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

[atyy]

Well, Peres's and Ballentine's books have something in common. If you read the fine details of the books, you can find erroneous statements. But if you make an appropriate blurring of the books, the fine errors cancel out and the books as a whole become great.

The problem is that the errors are fundamental, not incidental.

However, I do love Peres's book, although I dislike Ballentine's. In other words, I forgive Peres because he is so charming, at least in writing :)

 

[Demystifier]

The problem is that the errors are fundamental, not incidental.

Yes, but they are not visible after the blurring.

However, I do love Peres's book, although I dislike Ballentine's. In other words, I forgive Peres because he is so charming, at least in writing :)

I find Ballentine charming too.

 

[atyy]

I find Ballentine charming too.

But in a way, Ballentine is anti-Bohmian. If Ballentine were correct, there is no measurement problem, and Bohmian mechanics is pointless.

 

[martinbn]

The problem is that the errors are fundamental, not incidental.

You say errors, but I have the feeling that you mean opinions different than yours. Can you give examples?

However, I do love Peres's book, although I dislike Ballentine's. In other words, I forgive Peres because he is so charming, at least in writing :)

Have you read the whole book of Ballentine? May be you only dislike some parts.

 

[Demystifier]

But in a way, Ballentine is anti-Bohmian. If Ballentine were correct, there is no measurement problem, and Bohmian mechanics is pointless.

I strongly disagree, see Secs. 14.2 and 14.3. of his book. I would rather say that he is agnostic about Bohmian QM. He does not say that there is no measurement problem, but demonstrates that a lot can be understood without talking about it explicitly.

 

[atyy]

I strongly disagree, see Secs. 14.2 and 14.3. of his book. I would rather say that he is agnostic about Bohmian QM. He does not say that there is no measurement problem, but demonstrates that a lot can be understood without talking about it explicitly.

Yes, Ballentine himself is slightly sympathetic to Bohmian mechanics. But if the main point of Ballentine's book is correct, then Bohmian mechanics is pointless - vanhees71 has drawn the logical conclusion from Ballentine.

Thus in fact, it is Copenhagen - which Ballentine hates - that promotes Bohmian mechanics. As your Insights article explains, there is no reason for a Bohmian not to love Copenhagen.

 

[Demystifier]

Yes, Ballentine himself is slightly sympathetic to Bohmian mechanics. But if the main point of Ballentine's book is correct, then Bohmian mechanics is pointless - vanhees71 has drawn the logical conclusion from Ballentine.

Thus in fact, it is Copenhagen - which Ballentine hates - that promotes Bohmian mechanics. As your Insights article explains, there is no reason for a Bohmian not to love Copenhagen.

You really like to push your conclusions to the extreme.
Your style reminds me of the great philosopher of science, Feyerabend.

 

[bahamagreen]

I still don't understand what do you mean by "one particle". That the whole universe contains only one particle? That's excluded.
Concerning the number of particle types, I cannot exclude any possibility.

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?

 

[Greg Bernhardt]

Interesting thoughts Demystifier, thanks!

 

[PeterDonis]

The default hypothesis is that it persists, but a hypothesis that it doesn't is also legitimate

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.

 

[bluecap]

vanhees71 does not accept the classical-quantum cut, which even Peres does.

Peres is an excellent book, but it is not quantum orthodoxy. Although he hides it very well, ultimately his flawed sympathy with Ballentine shows itself in his lack of a clear statement of the measurement problem, and statements about fuzzy Wigner functions that try to avoid the measurement problem. Basically, unless a book about foundations talks about the measurement problem, it is useless as a book about foundations. The measurement problem is the most important problem in the foundations of quantum mechanics.

When we treat the wave function as objective.. we have problems with the consequence of real collapse (a varient of Copenhagen) or all the terms existing (MWI) or all terms existing but they are not real and only one particle being pushed around (Bohmian).. all of these are greatly disturbing. So why don't we just treat the wave function as just for calculation purposes and all of us happy.. I mean.. does this eliminate the classical-quantum cut by making all classical.. how does treating the wave function as calculational tool only affect the need or requirement of the classical-quantum cut?

 

[atyy]

When we treat the wave function as objective.. we have problems with the consequence of real collapse (a varient of Copenhagen) or all the terms existing (MWI) or all terms existing but they are not real and only one particle being pushed around (Bohmian).. all of these are greatly disturbing. So why don't we just treat the wave function as just for calculation purposes and all of us happy.. I mean.. does this eliminate the classical-quantum cut by making all classical.. how does treating the wave function as calculational tool only affect the need or requirement of the classical-quantum cut?

In all versions of Copenhagen, the classical/quantum cut does exists whether the wave function is real or calculational. If the wave function is not real, but the measurement apparatus and the results are real, quantum mechanics needs a reality/non-reality cut. The unitary evolution of the non-real wave function does not say what is measured, nor when a measurement occurs - it does not say when reality pops out of non-reality. There is a cut somewhere - different people call it by different names.

 

[bluecap]

In all versions of Copenhagen, the classical/quantum cut does exists whether the wave function is real or calculational. If the wave function is not real, but the measurement apparatus and the results are real, quantum mechanics needs a reality/non-reality cut. The unitary evolution of the non-real wave function does not say what is measured, nor when a measurement occurs - it does not say when reality pops out of non-reality. There is a cut somewhere - different people call it by different names.

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...

 

[bluecap]

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...

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..

 

[Nugatory]

but the cost is being unable to determine the reality/non-reality cut in the sense we don't know what happens between emission and detection?

Yes, you're pretty much there. Now, if you could just manage to be happy about paying that cost...

 

[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?