Nobody understands quantum physics?

[LittleSchwinger]

Just saying. It is nice to try to defend Bohr, because if you are able to understand what he was trying to say, then you see that he had a very good understanding of how nature works. But to imply that understanding Bohr would in any way be similar to understanding Feynman, that is frankly ridiculous.

I'm not sure how to respond to this. Certainly Feynman was a clearer writer than Bohr. I wasn't even "defending" Bohr. Just confirming that in this case Bohr meant what vanhees71 guessed at, since he states it pretty plainly in the relevant essay and that in general among most authors it seems to me that when you read their essays in context it's fairly clear what they meant.

Neither an entire "defense of Bohr" or a detailed comparison of Bohr and Feynman as authors was intended.

 

[Paul Colby]

This is also the problem of CH - you need a "macroscopic/classic" context to even define QM as it stands.

This IMO is the heart of the problem. To observe the microscopic, one needs a microscope. One must build a macroscopic device for the measurement that is of necessity coupled to and a definite part of the system being observed. The wonder of QM is a formalism that allows one to abstract this macroscopic part of the system away. Want to measure the x-component of spin? You need to build a suitably prepared system along with the x-component spin measurement device. Want to measure the z-component? Well, build a z-component measurement system. It's a whole different system.

BTW, is no sense do I see this as a problem with CH.

 

[Couchyam]

The main conceptual hurdle posed by quantum mechanics seems to be that it imposes a hard limit on the nature of measurement. We know that wave functions probably exist, but physically there’s no experimental way to probe or dissect the process of how wave function collapse occurs, or even to verify that the dynamics of wave functions that don’t collapse intermittently (i.e. the wave functions we never ever ever see) are anything like the dynamics of wave functions that do produce an observable signature. It’s incredibly weird that the ‘unitary’ dynamics of quantum mechanics plays out over a seemingly very non-unitary background statistical ensemble of many tiny rapidly and repeatedly collapsing wave functions that probably define our perception of classical time evolution. The one approach to examining this (the classical-quantum divide) that comes to mind, short of a double slit apparatus for very small black holes, would be to try to iteratively extend EPR style experiments to include more and possibly more complex degrees of freedom. But that would be expensive and laborious…

 

[Fra]

Agree with all except last paragraph at least in the context of unification.

This IMO is the heart of the problem. To observe the microscopic, one needs a microscope. One must build a macroscopic device for the measurement that is of necessity coupled to and a definite part of the system being observed. The wonder of QM is a formalism that allows one to abstract this macroscopic part of the system away. Want to measure the x-component of spin? You need to build a suitably prepared system along with the x-component spin measurement device. Want to measure the z-component? Well, build a z-component measurement system. It's a whole different system.

BTW, is no sense do I see this as a problem with CH.

This is IMO the precise problematic way we mix up and hide and as you say do away with physics background conditional complexions in a mathematical structure, that we soon forget has physical correpondence and thus potentially evolving. And from that point on the "effective math" some of us take for "proven structure" may misguide us forward

I feel that on this point Bohr did not mix it up like some seduced by its math, this is what i meant with honest. Note that im not saying here we dont need math that is obvious.

/Fredrik

 

[vanhees71]

Nice post. It probably just comes down to what styles of writing fit with one personally. One of my personal favourite essays on quantum theory is Schwinger's at the start of his "Symbolism of Atomic Measurement" book.

Yes, this chapter (and the entire book) is a masterpiece. It's utmost clear and no mysteries. It's clearly science and not philosophy ;-))!

 

[vanhees71]

This IMO is the heart of the problem. To observe the microscopic, one needs a microscope. One must build a macroscopic device for the measurement that is of necessity coupled to and a definite part of the system being observed. The wonder of QM is a formalism that allows one to abstract this macroscopic part of the system away. Want to measure the x-component of spin? You need to build a suitably prepared system along with the x-component spin measurement device. Want to measure the z-component? Well, build a z-component measurement system. It's a whole different system.

BTW, is no sense do I see this as a problem with CH.

You just need, e.g., a Penning trap with its magnetic field to decide which spin component (or rather which component of the magnetic moment $\vec{\mu}$ you want to measure. You can do that with an amazing accuracy. There's nothing mystic with this but very well understood (even analytically solvable!).

 

[vanhees71]

The main conceptual hurdle posed by quantum mechanics seems to be that it imposes a hard limit on the nature of measurement.

There's no limit on measurement. I don't know, where this fairy tale comes from. It's often written in popular-science textbooks, but it's wrong.

What quantum theory tells us is that it is impossible to prepare a quantum system such that all observable take determined values. A state of "complete knowledge" is a pure state, and it's uniquely determined, when preparing the system such that a complete set of compatible observables take determined values. Usually observables which are not compatible to this complete set then to not take determined values.

We know that wave functions probably exist, but physically there’s no experimental way to probe or dissect the process of how wave function collapse occurs, or even to verify that the dynamics of wave functions that don’t collapse intermittently (i.e. the wave functions we never ever ever see) are anything like the dynamics of wave functions that do produce an observable signature. It’s incredibly weird that the ‘unitary’ dynamics of quantum mechanics plays out over a seemingly very non-unitary background statistical ensemble of many tiny rapidly and repeatedly collapsing wave functions that probably define our perception of classical time evolution. The one approach to examining this (the classical-quantum divide) that comes to mind, short of a double slit apparatus for very small black holes, would be to try to iteratively extend EPR style experiments to include more and possibly more complex degrees of freedom. But that would be expensive and laborious…

At the present stage of our knowledge, we cannot say whether there is a collapse of the quantum state that goes beyond standard QT or not. For sure it's not the hand-waving addition to the well-defined formalism of QT one often reads in textbooks promoting some flavors of the Copenhagen interpretation, which include a collapse postulate. I've never found any necessity to assume a collapse to apply QT to the description of real-world experiments.

Further there's no hint at a "classical-quantum divide" aka "Heisenberg cut". Today ever larger systems have been used to verify "quantum effects". E.g., the ~10kg mirrors of the LIGO experiment show quantum behavior.

https://arxiv.org/abs/2102.12665
https://doi.org/10.1126/science.abh2634

 

[Fra]

There's nothing mystic with this but very well understood (even analytically solvable!).

I agree, but it's well understood and solid thanks to that it "connects one classically specified set of conditions to another". Fortunately this holds for what is dicussed here.

(But for a more complete reflection: when the macroscopic conditions are in motion, due ot considering gravity/unifications it seems to me it's less "well understood" as the clarity rests on the fixed, non-dynamical context. But this is beyond a B-level discussion of course)

Edit: I don't think "classicaly specific set of conditions" means that someone suggests "classical mechanics" is correct(or better than QM), it's just a metaphor for a set of conditions nthat we have solid confidence in, on par with how we did think of things in classical mechanics. So this does not mean we still don't have decoherence explanations, I don't think there is a contradiction.

/Fredrik

 

[Lord Jestocost]

The problem with Bohr is that he is so unclear in his writing that it invites such "philosophing" about "what might the author have wanted to say", and that's why QT till today is often displayed as something mystic.

In order to understand Bohr, it maybe could be helpful to delve into the philosophy of radical constructivism (https://en.wikipedia.org/wiki/Radical_constructivism).

In “Farewell to Objectivity” (Systems Research, 13(3), 279–286, 1996. 187), von Glasersfeld remarks:

“The conceptual revolution that has shaken the 20th century is more profound than the one initiated by Copernicus, who dislocated the human being from the cherished position at the hub of the universe. But even if mankind was relegated to an insignificant minor planet, it could still maintain the belief that it represented the crowning achievement of God’s creation and that the human mind towered over everything else because it was able to perceive and understand God’s work, at least in its great lines. The 20th century has shown this belief to be illusory. Whatever the stuff is that we call knowledge, it can no longer be considered a picture or representation of an experiencer-independent world. Heinz von Foerster has said this with consummate elegance and precision: ‘Objectivity is the delusion that observations could be made without an observer.’” [bold by LJ]

In “Towards a radical constructivist understanding of science” (Foundations of Science 6, 1–30 (2001), Riegler writes:

“Radical Constructivism (RC) is the insight that we cannot transcend the horizon of our experiences. Experiences are all we can work with; out of experiences we construct our world. Thus, there are no mind-independent entities on which our cognition is based. This does not imply that Radical Constructivists deny the existence of such an objective world populated by mind-independent entities, the reality. Neither do they assert its existence. RC is agnostic.“

I think that Bohr held an epistemological position that is compatible with radical constructivism, i.e., that any mind-independent reality is inaccessible.

 

[Fra]

In order to understand Bohr, it maybe could be helpful to delve into the philosophy of radical constructivism (https://en.wikipedia.org/wiki/Radical_constructivism).

In “Farewell to Objectivity” (Systems Research, 13(3), 279–286, 1996. 187), von Glasersfeld remarks:

“The conceptual revolution that has shaken the 20th century is more profound than the one initiated by Copernicus, who dislocated the human being from the cherished position at the hub of the universe. But even if mankind was relegated to an insignificant minor planet, it could still maintain the belief that it represented the crowning achievement of God’s creation and that the human mind towered over everything else because it was able to perceive and understand God’s work, at least in its great lines. The 20th century has shown this belief to be illusory. Whatever the stuff is that we call knowledge, it can no longer be considered a picture or representation of an experiencer-independent world. Heinz von Foerster has said this with consummate elegance and precision: ‘Objectivity is the delusion that observations could be made without an observer.’” [bold by LJ]

In “Towards a radical constructivist understanding of science” (Foundations of Science 6, 1–30 (2001), Riegler writes:

“Radical Constructivism (RC) is the insight that we cannot transcend the horizon of our experiences. Experiences are all we can work with; out of experiences we construct our world. Thus, there are no mind-independent entities on which our cognition is based. This does not imply that Radical Constructivists deny the existence of such an objective world populated by mind-independent entities, the reality. Neither do they assert its existence. RC is agnostic.“

I think that Bohr held an epistemological position that is compatible with radical constructivism, i.e., that any mind-independent reality is inaccessible.

I agree with one caveat: no matter what words used by tradition, the word "mind" is bad because it makes some people think that this litteraly has to do with "human observer". My impression is that this is not what Bohr ever meant. Instead Bohr thought that the "classical background" together, makes up the "observer". I think Heisenberg had a different angle to it, I think Bohr was more clear.

/Fredrik

 

[LittleSchwinger]

Yes, this chapter (and the entire book) is a masterpiece. It's utmost clear and no mysteries. It's clearly science and not philosophy ;-))!

Another nice one come to think of it is Kemble's textbook. It actually came out in 1937, but there were very few copies until its Dover reprint in 1958. He has a very good conceptual exposition of quantum theory, focusing on the fact that one cannot prepare a system so all quantities take definite values.

 

[lodbrok]

The mathematics is well understood by many. The physics, not so much. I think it is still relevant to say "nobody understands QM" because 99% of those who think they understand it, don't understand the physics, their abilities to wield the mathematics notwithstanding.

No other theory, attracts so much argument about interpretations of the meaning of the mathematics as QT, without any way to distinguish between interpretations.

 

[Paul Colby]

because 99% of those who think they understand it, don't understand the physics, their abilities to wield the mathematics notwithstanding.

I think 97% might disagree but I wouldn't want to speak for them.

 

[Demystifier]

The primary purpose of a theory is to make testable predictions, and it does that extremely well.
Comprehensibility is a secondary function.

Yes. And the primary purpose of economy is to make money, making goods is its secondary purpose. And the primary purpose of education is to pass exams, getting knowledge is its secondary purpose. And the primary purpose of science is to publish scientific papers, creating new knowledge is its secondary purpose. It is quite common in this society that the method of verification (of achievement of the original abstract purpose) has turned into a primary purpose, just because it's more concrete and measurable.

What's the purpose, for example, of predicting the probability of Higgs decay?

 

[Demystifier]

The mathematics is well understood by many. The physics, not so much. I think it is still relevant to say "nobody understands QM" because 99% of those who think they understand it, don't understand the physics, their abilities to wield the mathematics notwithstanding.

No other theory, attracts so much argument about interpretations of the meaning of the mathematics as QT, without any way to distinguish between interpretations.

Mathematics also has its deep conceptual problems that are not understood by many. How many people understand Godel theorems, or continuity hypothesis, or Banach-Tarski paradox, for instance?

 

[CoolMint]

This IMO is the heart of the problem. To observe the microscopic, one needs a microscope. One must build a macroscopic device for the measurement that is of necessity coupled to and a definite part of the system being observed. The wonder of QM is a formalism that allows one to abstract this macroscopic part of the system away. Want to measure the x-component of spin? You need to build a suitably prepared system along with the x-component spin measurement device. Want to measure the z-component? Well, build a z-component measurement system. It's a whole different system.

BTW, is no sense do I see this as a problem with CH.

The “answer to the question that does not answer the question” is emergence - properties of the macroscopic system (such as “phases of matter”) exist that are not properties of the subatomic system.
They are "properties" of the field l(as solid particles have not been found yet in this part of the galaxy).
The field is not the chair, the chair is emergent from the field, a partucular configuration and this is the only consistent way to think about the world. We have something that manifests as something which it is not there in its constituents with defined properties. This brings us closer to raising the concept of observation to a level that is more fundamental than the concept of chairs.
Cogito ergo sum

 

[Vanadium 50]

Mathematics also has its deep conceptual problems that are not understood by many.

But how many people worry about this when balancing their checkbook?

 

[Jarvis323]

We should probably also define what it means to understand the math of QM. Does that mean knowing how to perform the common calculations? Or does it include understanding the foundations? Does it include understanding the infinite space of mathematical implications (e.g., theorems in the theory of quantum computing)?

Also, is understanding the theory without interpretation the same as understanding the math? Is there even an agreement about the mathematical structures QM should use (or which ones correspond to nature)? I still see people writing papers claiming that different variations of Hilbert space might have different implications for things like quantum gravity.

 

[DaveC426913]

Mathematics also has its deep conceptual problems that are not understood by many. How many people understand Godel theorems, or continuity hypothesis, or Banach-Tarski paradox, for instance?

Popular understanding follows in time.

There was once a time when Einsteinian relativity was brain-breaking stuff.
There was once a time when Newtonian motion was beyond the ken of most.

 

[PhDeezNutz]

I also believe Feynman said in one of his documentaries something to the effect of “science has no obligation to be psychologically pleasing”.

 

[Spinnor]

Listen just 1 minute, what does it mean when he said nobody understand quantum mechanics?
This sound like comedy

If you watch from 1:08:08 to 1:10:00 I think Feynman answers your question.

 

[Fra]

What's the purpose, for example, of predicting the probability of Higgs decay?

And ultimately...

What is agents purpose of predicting the future?
Does it have a choice?
Under what conditions can the agent learn?
Are bad learners and bad predicting agents likely to be ubiquitous in nature?

/Fredrik

 

[gentzen]

Mathematics also has its deep conceptual problems that are not understood by many. How many people understand Godel theorems, or continuity hypothesis, or Banach-Tarski paradox, for instance?

But it is known material that can be taught, and the info&math students typically understand it. Well, maybe except for Banach-Tarski paradox, where they only understand the math, but not how to "resolve" it.

What is also not so well understood (by the info&math students) is why Gerhard Gentzen (by proving that $\epsilon_0$ is well ordered) and later Kurt Gödel (by using his higher order dialectica interpretation) could still prove the consistency of Peano arithmetic. But I guess those are not deep conceptual problems, but simply less standard material.

The Banach-Tarski paradox might indeed be a conceptual problem, because how do you deal with the fact that your mathematical statements "there exists ..." don't mean what they are supposed to mean? You introduce measurable sets, and hope that this reduces the gap. But can you ever prove that your mathematical statements now mean what you believe that they say?

 

[DaveC426913]

This is beginning to sound a lot like philosophy of science.

 

[Demystifier]

What is also not so well understood (by the info&math students) is why Gerhard Gentzen (by proving that ϵ0 is well ordered)

Are you named after him?

 

[Demystifier]

But how many people worry about this when balancing their checkbook?

How many worry about quantum interpretations when using their electronic devices?

 

[Demystifier]

But it is known material that can be taught, and the info&math students typically understand it.

How many students understand Cohen's forcing showing that continuity hypothesis is undecidable in ZF(C)?

 

[Demystifier]

This is beginning to sound a lot like philosophy of science.

What do you mean - beginning?

 

[martinbn]

An anagram of Banach-Tarski is Banach-Tarski Banach-Tarski.

 

[Demystifier]

An anagram of Banach-Tarski is Banach-Tarski Banach-Tarski.

What does B. stand for in Benoit B. Mandelbrot? Benoit B. Mandelbrot.

 

[gentzen]

Are you named after him?

Of course. You mean why? Initially, it was just an accident related to my "fascination" with the impact of languages (and representation). Later, I first decided that I didn't care too much whether anybody would draw wrong conclusions based on that name. Then I decided that I liked his sequent calculus, and would like to better understand its deeper meaning and properties. When I created my email address based on that name, I actively engaged with some darker associations of that name.

 

[A. Neumaier]

there's no hint at a "classical-quantum divide" aka "Heisenberg cut". Today ever larger systems have been used to verify "quantum effects". E.g., the ~10kg mirrors of the LIGO experiment show quantum behavior.

You misunderstand the Heisenberg cut. It is not a physical cut but something done when one applies theory to interpret experiments. Then one has to choose which part of the universe to describe by quantum mechanics (using a QM model) and which part to treat classically (using classical computations to relate to experimental data). The Heisenberg cut is the dividing region, and it exists quite visibly in all applications of quantum mechanics.

 

[vanhees71]

I never understood this argument. E.g., using a silicon detector to detect photons doesn't mean to use a "classical device". It's based on the photoeffect and thus relies on (at least semiclassical) quantum theory to be understood.

 

[A. Neumaier]

I never understood this argument. E.g., using a silicon detector to detect photons doesn't mean to use a "classical device". It's based on the photoeffect and thus relies on (at least semiclassical) quantum theory to be understood.

Classical calculations and classical language is used everywhere on the experimental side, and many quantum mechanical models treat many variables classically. For example:

• In quantum information theory, the Hilbert space is always finite-dimensional, and all position information is classical.
• In entangled photon experiments, the paths are treated classically.
• In condensed matter physics, the lattice of atomic sites is treated as classical, and only small oscillations are treated quantum mechanically.
• In collision experiments at CERN, the paths of the colliding particle bunches are also treated classically.

The Heisenberg cut is always very visible. Not using it would amount to having to consider the whole universe quantum-mechanically for any quantum prediction!

 

[vanhees71]

This is just, because the classical approximations are accurate enough for these purposes (though the item concerning the Born-Oppenheimer approximation in solid-state physics is not always applicable). Nothing in this proves the necessity of a Heisenberg cut. It's hard to accept for Copenhagenianers, but there's no hint at the claimed dichotomy between a classical and a quantum world. The classical behavior of macroscopic systems is an emergent phenomenon!