general. Though experiment: You are inside a closed box kept isolated from the environment (assume that the outside of the box is at zero K and a vacuum and that the boundary of the box is also at zero K).

In that closed box you do measurements. So if the many particle state of the box was intitially a pure state, it will become a mixed state (according to collapse interpretations). But no external observer is measuring the state of the box.

So, the conclusion must be that in collapse interpretations of QM, closed systems will evolve from pure states to mixed states, even if there are kept perfectly isolated.

So, collapse interpretations must lead to violations of unitary time evolution also inbetween measurements. Then it is fair to ask about the experimental evidence. E.g you can try to measure decoherence rates of large molecules and then see if you get a larger decpoherence rate than predicted by standard quantum mechanics. AFAIK, no deviations of theoretical predictions have ever been found.

 

[Dmitry67]

Count, check this: http://en.wikipedia.org/wiki/Wigner's_friend
Even I hate CI I have to admit that it leads to the same results and can not be distinguished experimantally, because wavefunction in CI is not real but just a "knowledge of an observer about the system". So it is perfectly valid to have many different (and inconsisent) wavefunctions describing the SAME system. So your argument is valid only for the objective collapse theories.

 

[Coldcall]

I like CI because while it is not explicit, it sort of infers that our ability to interact with quantum states is based purely on our reality, here and now. We are human and cannot escape the fact that whatever we see happen in this universe is based on our version of reality. Do two particles have a reality? I don't think so, as i believe one needs a brain or some sort of awareness to experience reality in the first place. Our interactions with quantum states are thus based on experiences.

Hence why I've never felt comfortable with the idea of decoherence, other than as a FAPP interpretation. We can never know if a particle can decohere through interaction with another particle - in the absence of an observer/definer's reality or version of reality.

Its impossible for us to know whether a universe such as ours can exist without observers/definers, or at least with the conditions suitable that such observers/definers will exist in that universe at some point in the future.

These are just my personal views :-)

 

[Fra]

So, collapse interpretations must lead to violations of unitary time evolution also inbetween measurements.

I didn't quite get your comment nor your argument here, and in what direction you argue. But I think Dmitry's comment that the wavefuntion is not real, in the sense of objective realism is part of the key.

You seems to mix up different views.

I do not quite adhere to CI. I am partly close to CI, but my personal view suggest that the quantum formalism needs to be reconstructed, and changed. But this reconstruction does not violate the current limits. Ie. it would not contradict the current formalism as a limiting/special case scenario.

I like to think that what I suggest is more than just an interpretation.

Let me see if I got what you said.

If it doesn't apply after a measurement is made, then it doesn't apply in general. Though experiment: You are inside a closed box kept isolated from the environment (assume that the outside of the box is at zero K and a vacuum and that the boundary of the box is also at zero K).

If the box is closed, and the observer is on the inside. Your description of the outside obviously introduces another observers. And "box is closed" I assume you mean these can not communicate, right?

In that closed box you do measurements. So if the many particle state of the box was intitially a pure state, it will become a mixed state (according to collapse interpretations). But no external observer is measuring the state of the box.

I'm not sure I see what you say.

As far as I am concerned, the collapse is not an "assumption". It is new evidence beeing thrown in the face of an observer. So the collapse is something inherent to an observer. This collapse, is not a collapse from the point of view of a third observer.

Dmitry said it already. The wavefunction is not representing an objective state of realist information. It's a relational kind of information. The collapse, is the "inside view". The outside view is not a collapse. It's more like an emergent decoherence view, if the third observer is large enough to hold so much information. If not, there are collapse phenomena even there, but the collapses are not objective events.

As I see it, it's the fact that they are not, that leads to interaction between the parts in the universe (or between different "observers" - and observer is simply a subsystem, it need not have a brain or even be biological). To me, "observer" is simply an abstraction for any part of the universe, that interacts with it's environment.

/Fredrik

 

[Coldcall]

Fredrik,

"To me, "observer" is simply an abstraction for any part of the universe, that interacts with it's environment."

How does an inanimate particle observe another particle? With what does it do the observing or measuring?

Are you really suggesting that non-living matter can experience reality?

 

[Fra]

Are you really suggesting that non-living matter can experience reality?

In the restriced sense we are talking about - absolutely. The laws of nature does not IMO distinguish in a fundamental sense, a general physical system, from biological systems.

If you think that "observer" means human, then clearly CI is baloney. But this is not what I have in mind. Observer has a wider meaning, having no relation per see to the human brain.

I'll comment more later...on my way out.

/Fredrik

 

[mn4j]

Example: double slit diffraction with electrons. How does MWI explain the build-up of the pattern spec by spec. Unless the interpretation can explain what is ACTUALLY happening, it has not solved the measurement problem.

Can any MWI'er explain it?