Insights How I Stopped Worrying and Learned to Love Orthodox Quantum Mechanics - Comments

[atyy]

To elaborate my concern. I was hoping that making the wave function as simply calculation tool would make all classical.. without needing any quantum cut.. but in the double slit experiment, treating the wave function as simply calculation tool means we only deal with the output or the detector screen.. we ignore what happens between emission and detection.. is this what you mean or a good example.. and the reason we still need the cut is because we need to know what happens inbetween.. or how reality comes from non-reality?

It's really a headache to treat the wave function as objective.. I have headaches for many days lol.. so treating it as calculational tool can give us relief.. but the cost is unable to determine the reality/non-reality cut in the sense we don't know what happens between emission and detection? Is this the exact reason so I know.. thanks..

Yes, you can try to make everything classical, eg. Bohmian mechanics or MWI are famous approaches to making quantum mechanics classical. But if you only make everything notionally classical, without equations (ie. without Bohmian Mechanics or MWI), you have no way to interact with quantum mechanics and pull quantitative predictsion about reality (or the "classical" or "macroscopic" world). Basically, quantum mechanics does not tell us when a measurement occurs, and we need that input to pull a quantitative prediction out.

 

[martinbn]

What is the problem with having a classical/quantum cut?

 

[atyy]

What is the problem with having a classical/quantum cut?

It has been stated in various equivalent ways.

- It means that classical mechanics cannot be derived from quantum mechanics without assuming classical mechanics. Thus quantum mechanics is not the most fundamental theory.

- If there is a classical/quantum cut, that means that means that quantum mechanics cannot describe the observer himself. If we extend the quantum boundary to include the observer, we need yet a second observer to observe the first observer, so there is always some bit of the universe that is excluded from quantum mechanics, again suggesting that quantum mechanics is incomplete.

It is not a problem of internal coherence of the theory itself. Rather, from how we understand theories of physics to describe reality, it appears that quantum mechanics is incomplete, even if no experimental evidence contradicts it. Thus there is a theoretical opportunity to devise more complete theories (eg. BM) or alter our understanding of reality (eg. MWI).

 

[vanhees71]

Yes, that blurring is exactly where Peres reveals his mistaken sympathies with Ballentine. It is simply wrong.

It's only your lonely opinion that the minimal interpretation is wrong. Instead, it's the interpretation nearly all physicists use in the labs and theory institutes around the world.

 

[martinbn]

- It means that classical mechanics cannot be derived from quantum mechanics without assuming classical mechanics. Thus quantum mechanics is not the most fundamental theory.

That by itself is not a problem. No matter what people prefer, this may be how nature is.

- If there is a classical/quantum cut, that means that means that quantum mechanics cannot describe the observer himself. If we extend the quantum boundary to include the observer, we need yet a second observer to observe the first observer, so there is always some bit of the universe that is excluded from quantum mechanics, again suggesting that quantum mechanics is incomplete.

It only means that in any description there will be classical mechanics involved. Why is that a problem?

It is not a problem of internal coherence of the theory itself. Rather, from how we understand theories of physics to describe reality, it appears that quantum mechanics is incomplete, even if no experimental evidence contradicts it. Thus there is a theoretical opportunity to devise more complete theories (eg. BM) or alter our understanding of reality (eg. MWI).

I don't see why it is incomplete? Any experiment can in principle be described. It may not be the way some people want it but that is irrelevant.

 

[vanhees71]

First of all QT is incomplete, because there's no satisfactory quantum description of gravity, but that's not the issue.

The state of affairs, however, nowadays is that where QT is applicable, it is more comprehensive than classical physics, whose validity is limited by empirical facts where QT effects become important. In the standard way of thinking in the physics community thus quantum theory is the more fundamental theory, and classical behavior of macroscopic systems is considered as an "emergent phenomenon", i.e., it can be understood as an effective description of macroscopic observables, which are defined by coarse-graining microscopic observables in the sense of quantum statistical mechanics. As far as I know, there's no evidence that this point of view is wrong.

 

[Demystifier]

If one type of fundamental particle were manifest as all various types of particles/quasi-particles,
what prevents associating one particle only of only one type to all the world lines in space time?

I still have no idea what are you talking about. How can one particle be associated with many world lines?

To other readers, does somebody else understand the question?

 

[Demystifier]

Isn't it known to be possible for Lorentz invariance to emerge at large distance scales from a quantum field theory that is non-relativistic on small distance scales? IIRC (I think I first came across this in Zee's QFT textbook), the Lorentz invariant speed is something like a "sound speed" in an underlying medium that emerges from the non-relativistic QFT.

Yes, exactly.

 

[Demystifier]

What is the problem with having a classical/quantum cut?

Let me use an analogy from pure mathematics. Consider naive set theory, which is known to suffer from certain paradoxes. There is a small group of mathematicians called finitists who resolve the problems by postulating that infinity doesn't exist. At first sight that looks reasonable, for no one ever seen infinity, nor ever will. However, such an approach requires a cut, namely the existence of the largest natural number. What that number is? Nobody has a clue. Whatever specific number you choose (e.g. Graham number or whatever), a smart combinatorist may find a good practical reason for existence of an even larger number.

The situation with classical/quantum cut is similar. If there is a cut, you must say where exactly the cut is. But wherever you place the cut, a smart physicist may find compelling experimental evidence that the cut cannot be there. For instance, if you say that the cut is 100 atoms, there are situations in which molecules with more than 100 atoms behave quantum mechanically. If you say that cut is 1000 atoms, there are situations in which molecules with less than 1000 atoms behave classically. Perhaps the cut is not defined by the number of atoms but by something else, but then what that else is? The amount of complexity? The existence of consciousness (something similar was proposed by mathematician von Neumann)? There are various ideas, but neither looks very convincing.

 

[martinbn]

But that by itself is not a problem, at least not in my eyes. If we go back to the analogy and we decide to use only finite sets, that doesn't mean that there will be a universal bound for the number of elements. It only means that for every problem you want to consider you will use only finite sets. For the next question you may need a bigger set, no problem as long as it is finite.

 

[Demystifier]

But that by itself is not a problem, at least not in my eyes. If we go back to the analogy and we decide to use only finite sets, that doesn't mean that there will be a universal bound for the number of elements. It only means that for every problem you want to consider you will use only finite sets. For the next question you may need a bigger set, no problem as long as it is finite.

So in physics, for each case you would use a different cut. Well, in a sense you are right because this is what physicists often do in practice (usually without being aware of it). To decide where to put the cut they use intuition (rather than a physical law) and more often than not they got results which agree with experiments. But, similarly to naive set theory, this means that one must abandon axiomatization of the most fundamental physical theory we have. This may not be a problem from a practical point of view, but physicists who like to think that physics is more than a practical tool don't find it satisfying.

 

[jerromyjon]

If one type of fundamental particle were manifest as all various types of particles/quasi-particles,
what prevents associating one particle only of only one type to all the world lines in space time?

I still have no idea what are you talking about. How can one particle be associated with many world lines?

To other readers, does somebody else understand the question?

I think it refers to having only one single particle, as if the standard model were constructed from one single type of particle, but it would only help the situation if you quantitatively knew every possible state of every particle that any system were composed of with no uncertainty and that isn't even remotely possible as per Heisenburg uncertainty...

 

[martinbn]

So in physics, for each case you would use a different cut. Well, in a sense you are right because this is what physicists often do in practice (usually without being aware of it). To decide where to put the cut they use intuition (rather than a physical law) and more often than not they got results which agree with experiments. But, similarly to naive set theory, this means that one must abandon axiomatization of the most fundamental physical theory we have. This may not be a problem from a practical point of view, but physicists who like to think that physics is more than a practical tool don't find it satisfying.

But the theory shouldn't tell you where to put the cut. It should be able to handle all possible scenarios. It's like asking from classical mechanics to tell you what the forces are, or what coordinates you should use.

 

[haushofer]

I liked your article a lot because of its freshness and because I share your opinion about the "such a mechanism is possible" interpretation of Bohmian mechanics . Thanks.

And as a defendor of Newton-Cartan theory I wholeheartly subscribe the idea that the world is fundamentally non-relativistic :P

 

[atyy]

That by itself is not a problem. No matter what people prefer, this may be how nature is.

It only means that in any description there will be classical mechanics involved. Why is that a problem?

I don't see why it is incomplete? Any experiment can in principle be described. It may not be the way some people want it but that is irrelevant.

You are allowed to think it is not a problem. However, what must be agreed upon is that there is a cut. From there one can take either of two classic positions: (1) it is not a problem (eg. Bohr), or (2) it is a problem (eg. Dirac).

Bohr's position is very beautiful. Dirac's position means that one may use this as an opportunity to think about new physics, even though there is not yet evidence against quantum mechanics. It is similar to working on string theory, even though there is no evidence yet for the failure of quantum general relativity.

Ballentine's error is that the cut is not acknowledged.

Although one is allowed to say that there is no problem with a classical/quantum cut, I don't believe you have given a right reason for saying that it is not a problem: you cannot claim that that is how nature is. The point of physics that says that the cut is not a problem, is that physics is not about what nature is.

 

[martinbn]

You are allowed to think it is not a problem. However, what must be agreed upon is that there is a cut. From there one can take either of two classic positions: (1) it is not a problem (eg. Bohr), or (2) it is a problem (eg. Dirac).

Bohr's position is very beautiful. Dirac's position means that one may use this as an opportunity to think about new physics, even though there is not yet evidence against quantum mechanics. It is similar to working on string theory, even though there is no evidence yet for the failure of quantum general relativity.

Although one is allowed to say that there is no problem with a classical/quantum cut, I don't believe you have given a right reason for saying that it is not a problem: you cannot claim that that is how nature is. The point of physics that says that the cut is not a problem, is that physics is not about what nature is.

But if one claims that there is a problem, he has to explain why it is a problem. To me it seems that you consider the cut to be a problem simply because you want the theory to be of curtain type i.e. to not have a cut. And you can have that view and look for such a theory, but it is strange to say that the existent theory has a problem because of this.

Ballentine's error is that the cut is not acknowledged.

This doesn't sound like an error. At best it is an omission. Given that the whole subject is vast, the author can decide what to include.

 

[atyy]

But if one claims that there is a problem, he has to explain why it is a problem. To me it seems that you consider the cut to be a problem simply because you want the theory to be of curtain type i.e. to not have a cut. And you can have that view and look for such a theory, but it is strange to say that the existent theory has a problem because of this.

As I said, that is not my technical quarrel with Ballentine. If you choose not to say the cut is not a problem, that is intellectually coherent, and I respect this view. I have not stressed these two alternatives, because they are well known, and I don't need to repeat all tiny well known caveats all the time. Most physicists have not agreed with Bohr, and believe that there is a measurement problem eg. Dirac, Einstein, Bohm, Bell, Weinberg ... Thus measurement problem is standard terminology in physics, and includes the acknowledgment that Bohr's position is tenable.

This doesn't sound like an error. At best it is an omission. Given that the whole subject is vast, the author can decide what to include.

It is an error. This is the most important subject of foundations and interpretation of QM, and Ballentine is supposed to be a book about foundations. Ballentine explicitly attacks Copenhagen - the standard interpretation of QM - and makes calculational errors because of the lack of a cut. Ballentine's book is rotten in its foundations.

 

[bluecap]

As I said, that is not my technical quarrel with Ballentine. If you choose not to say the cut is not a problem, that is intellectually coherent, and I respect this view. I have not stressed these two alternatives, because they are well known, and I don't need to repeat all tiny well known caveats all the time. Most physicists have not agreed with Bohr, and believe that there is a measurement problem eg. Dirac, Einstein, Bohm, Bell, Weinberg ... Thus measurement problem is standard terminology in physics, and includes the acknowledgment that Bohr's position is tenable.
It is an error. This is the most important subject of foundations and interpretation of QM, and Ballentine is supposed to be a book about foundations. Ballentine explicitly attacks Copenhagen - the standard interpretation of QM - and makes calculational errors because of the lack of a cut. Ballentine's book is rotten in its foundations.

Why do you guys still discussing about the classical-quantum cut. Isn't it decoherence has already already removed the need for the cut.. so everything is quantum and outcome occurs after decoherence? Or maybe you are saying decoherence is not yet even proven yet? isn't it we could observe transition from quantum to classical in experiments. Or are such experiments open to debate and could be interpretated other ways disproving decoherence?

 

[atyy]

Why do you guys still discussing about the classical-quantum cut. Isn't it decoherence has already already removed the need for the cut.. so everything is quantum and outcome occurs after decoherence? Or maybe you are saying decoherence is not yet even proven yet? isn't it we could observe transition from quantum to classical in experiments. Or are such experiments open to debate and could be interpretated other ways disproving decoherence?

Decoherence does not remove the need for a cut, unless it is used in the context of BM or MWI. It is sometimes said that decoherence specifies where the cut is, but this is not true without additional assumptions eg. the predictability sieve (not sure what the current status of that idea is). As far as I know, Zurek has not removed the need for collapse, and if collapse is present, there is a classical-quantum cut.

https://arxiv.org/abs/quant-ph/0010011
Environment-Induced Decoherence and the Transition From Quantum to Classical
Juan Pablo Paz, Wojciech Hubert Zurek
"A sharp distinction between the classical core and the rest of the Hilbert space is possible only in idealized situations (or in an even more idealized “mathematical classical limit” ... In realistic situations, all that will be required is a clear contrast between the rates of the entropy production between the inside and the outside of the classical core. We shall refine such criteria in the discussion of the predictability sieve"

https://arxiv.org/abs/1412.5206
Quantum Darwinism, Classical Reality, and the Randomness of Quantum Jumps
Wojciech H. Zurek

 

[bluecap]

Decoherence does not remove the need for a cut, unless it is used in the context of BM or MWI. It is sometimes said that decoherence specifies where the cut is, but this is not true without additional assumptions eg. the predictability sieve (not sure what the current status of that idea is). As far as I know, Zurek has not removed the need for collapse, and if collapse is present, there is a classical-quantum cut.

https://arxiv.org/abs/quant-ph/0010011
Environment-Induced Decoherence and the Transition From Quantum to Classical
Juan Pablo Paz, Wojciech Hubert Zurek
"A sharp distinction between the classical core and the rest of the Hilbert space is possible only in idealized situations (or in an even more idealized “mathematical classical limit” ... In realistic situations, all that will be required is a clear contrast between the rates of the entropy production between the inside and the outside of the classical core. We shall refine such criteria in the discussion of the predictability sieve"

https://arxiv.org/abs/1412.5206
Quantum Darwinism, Classical Reality, and the Randomness of Quantum Jumps
Wojciech H. Zurek

Actually Zurek has removed the need for collapse. His "classical core" in the paper you quote are also quantum but Einselected (imagine smoke inside balloon, the smoke is quantum (shapeless), the balloon is just the Einselected states to give shape or classical reality). The following are the direct quotes from Zurek other papers that clearly stated Quantum Darwinism is quantum to the core.

https://arxiv.org/pdf/0903.5082.pdf

"The collection of ideas discussed here allows one to understand how “the classical” emerges from the quantum substrate staring from more basic assumptions than decoherence. We have bypassed a related question of why is our Universe quantum to the core. The nature of quantum state vectors is a part of this larger mystery. Our focus was not on what quantum states are, but on what they do. Our results encourage a view one might describe (with apologies to Bohr) as “complementary”. Thus, $|\psi\rangle$ is in part information (as, indeed, Bohr thought), but also the obvious quantum object to explain “existence”. We have seen how Quantum Darwinism accounts for the transition from quantum fragility (of information) to the effectively classical robustness. One can think of this transition as “It from bit” of John Wheeler [39]."

And in paper: https://arxiv.org/pdf/quant-ph/9805065.pdf

"The first resolution (championed by Bohr45) was to outlaw “by fiat" the use of quantum theory for the objects which were classical. This Copenhagen Interpretation (CI) had several flaws: It would have forced quantum theory to depend on classical physics for its very existence. It would have also meant that neither quantum nor classical theory were universal. Moreover, the boundary between them was never clearly delineated (and, according to Bohr, had to be “movable” depending on the whims of the observer). Last not least, with the collapse looming on the quantum-classical border, there was little chance for a seemless extension." ...

and in the Conclusion section:

"What we have described above is a fairly complete sketch of the physics involved in the transition from quantum to classical. Whether one would now claim that the emerging picture fits better Bohr’s “Copenhagen” framework or Everett’s “Many Worlds” interpretation seems to be a semantic rather than a substantial issue. To begin with, decoherence was not a part of either of these interpretations. Thus, what we have presented here is clearly beyond either CI or MWI."
"The existential interpretation owes Bohr the central question which was always implicit in the early discussions. This question — about the location of the quantum-classical border — is really very similar to questions about “existence.” We have posed and settled these questions operationally and, thus, provided a quantum justification for some of the original CI program."

So you see Quantum Darwinism or Existential Interpretation is quantum to the core. So add to it MWI or BM. Bohr original collapse based formalism is almost superseded already. Unless if all these Quantum Darwinism, MWI and BM are wrong? you mean there is still possibility Bohr objective collapse is real? But experiments have proven Decoherence exist. As Zurek said. Decoherence is beyond CI. Unless you want to use updated CI where decoherence is included? Also most of our physicists in the field when they talk about quantum, they assume the collapse is just subjective.. and not real.. so you have the problem that you can't describe what goes on between the double slit experiment's emitter and detector. Seriously.. does anyone still think objective collapse is possible? the classical-quantum cut only exist if objective collapse exists. But won't this violate relativity as objective collapse is instantaneous across the universe? maybe non-realism can still save it?

 

[atyy]

Actually Zurek has removed the need for collapse.

Zurek work within a version of MWI or the Everett interpretation. It remains unclear if his attempted solution of the measurement problem is correct.

 

[bluecap]

Zurek work within a version of MWI or the Everett interpretation. It remains unclear if his attempted solution of the measurement problem is correct.

Yup his stuff is Everettian.. but seriously how many percentage of physicists really believe in objective collapse? I think most just are into subjective collapse. You only need the classical-quantum cut if there is objective collapse as you said. For the rest of the physicists who are into subjective collapse. It's actually "shut up and calculate". Isn't it?

 

[atyy]

Yup his stuff is Everettian.. but seriously how many percentage of physicists really believe in objective collapse? I think most just are into subjective collapse. You only need the classical-quantum cut if there is objective collapse as you said. For the rest of the physicists who are into subjective collapse. It's actually "shut up and calculate". Isn't it?

Subjective collapse requires the cut.

 

[bluecap]

Subjective collapse requires the cut.

Oh I didn't know that because you didn't reply to my message number #48 where I asked you concerning it:

"If the wave function is not real, but the measurement apparatus and the results are real, why do quantum mechanics still need a reality/non-reality cut.. (by reality you mean classical and nonreality quantum? why didn't you use the word classica/quantum and instead use reality/non-reality?) Can you please give an example why the cut is still needed. We can treat all as classical, and the quantum result only a tool to produce probability... "

do you have a clear example? I gave the double slit example but your reply was:

"Yes, you can try to make everything classical, eg. Bohmian mechanics or MWI are famous approaches to making quantum mechanics classical. But if you only make everything notionally classical, without equations (ie. without Bohmian Mechanics or MWI), you have no way to interact with quantum mechanics and pull quantitative predictsion about reality (or the "classical" or "macroscopic" world). Basically, quantum mechanics does not tell us when a measurement occurs, and we need that input to pull a quantitative prediction out."

I thought you meant no cut was needed if wave function is just calculational tool. Thanks.

 

[bluecap]

Oh I didn't know that because you didn't reply to my message number #48 where I asked you concerning it:

"If the wave function is not real, but the measurement apparatus and the results are real, why do quantum mechanics still need a reality/non-reality cut.. (by reality you mean classical and nonreality quantum? why didn't you use the word classica/quantum and instead use reality/non-reality?) Can you please give an example why the cut is still needed. We can treat all as classical, and the quantum result only a tool to produce probability... "

do you have a clear example? I gave the double slit example but your reply was:

"Yes, you can try to make everything classical, eg. Bohmian mechanics or MWI are famous approaches to making quantum mechanics classical. But if you only make everything notionally classical, without equations (ie. without Bohmian Mechanics or MWI), you have no way to interact with quantum mechanics and pull quantitative predictsion about reality (or the "classical" or "macroscopic" world). Basically, quantum mechanics does not tell us when a measurement occurs, and we need that input to pull a quantitative prediction out."

I thought you meant no cut was needed if wave function is just calculational tool. Thanks.

the reason I thought no cut needed if wave function is just calculational tool was because you blank the entire process in between emission and detection.. so is cut even the right word.. but then maybe you needed a cut so you can guess where between emission and detection the objective collapse occurs.. lol.. right? now I'll give the floor back to you and Martin and Vanheez71 for more professional discussions.. thanks..

 

[bluecap]

the reason I thought no cut needed if wave function is just calculational tool was because you blank the entire process in between emission and detection.. so is cut even the right word.. but then maybe you needed a cut so you can guess where between emission and detection the objective collapse occurs.. lol.. right? now I'll give the floor back to you and Martin and Vanheez71 for more professional discussions.. thanks..

btw.. I mentioned all this cut thing because I was reading Bhobba messages this morning and it make sense when he said that:

"Well Von-Neumann died early, but Wigner was around when the flaw in Von-Neumann's reasoning was found. There is a place that's different - just after decoherence. When reading some early papers about it by Zeth he did 180% about face and realized you simply place the cut after decoherence - no consciousness required. He then believed in real collapse type interpretations such as GRW but that's a whole new story. That's the error Von-Neumann made - there is a place that's different and the logical place to put it. Its now a very backward (though still valid) interpretation."

Reference: https://www.physicsforums.com/threa...iousness-causes-collapse.902721/#post-5684686

Bhobba said the cut occurs after decoherence.. so since Copenhagen doesn't have decoherence and we do now... decoherence then simply says all is quantum and the cut occurs after decoherence.. so it's not really movable when you consider decoherence. So with this in mind.. the classical-quantum cut can be determined.. right after decoherence.. is there a problem with this view?

 

[atyy]

the reason I thought no cut needed if wave function is just calculational tool was because you blank the entire process in between emission and detection.. so is cut even the right word.. but then maybe you needed a cut so you can guess where between emission and detection the objective collapse occurs.. lol.. right? now I'll give the floor back to you and Martin and Vanheez71 for more professional discussions.. thanks..

the word "cut" is not always the best - but the basic idea is in Copenhagen, the unitary evolution of the state vector is insufficient, and we need outside input to say when to apply the Born rule. That outside input is the cut. The measurement problem asks whether that outside input can be described by the laws of physics.

The various options are something like:
Copenhagen needs a cut - usually, Copenhagen is agnostic about whether collapse is objective or subjective
Copenhagen V1: Who cares? The theory works great!
Copenhagen V2: It works great, but it shows that QM is incomplete
Copenhagen V2.1: Bohmian Mechanics, GRW etc - keep the normal view of reality, but remove the cut by introducing new physics
Copenhagen V2.2: Many Worlds Interpretation etc - keep the "normal" view of reality, but remove the cut by saying, eg. by saying that all outcomes occur

 

[atyy]

Bhobba said the cut occurs after decoherence.. so since Copenhagen doesn't have decoherence and we do now... decoherence then simply says all is quantum and the cut occurs after decoherence.. so it's not really movable when you consider decoherence. So with this in mind.. the classical-quantum cut can be determined.. right after decoherence.. is there a problem with this view?

No, of course Copenhagen has decoherence. Decoherence alone is common to all interpretations of QM.

 

[bluecap]

No, of course Copenhagen has decoherence. Decoherence alone is common to all interpretations of QM.

So what's the problem with this view that the collapse in Copenhagen occurs after Decoherence, then you can know the location of the cut.. after decoherence.. as bhobba seemed to be saying above.. so there is no need to figure out where is the classical-quantum cut..

 

[atyy]

So what's the problem with this view that the collapse in Copenhagen occurs after Decoherence, then you can know the location of the cut.. after decoherence.. as bhobba seemed to be saying above.. so there is no need to figure out where is the classical-quantum cut..

Decoherence is not exact, so it is unclear what one means by "after decoherence" without additional specification.

 

[bluecap]

Decoherence is not exact, so it is unclear what one means by "after decoherence" without additional specification.

That's right. For a week I kept wondering how the wave function decide to collapse after it is decohered.. my analogy (silly as it is) is like wave function is very sensitive and commit suicide (collapse) when any of its secret is known (or loss phase coherence). I'd continue to think but won't mention in this thread again.

So as not to be off topic. Demystifier idea of our particles like electron, quark as relativistic quasiparticles (like phonons) from condense matter physics is great with the real Bohmian particles as non-relativistic ontology.. actually I first heard of it early this year from his paper... and I'd like to ask Demystifier what is the speed limit of the real bohmian particles.. is it not limited by c? If you don't know. Hope Demystifer can answer this when he gets back. Thanks.

 

[vanhees71]

Well, there is no cut, at least nobody could empirically prove that there is anything that doesn't follow quantum theory but must be described classically. Classical physics is understood as an effective description of quantum physics for sufficiently coarse-grained observables of macroscopic objects, and decoherence is among the strongest mechanisms at work to let macroscopic objects occur as classical. Another hint is that the classical-quantum cut is artificial and can be often shifted from one part of the description of a system as applicable. E.g., in the standard textbook description of the Stern-Gerlach experiment the motion of the center of mass of the atom is usually done as classical mechanics, which is a legitimate approximation for the usual setup. Of course, you can as well describe the entire dynamics with the Pauli equation and solve the time-dependent Schrödinger equation (numerically), i.e., purely quantum. The results are, of course, compatible since in this case the center-of mass motion can be described classically, i.e., it is sufficient to study the motion of its expectation value using Ehrenfest's theorem in this case.

 

[bluecap]

Well, there is no cut, at least nobody could empirically prove that there is anything that doesn't follow quantum theory but must be described classically. Classical physics is understood as an effective description of quantum physics for sufficiently coarse-grained observables of macroscopic objects, and decoherence is among the strongest mechanisms at work to let macroscopic objects occur as classical. Another hint is that the classical-quantum cut is artificial and can be often shifted from one part of the description of a system as applicable. E.g., in the standard textbook description of the Stern-Gerlach experiment the motion of the center of mass of the atom is usually done as classical mechanics, which is a legitimate approximation for the usual setup. Of course, you can as well describe the entire dynamics with the Pauli equation and solve the time-dependent Schrödinger equation (numerically), i.e., purely quantum. The results are, of course, compatible since in this case the center-of mass motion can be described classically, i.e., it is sufficient to study the motion of its expectation value using Ehrenfest's theorem in this case.

The greatest puzzle causing many physicists sleepless nights is the so called Problem of Outcome..
For the Stern-Gerlach setup.. let's say you have an electron in superposition of spin up and spin down and you make a measurement (or decoherence with environment or whatever)..

1. Does the spin up or spin down result because the wave function collapses into spin up or spin down (Copenhagen)..
2. Do both spin up and spin down occurs as they entangle with the measuring device or environment (Many worlds)..
3. Do both spin up and spin down occurs but only in configuration space with the quantum potential pushing it to be either spin up or down (Bohmians)..

This is one of the world's greatest mysteries. Physicists who want to bypass the problem simply says mention the ensemble interpretation that says to simply ignore it and only tells you to do many identical measurements and after 100 trials.. tells you.. "see.. it's 50% spin up and 50% spin down"...

Why did we need to solve the problems of outcome.. because it can help solve other mysteries in physics such as the nature of spacetime and other stuff still banned in the mainstream.

 

[vanhees71]

You can just sleep fine by just taking Born's rule as an irreducible natural law, found by observation. There is no reason that a specific outcome of a spin-$z$ measurement occurs since the spin-$z$ component is indetermined due to your determination of the state, which is described by $\hat{\rho}=|\psi \rangle \langle \psi |$ with
$$|\psi \rangle=a |\sigma_z=+\hbar/2 \rangle + b |\sigma_z=-\hbar/2 \rangle, \quad |a|^2+|b|^2 = 1.$$
Then all you can say is that with probality $|a|^2$ you get $\sigma_z=+\hbar/2$ and with probability $|b|^2=1-|a|^2$ you get $\sigma_z=-\hbar/2$. There's just not more to say about the spin-$z$ component than that, and a single measurement can result in either of both values. You need to prepare a suffciently large ensemble to be able to say that you description is correct, i.e., you get with a certain given significance (which determines how large you must make your ensemble to reach this given significance level) the said probabilities as "frequencies of outcomes".

There is no mystery in this indeterminism. It's just an empirically found fact about how nature behaves, by looking accurately enough at small enough systems which we can prepare accurately enough to "see" quantum effects. We are just not used to this irreducibly probabilistic behavior and indeterminism of the observables' values via our everyday experience with macroscopic objects, which occur to "behave classically" since we don't look accurately enough (i.e., we course grain with our senses enough to "blur out" quantum effects).

Science is there to get rid of mysteries. Only science fiction and esoterical philosophers uses apparently weird findings of the sciences to create them.

 

[bluecap]

You can just sleep fine by just taking Born's rule as an irreducible natural law, found by observation. There is no reason that a specific outcome of a spin-$z$ measurement occurs since the spin-$z$ component is indetermined due to your determination of the state, which is described by $\hat{\rho}=|\psi \rangle \langle \psi |$ with
$$|\psi \rangle=a |\sigma_z=+\hbar/2 \rangle + b |\sigma_z=-\hbar/2 \rangle, \quad |a|^2+|b|^2 = 1.$$
Then all you can say is that with probality $|a|^2$ you get $\sigma_z=+\hbar/2$ and with probability $|b|^2=1-|a|^2$ you get $\sigma_z=-\hbar/2$. There's just not more to say about the spin-$z$ component than that, and a single measurement can result in either of both values. You need to prepare a suffciently large ensemble to be able to say that you description is correct, i.e., you get with a certain given significance (which determines how large you must make your ensemble to reach this given significance level) the said probabilities as "frequencies of outcomes".

This may work if the wave function is just calculational tool or aid. But is it not possible the wave function is really the particle itself and really there?
This is what drives all interpretations. Is there any empirical evidence or arguments the wave function can't be objective?

But other scientists like Bill Hobba still go for the ensemble interpretation because like GR no prior geometry.. he said nature may be like that and the mechanism may even be more bizarre than than simply Copenhagen, Bohmian Mechanics, Many Worlds.. sometimes I think he has a point.. and for those that don't want to delve endlessly or pointlessly into interpretations that may not be true.. then the best tactical retreat may be the Ensemble Interpretation.. but according to Lee Smolin.. it may be difficult to get into right quantum gravity without going back into quantum foundations and rethinking it. So I guess only quantum gravity folks need to worry about interpretations?

Sometimes I think I'd just be an ensemble interpretation proponent too so I don't have to think about all these.

There is no mystery in this indeterminism. It's just an empirically found fact about how nature behaves, by looking accurately enough at small enough systems which we can prepare accurately enough to "see" quantum effects. We are just not used to this irreducibly probabilistic behavior and indeterminism of the observables' values via our everyday experience with macroscopic objects, which occur to "behave classically" since we don't look accurately enough (i.e., we course grain with our senses enough to "blur out" quantum effects).

Science is there to get rid of mysteries. Only science fiction and esoterical philosophers uses apparently weird findings of the sciences to create them.

 

[vanhees71]

This may work if the wave function is just calculational tool or aid. But is it not possible the wave function is really the particle itself and really there?

All the math used in theoretical physics are just calculational tools to describe nature. It's just the language which by experience is the best suited to do so. I consider any attempt to identify the wave function with the particle itself is doomed to lead to contradictions with observations. That's why this idea (Schrödinger 1926) has been given up for more than 90 years now (Born 1926).

It is also important to keep in mind that not the wave function represents a pure state of a quantum system but the corresponding density matrix/statistical operator (or equivalently the ray in Hilbert space), but that's another subtlety.

 

[bluecap]

All the math used in theoretical physics are just calculational tools to describe nature. It's just the language which by experience is the best suited to do so. I consider any attempt to identify the wave function with the particle itself is doomed to lead to contradictions with observations. That's why this idea (Schrödinger 1926) has been given up for more than 90 years now (Born 1926).

It is also important to keep in mind that not the wave function represents a pure state of a quantum system but the corresponding density matrix/statistical operator (or equivalently the ray in Hilbert space), but that's another subtlety.

Oh I didn't mean Schrodinger Wave Function written in the position basis. I meant the state vectors (or whatever) used by Many Worlds and Bohmian where they are objective. If MWI and Bohmians can make them objective.. why can't Copenhagen make them objective?

Sorry for these basic questions (but I'd not ask more in this thread). I'll leave you experts to discuss stuff more professionally in this professional Insight thread.. thanks..

 

[martinbn]

As I said, that is not my technical quarrel with Ballentine. If you choose not to say the cut is not a problem, that is intellectually coherent, and I respect this view. I have not stressed these two alternatives, because they are well known, and I don't need to repeat all tiny well known caveats all the time. Most physicists have not agreed with Bohr, and believe that there is a measurement problem eg. Dirac, Einstein, Bohm, Bell, Weinberg ... Thus measurement problem is standard terminology in physics, and includes the acknowledgment that Bohr's position is tenable.

My question was why the classical/quantum cut is a problem. Now you are just making general statements about the measurement problem.

It is an error. This is the most important subject of foundations and interpretation of QM, and Ballentine is supposed to be a book about foundations. Ballentine explicitly attacks Copenhagen - the standard interpretation of QM - and makes calculational errors because of the lack of a cut. Ballentine's book is rotten in its foundations.

That is very strange. It's like complaining that in such and such book on algebraic geometry, where sets are used, there is no reference to Russel's paradox. If there are calculational mistakes you can point them out. But there is a difference between the foundations as the basics and the logical foundations. A book on the foundations of differential geometry will likely not talk about set theory and mathematical logic, and that is not not error.

 

[bluecap]

Oh I didn't mean Schrodinger Wave Function written in the position basis. I meant the state vectors (or whatever) used by Many Worlds and Bohmian where they are objective. If MWI and Bohmians can make them objective.. why can't Copenhagen make them objective?

Sorry for these basic questions (but I'd not ask more in this thread). I'll leave you experts to discuss stuff more professionally in this professional Insight thread.. thanks..

To avoid silliness like asking whether the ket vector being real in Many World. I found the following interesting classification so let us use it to finish our particular discussions with a question at bottom.

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

"Classification adopted by Einstein[edit]
An interpretation (i.e. a semantic explanation of the formal mathematics of quantum mechanics) can be characterized by its treatment of certain matters addressed by Einstein, such as:

• Realism
• Completeness
• Local realism
• Determinism

To explain these properties, we need to be more explicit about the kind of picture an interpretation provides. To that end we will regard an interpretation as a correspondence between the elements of the mathematical formalism M and the elements of an interpreting structure I, where:

• The mathematical formalism M consists of the Hilbert space machinery of ket-vectors, self-adjoint operators acting on the space of ket-vectors, unitary time dependence of the ket-vectors, and measurement operations. In this context a measurement operation is a transformation which turns a ket-vector into a probability distribution (for a formalization of this concept see quantum operations).
• The interpreting structure I includes states, transitions between states, measurement operations, and possibly information about spatial extension of these elements. A measurement operation refers to an operation which returns a value and might result in a system state change. Spatial information would be exhibited by states represented as functions on configuration space. The transitions may be non-deterministic or probabilistic or there may be infinitely many states.

The crucial aspect of an interpretation is whether the elements of I are regarded as physically real. Hence the bare instrumentalist view of quantum mechanics outlined in the previous section is not an interpretation at all, for it makes no claims about elements of physical reality.

The current usage of realism and completeness originated in the 1935 paper in which Einstein and others proposed the EPR paradox.[12] In that paper the authors proposed the concepts element of reality and the completeness of a physical theory. They characterised element of reality as a quantity whose value can be predicted with certainty before measuring or otherwise disturbing it, and defined a complete physical theory as one in which every element of physical reality is accounted for by the theory. In a semantic view of interpretation, an interpretation is complete if every element of the interpreting structure is present in the mathematics. Realism is also a property of each of the elements of the maths; an element is real if it corresponds to something in the interpreting structure. For example, in some interpretations of quantum mechanics (such as the many-worlds interpretation) the ket vector associated to the system state is said to correspond to an element of physical reality, while in other interpretations it is not."

My question Vanheez71 is.. if the ket vector associated to the system state is said to correspond to an element of physical reality in MWI.. why can't it in Copenhagen.. remember Copenhagen or even BM is just one world chosen in Many Worlds (in BM, particles being pushed around by quantum potential while in Copenhagen, the particle is conjured into existence (?))

 

[stevendaryl]

I still have no idea what are you talking about. How can one particle be associated with many world lines?

To other readers, does somebody else understand the question?

I'm confused, as well, about whether the issue is one particle or one TYPE of particle. String theory is an attempt to have a theory in which there is only one type of object (not a particle, I guess, since it's not a point-mass).

A theory with just one particle would be pretty bizarre. But it might be possible, if that one particle travels back and forth through time (if you take literally the idea that an anti particle is a particle moving back in time).

 

[jerromyjon]

(quoted from Wikipedia) Spatial information would be exhibited by states represented as functions on configuration space. The transitions may be non-deterministic or probabilistic or there may be infinitely many states.

This is my whole "worry" of quantum mechanics. How is classic physics "realized" from "infinitely many states"? It is quite simple to understand how one degree of freedom (spin up/down) can have only two possible outcomes in the real, physical world we know and trust but are there any constraints within quantum physics to only allow classically physical results of more complex systems or is that just "shut up and calculate" and the answers are always realistic once applied?

String theory is an attempt to have a theory in which there is only one type of object (not a particle, I guess, since it's not a point-mass).

I made that "point" and demystifier replied that a string can split into 2 strings...

 

[jerromyjon]

A theory with just one particle would be pretty bizarre. But it might be possible, if that one particle travels back and forth through time (if you take literally the idea that an anti particle is a particle moving back in time).

Any further info about this requirement would be appreciated! In my mind retro-causality could be avoided with complete knowledge of the variables, is that not the case?

 

[vanhees71]

Oh I didn't mean Schrodinger Wave Function written in the position basis. I meant the state vectors (or whatever) used by Many Worlds and Bohmian where they are objective. If MWI and Bohmians can make them objective.. why can't Copenhagen make them objective?

Sorry for these basic questions (but I'd not ask more in this thread). I'll leave you experts to discuss stuff more professionally in this professional Insight thread.. thanks..

Of course, all I said about the wave function is equally valid for the representation free formulation, which makes QT indeed much more clear.

Of course, the quantum state is objective also in the minimal interpretation. We haven't even discussed about this question in the entire thread yet. The state is operationally defined by an equivalence class of preparation procedures and as such independent of any subjective influence.

 

[stevendaryl]

So what's the problem with this view that the collapse in Copenhagen occurs after Decoherence, then you can know the location of the cut.. after decoherence.. as bhobba seemed to be saying above.. so there is no need to figure out where is the classical-quantum cut..

Maybe I'm misunderstanding something about decoherence, but in my superficial way of thinking about it, there isn't an objective, precise moment of decoherence. Somebody will please correct me if I'm wrong about this, but the way I think of it is that in any experiment, there is a division of the universe into:

1. The system being studied (say, a particle)
2. The apparatuses (apparati?) used to prepare and later measure the system.
3. The rest of the universe (the "environment")

The system being studied can only briefly be described using a wave-function (pure state). After it interacts with systems 2 and 3, its state becomes entangled with the states of other (generally macroscopic) systems. At the point, unless you are using a wave function for the entire universe, you are forced to describe the system of interest using mixed states (density matrices), where the degrees of freedom due to systems 2 and 3 are "traced over". A density matrix can be interpreted using classical probability: the system is in this or that state, we just don't know which, and the density matrix gives the various probabilities. After you've switched to a mixed state description, you're free to think that the wave function of the system of interest has "collapsed", and you just don't know what state it's collapsed into. (This is slightly different from the "collapse" interpretation which says that the act of measurement causes the collapse. There doesn't actually have to be a measurement or observation, as long as the system of interest gets entangled with the environment).

Decoherence is just the process by which one system becomes hopelessly entangled with an environment so that for practical purposes, we switch from a pure state description to a mixed state description. But the whole decoherence process as I understand it (which I very well may not) depends on our splitting the universe into a system of interest plus everything else. So there is no objective decoherence process.

 

[stevendaryl]

Of course, the quantum state is objective also in the minimal interpretation. We haven't even discussed about this question in the entire thread yet. The state is operationally defined by an equivalence class of preparation procedures and as such independent of any subjective influence.

I don't think that defining a state as an equivalence class of preparations procedures eliminates subjectivity. The notion of equivalence of preparation procedures requires a judgement of when two preparation procedures are the same. That seems subjective to me.

 

[bluecap]

Maybe I'm misunderstanding something about decoherence, but in my superficial way of thinking about it, there isn't an objective, precise moment of decoherence. Somebody will please correct me if I'm wrong about this, but the way I think of it is that in any experiment, there is a division of the universe into:

1. The system being studied (say, a particle)
2. The apparatuses (apparati?) used to prepare and later measure the system.
3. The rest of the universe (the "environment")

The system being studied can only briefly be described using a wave-function (pure state). After it interacts with systems 2 and 3, its state becomes entangled with the states of other (generally macroscopic) systems. At the point, unless you are using a wave function for the entire universe, you are forced to describe the system of interest using mixed states (density matrices), where the degrees of freedom due to systems 2 and 3 are "traced over". A density matrix can be interpreted using classical probability: the system is in this or that state, we just don't know which, and the density matrix gives the various probabilities. After you've switched to a mixed state description, you're free to think that the wave function of the system of interest has "collapsed", and you just don't know what state it's collapsed into. (This is slightly different from the "collapse" interpretation which says that the act of measurement causes the collapse. There doesn't actually have to be a measurement or observation, as long as the system of interest gets entangled with the environment).

Decoherence is just the process by which one system becomes hopelessly entangled with an environment so that for practical purposes, we switch from a pure state description to a mixed state description. But the whole decoherence process as I understand it (which I very well may not) depends on our splitting the universe into a system of interest plus everything else. So there is no objective decoherence process.

I think what Bill meant was that the cut occurred the moment the system lost phase coherence when it becomes hopelessly entangled with an environment, then the stressed wave function (or state vector) collapses into one value (in collapse interpretation). We use the density matrix only as tools to trace the environment even if the superposition is still theoretically (what we think) there.. so the objective decoherence process occurs when the coherence of the system become decoherent (or lost phase coherence)... which may occur before we do any tracing.. maybe Bill can clarify this as he is well verse in decoherence and the cut...

 

[atyy]

My question was why the classical/quantum cut is a problem. Now you are just making general statements about the measurement problem.

The classical/quantum cut is the very definition of the measurement problem. They are equivalent.

That is very strange. It's like complaining that in such and such book on algebraic geometry, where sets are used, there is no reference to Russel's paradox. If there are calculational mistakes you can point them out. But there is a difference between the foundations as the basics and the logical foundations. A book on the foundations of differential geometry will likely not talk about set theory and mathematical logic, and that is not not error.

No, it's complaining about a book on mathematics that claims that standard mathematics is wrong! Ballentine claims standard physics is wrong. Sorry, but standard physics is right, and Ballentine is rubbish.

 

[vanhees71]

I don't think that defining a state as an equivalence class of preparations procedures eliminates subjectivity. The notion of equivalence of preparation procedures requires a judgement of when two preparation procedures are the same. That seems subjective to me.

You can decide about the state only by measurement, and there's nothing subjective about it. Complete state determination can, of coarse, only be done on ensembles, never by just a single measurement due to the probabilistic nature of the quantum state, but what do you think is subjective about it? E.g., you can determine a system to be in a pure state by doing a simultaneous von Neumann filter measurement of a complete set of compatible observables. This is an objective procedure, but it can be realized in different ways using different measurement and filter devices. That's why I talked about "an equivalence class of preparation procedures".

 

[stevendaryl]

You can decide about the state only by measurement, and there's nothing subjective about it.

I don't agree that that's true. A measurement occurs when the state of the system of interest becomes correlated with a macroscopic variable that we can check ourselves. The subjectivity is the choice of which variable will count as a measurement.

 

[haushofer]

I made that "point" and demystifier replied that a string can split into 2 strings...

String theory is not only a theory of strings. Perturbative string theory is just an approximation of M-theory, which contains branes.