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

[Demystifier]

Demystifier submitted a new PF Insights post

How I Stopped Worrying and Learned to Love Orthodox Quantum Mechanics

Continue reading the Original PF Insights Post.

 

[Dr. Courtney]

Nice article. Thanks!

 

[Physics Footnotes]

Thanks for the stimulating read. I too have come to the conclusion in recent years that anyone serious about the Foundations of Physics must thoroughly acquaint themselves with Bohmian Mechanics; not necessarily because it will turn out to be correct, but because it provides the most coherent and well fleshed-out alternative to the usual bare-bones view of QM.

I find your idea of taking QM as fundamental, while QFT as emergent, particularly interesting, except for one thing. You talk of non-relativistic particles, but I'm not sure how such things could exist, since we know spacetime, even locally, is not Galilean. Wouldn't the fundamental theory, in your assumed view, have to relativistic QM rather than non-relativistic? To put it a different way, how could we have particles existing in the world which respect Galilean spacetime, but not Minkowskian spacetime, when the latter is the one we know to actually be the case (or at least to be closer to the truth than the former)?

 

[atyy]

Are you worried about the chiral fermion problem, which seems to me the remaining problem in realizing the standard model using non-relativistic QM?

 

[Demystifier]

Are you worried about the chiral fermion problem, which seems to me the remaining problem in realizing the standard model using non-relativistic QM?

I consider it to be a technical problem, with some proposed solutions already existing. So I do not worry too much.

But I would still like to see your insight article about the chiral fermion problem. Any chance that you write it down one day?

 

[Demystifier]

since we know spacetime, even locally, is not Galilean

What we know is that spacetime does not appear Galilean at "large" distances (e.g. distances much larger than the Planck distance). How does it appear at very small distances, we don't know that.

 

[bahamagreen]

"What does have Bohmian trajectories are some more fundamental particles..."

Above, at, or below string level?
Multiples of multiple types?
Multiples of one type?
One each of multiple types?
One only of only one type? It would have to really get around, but how elegant.

 

[zonde]

Even if this mechanism is not exactly how Nature really works, the simple fact that such a mechanism is possible is sufficient to stop worrying and start to love instrumental QM as a useful tool that somehow emerges from a more fundamental mechanism, even if all the details of this mechanism are not (yet) known.

Well said.

 

[zonde]

You talk of non-relativistic particles, but I'm not sure how such things could exist, since we know spacetime, even locally, is not Galilean. Wouldn't the fundamental theory, in your assumed view, have to relativistic QM rather than non-relativistic? To put it a different way, how could we have particles existing in the world which respect Galilean spacetime, but not Minkowskian spacetime, when the latter is the one we know to actually be the case (or at least to be closer to the truth than the former)?

It is interesting that possibility of relativity principle not being fundamental is generally not considered.

 

[Demystifier]

"What does have Bohmian trajectories are some more fundamental particles..."

Above, at, or below string level?

There are 3 possibilities:
1) String theory is wrong. In this case the hypothetical fundamental distance $l_{\rm nr}$ at which Nature starts to look non-relativistic is not related to the string scale $l_{\rm string}$.
2) String theory is correct, but only as an effective theory. In this case $l_{\rm nr}\ll l_{\rm string}$.
3) String theory is correct as the fundamental theory of everything. In this case my theory is wrong and there is no such thing as $l_{\rm nr}$.

Multiples of multiple types?
Multiples of one type?
One each of multiple types?
One only of only one type? It would have to really get around, but how elegant.

Sorry, I don't understand the questions. Any hint?

 

[Demystifier]

It is interesting that possibility of relativity principle not being fundamental is generally not considered.

It is considered (but perhaps not enough). One rather famous work in that direction is Horava gravity
https://inspirehep.net/search?p=find+eprint+0901.3775
cited about 1500 times. Indeed, this work also significantly influenced may way of thinking, as can be seen in
https://inspirehep.net/search?p=find+eprint+0904.3412

 

[atyy]

There are 3 possibilities:
1) String theory is wrong. In this case the hypothetical fundamental distance $l_{\rm nr}$ at which Nature starts to look non-relativistic is not related to the string scale $l_{\rm string}$.
2) String theory is correct, but only as an effective theory. In this case $l_{\rm nr}\ll l_{\rm string}$.
3) String theory is correct as the fundamental theory of everything. In this case my theory is wrong and there is no such thing as $l_{\rm nr}$.

I think a variant of (2) is that string theory is correct only as an effective theory, but when string theory fails, there is no more spacetime, so $l_{\rm nr}$ does not exist, eg. gauge/gravity where the gauge theory is emergent from non-relativistic QM.

 

[bahamagreen]

Sorry, what I meant was have you excluded any of these possibilities for the more fundamental particle(s) ?

Multiple particles of multiple types?
Multiple particles of one type?
One particle each of multiple types?
One particle only of only one type?

 

[Demystifier]

I think a variant of (2) is that string theory is correct only as an effective theory, but when string theory fails, there is no more spacetime, so $l_{\rm nr}$ does not exist, eg. gauge/gravity where the gauge theory is emergent from non-relativistic QM.

Sure, in principle 2) has an infinite number of versions, including this one.

 

[Demystifier]

Sorry, what I meant was have you excluded any of these possibilities for the more fundamental particle(s) ?

Multiple particles of multiple types?
Multiple particles of one type?
One particle each of multiple types?
One particle only of only one type?

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.

 

[jerromyjon]

That the whole universe contains only one particle? That's excluded.

Isn't that strings in a nutshell?

 

[Demystifier]

Isn't that strings in a nutshell?

No. For instance, a string can split into two strings.

 

[vanhees71]

What I find not so convincing about the final conclusion of the article is the fact that obviously nature is not Newtonian but relativistic, as is shown also in the domain of physics, where classical approximations are valid. There seems to be really a limiting speed, $c$, and it seems to be universal no matter of which system is studied.

Of course, you have also in non-relativstic (condensed-matter) physics quasiparticles with relativistic dispersion relations and a lot of quite "exotic" features (Weyl fermions, magnetic monopoles, anyons ans what not has been discovered in the sense of quasiparticles but seem not to exist on a fundamental level), but these are only valid in the quasiparticle approximation and in fact describe collective low-energy excitations of the matter as a whole. At some point the non-relativistic approximation breaks down, and you have to use relativistic models.

 

[Demystifier]

There seems to be really a limiting speed, c, and it seems to be universal no matter of which system is studied.

It seems, but we don't know if this persists at even smaller distances than available by current experimental technology. The default hypothesis is that it persists, but a hypothesis that it doesn't is also legitimate and Bohmian mechanics is not the only motivation for such a "heretic" hypothesis. See e.g. Horava gravity.

 

[vanhees71]

Well, I don't see any merit of Bohmian mechanics to begin with. It just assumes unobservable "trajectories" in non-relativistic QT and otherwise predicts the same thing as QT in its minimal quantization. So this argument doesn't convince me too much. I've to check out what Horava gravity might be.

 

[Demystifier]

Well, I don't see any merit of Bohmian mechanics to begin with.

Of course. The insight article is about how I stopped worrying and learned to love orthodox QM. It does not say that everyone should follow the same path. When all think alike, then nobody thinks much.

 

[atyy]

Well, I don't see any merit of Bohmian mechanics to begin with. It just assumes unobservable "trajectories" in non-relativistic QT and otherwise predicts the same thing as QT in its minimal quantization. So this argument doesn't convince me too much. I've to check out what Horava gravity might be.

But you don't see the merit of orthodox QM either :P

 

[vanhees71]

No, orthodox QM includes the collapse, which is only making trouble without any other merit either. That's why I'm a minimal interpreter with great sympathies for it's simplified version called "shutup-and-calculate interpretation".

 

[Demystifier]

But you don't see the merit of orthodox QM either :P

In the article I defined orthodox QM as instrumental QM a la Peres (which does not involve collapse), and I think that @vanhees71 is OK with it.

 

[vanhees71]

Peres is among the best books on interpretational issues ever! A clear no-nonsense approach, which seems to me at least very close to the minimal interpretation.

 

[atyy]

In the article I defined orthodox QM as instrumental QM a la Peres (which does not involve collapse), and I think that @vanhees71 is OK with it.

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.

 

[vanhees71]

Well, what you don't like about Peres's book is precisely why I like it ;-)). As I said, it's a nice example for the "no-nonsense approach" to (quantum) physics.

 

[Demystifier]

Peres is an excellent book, but it is not quantum orthodoxy.

Why do you think that it is not orthodoxy? Just because it doesn't involve collapse? Why do you think that orthodoxy must involve collapse?

 

[atyy]

Why do you think that it is not orthodoxy? Just because it doesn't involve collapse? Why do you think that orthodoxy must involve collapse?

Not "collapse" - state reduction is fine - in fact state reduction is often synonymous with "collapse". Only some people misunderstand Copenhagen and believe that "collapse" is necessarily physical.

The flaw of Peres is that he fails to state the classical-quantum cut clearly. I believe he also does not include state reduction in his axioms.

 

[Demystifier]

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

I wouldn't say that Peres accepts a classical-quantum cut. What he accepts is something more like (abstract formalism)-(laboratory phenomena) cut. He says that quantum phenomena do not occur in Hilbert space, but in a laboratory. It would be akin to a statement that classical phenomena do not occur in phase space, but in a laboratory.

 

[vanhees71]

Not "collapse" - state reduction is fine - in fact state reduction is often synonymous with "collapse". Only some people misunderstand Copenhagen and believe that "collapse" is necessarily physical.

The flaw of Peres is that he fails to state the classical-quantum cut clearly. I believe he also does not include state reduction in his axioms.

In the last chapter he very clearly discusses measurements on the example of the Stern-Gerlach experiment (the "Drosophila" of quantum physicists ;-)), and it becomes very clear that his view on the "classicality of measurement apparati" to ensure an irreversible storage of the measurement result is seen in the sense of an emergent phenomenon through the usual coarse-graining argument of quantum statistics (he calls it "blurring").

As I said, for me this book has been a relief be cause it cleans up the QT-foundational discussion of all unnecessary philosophical complications.

 

[atyy]

In the last chapter he very clearly discusses measurements on the example of the Stern-Gerlach experiment (the "Drosophila" of quantum physicists ;-)), and it becomes very clear that his view on the "classicality of measurement apparati" to ensure an irreversible storage of the measurement result is seen in the sense of an emergent phenomenon through the usual coarse-graining argument of quantum statistics (he calls it "blurring").

As I said, for me this book has been a relief be cause it cleans up the QT-foundational discussion of all unnecessary philosophical complications.

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

 

[Demystifier]

The flaw of Peres is that he fails to state the classical-quantum cut clearly. I believe he also does not include state reduction in his axioms.

Why would he talk about classical-quantum cut clearly if he doesn't think that there is such a cut?

Concerning the axioms, he is developing a practical instrumental approach, not an axiomatic approach. This is like complaining that a handbook of civil engineering does not state axioms of stable building construction.

 

[atyy]

I wouldn't say that Peres accepts a classical-quantum cut. What he accepts is something more like (abstract formalism)-(laboratory phenomena) cut. He says that quantum phenomena do not occur in Hilbert space, but in a laboratory. It would be akin to a statement that classical phenomena do not occur in phase space, but in a laboratory.

The book is very good, because Peres for the most part accepts the classical quantum cut. But he cannot bring himself to articulate it explicitly. He accepts it implicitly when he talks about the need for a second classical measuring apparatus, if we treat one measuring apparatus as quantum. A book about foundations should be as explicit about axioms as possible, not hide the ones that he doesn't like in implicit statements. And because he dislikes the axiom, eventually he does make a misleading statement about blurring of the Wigner function. There may be a way to read it without being misled, but as you can see from vanhees71's post #31, Peres has managed to mislead an expert.

 

[Demystifier]

The book is very good, because Peres for the most part accepts the classical quantum cut. But he cannot bring himself to articulate it explicitly. He accepts it implicitly when he talks about the need for a second classical measuring apparatus, if we treat one measuring apparatus as quantum. A book about foundations should be as explicit about axioms as possible, not hide the ones that he doesn't like in implicit statements. And because he dislikes the axiom, eventually he does make a misleading statement about blurring of the Wigner function. There may be a way to read it without being misled, but as you can see from vanhees71's post, Peres has managed to mislead an expert.

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 become invisible and the books as a whole become great.

 

[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...