Particle vs Wave Interpretations of QM

[jeffn1]

I am curious whether undergrad physics students take courses on the philosophy of science. For example, does your syllabus include reading the seminal book, The Structure of Scientific Revolutions by Thomas Kuhn. Physics is sort of the cutting edge for these type of issues. Of course, Newton to Einstein was a classic example of a Kuhnian "paradigm shift". I might tend to argue that particle oriented quantum mechanics to field-based quantum field theory represents a similar shift. Thoughts?

 

[BillTre]

The Kuhn book The Structure of Scientific Revolutions came out in 1962. I read it and liked it in high school. I was a zoology major in college (no biology degree there) but took several courses on philosophy of science and medical history in the early 1970's. By then Kuhn's book was considered not cutting edge by the philosophy people where I was but he was not a philosopher originally, so... In the philosophy of science course they didn't even cover it much at all. In stead they had several philosophical responses to it dealing with inconsistencies in some of his definitions and some other philosophers vastly different refinements of it with out explaining his basic ideas. Not great.
Many scientists like his description of how science works and its easy to understand concepts to this day.

I don't think it is normal for any science students to take philosophy classes. There were none in mine.

 

[iste]

TL;DR: Physics and Philosophy of Knowledge

I am curious whether undergrad physics students take courses on the philosophy of science. For example, does your syllabus include reading the seminal book, The Structure of Scientific Revolutions by Thomas Kuhn. Physics is sort of the cutting edge for these type of issues. Of course, Newton to Einstein was a classic example of a Kuhnian "paradigm shift". I might tend to argue that particle oriented quantum mechanics to field-based quantum field theory represents a similar shift. Thoughts?

Yes, its definitely a Kuhnian shift as it introduces something that implies a new ontology or metaphysics that didn't exist before. I think another poster Bhobba said something like you cannot get non-relativistic quantum mechanics out of relativistic quantum field theory, only a relativistic field theory. This would be a another good argument that this is a Kuhnian shift as it shows that you cannot recover quantum mechanics from field theory without some extra implied ontology or metaphysics that wasn't originally there.

 

[jeffn1]

My impression is, for practicing physicists, the major difference between ordinary quantum physics and quantum field theory is the relativistic component. But, I think there is a very big difference in terms quantum interpretations. From a big picture perspective, things tend to make "more sense" if you view quantum behavior through the lens of quantum field theory ("there are no particles, just fields"). Art Hobson argues (at least how I understand him) that if you view quantum behavior from a QFT perspective, there simply is no need for all the different interpretations of quantum phenomenon (he mostly critcicizes consciousness-based theories and MWT). He even argues that the reason Feynman's said "nobody understands quantum mechanics" was because he was a "particle guy".

 

[Demystifier]

Hobson is wrong. QFT does not remove the need for quantum interpretations. And of course, Feynman was one of the developers of QFT, he even got the Nobel prize for that.

 

[Matterwave]

Art Hobson argues (at least how I understand him) that if you view quantum behavior from a QFT perspective, there simply is no need for all the different interpretations of quantum phenomenon (he mostly critcicizes consciousness-based theories and MWT). He even argues that the reason Feynman's said "nobody understands quantum mechanics" was because he was a "particle guy".

Feels just untrue to me. Do you have a concrete reference?

 

[jeffn1]

Feels just untrue to me. Do you have a concrete reference?

I'll try to find the exact quote.

In the Introduction, p. xiii of Fields and their Quanta, Hobson writes:

"Chapter 4 discusses wave-particle duality and the meaning of the quantum state. It explains the famous 2-slit experiment that Richard Feynman urged students not to think about because otherwise 'you will get "down the drain," into a blind alley from which nobody has yet escaped.' But Feynman's pessimism sprang from his obsession with an all-particles interpretation of the theory. The 2-slit experiment is natural and intuitive when analyzed in terms of universal quantum fields."

Sorry if my initial post was not exact (I think there are other similar quotes that I cannot find this minute). But I think the above quote captures the point pretty well. (I suspect Hobson was well aware of Feynman's work on QFT.)

 

[Demystifier]

The 2-slit experiment is natural and intuitive when analyzed in terms of universal quantum fields.

Except that it isn't. Of course, with a field picture in mind, it is natural and intuitive that it travels through both slits at once. However, then it is not natural and intuitive that at the screen we detect the field at one localized position only.

 

[Matterwave]

I'll try to find the exact quote.

In the Introduction, p. xiii of Fields and their Quanta, Hobson writes:

"Chapter 4 discusses wave-particle duality and the meaning of the quantum state. It explains the famous 2-slit experiment that Richard Feynman urged students not to think about because otherwise 'you will get "down the drain," into a blind alley from which nobody has yet escaped.' But Feynman's pessimism sprang from his obsession with an all-particles interpretation of the theory. The 2-slit experiment is natural and intuitive when analyzed in terms of universal quantum fields."

This is just a statement. Does Hobson actually show how "universal quantum fields" makes the 2-slit experiment natural and intuitive?

I hear sometimes that somehow QFT resolves the measurement problem or it resolves quantum weirdness but I have never actually seen a rigorous treatment that made things any more natural and intuitive for me. QFT has always just made things more complicated in general since the mathematical machinery is quite a bit more complicated. If such a rigorous treatment exists, I would certainly like to see it.

Maybe it's in chapter 4?

 

[jeffn1]

This is just a statement. Does Hobson actually show how "universal quantum fields" makes the 2-slit experiment natural and intuitive?

I hear sometimes that somehow QFT resolves the measurement problem or it resolves quantum weirdness but I have never actually seen a rigorous treatment that made things any more natural and intuitive for me. QFT has always just made things more complicated in general since the mathematical machinery is quite a bit more complicated. If such a rigorous treatment exists, I would certainly like to see it.

Maybe it's in chapter 4?

I'll dig into the book more. (I'm reading it occasionally and slowly). I suspect it doesn't make the math easier. The comparison to Newtonian and relatavistic physics may be apt here too. In a typical case on earth classical Newtonian physics will get you the answer easier. So, where you are not dealing with a high energy system, QFT math just complicates matters, as I understand it. But, Newtonian physics gives you a false impression of (our best interpretation of) what is actually happening. Similarly, particle-based QM makes it difficult to "conceptually" understand what is going with the a quantum system's interaction with a detector ("collapse") and entanglement. At least that's my understanding of Hobson's argument. I had a similar impression (based on FAR less knowledge) when I first learned (conceptually) about Quantum Field Theory. I'll report back.

 

[Matterwave]

I'll dig into the book more. (I'm reading it occasionally and slowly). I suspect it doesn't make the math easier.

It's ok if the math is more difficult but is still well founded and the logical argument is clear. Let's see what he really says about the double slit experiment from the perspective of universal quantum fields. I wouldn't presume to say he is wrong without reading and (at least making a good faith effort to) understanding his argument.

Similarly, particle-based QM makes it difficult to "conceptually" understand what is going with the a quantum system's interaction with a detector ("collapse") and entanglement.

Conceptual understanding is subjective and depends on the person. For me, an extra variables interpretation like Bohmian mechanics (about as "particle-based QM" as it gets) is conceptually easy to understand (there's particles, and they are guided by the wave function). The thing that needs stomaching there is the gross non-locality (and depending on your philosophical lean, one extra dynamical equation in the form of the guidance equation).

On your second point about interaction with a detector and collapse. MWI simply posits a universal wave function that evolves deterministically for all time. This is also conceptually clear to me. No collapse, and "observations" are just manifestations of decoherence. What does still bother me about MWI is its explanation of probability -- but of course this could be due to my personal ignorance.

Put another way, I don't view MWI to have a problem with "why do dots appear?". I view MWI as having a problem with "why did those dots appear?".

 

[jeffn1]

I need to understand MWI better. To the extent it really requires universes to come into existence every time there is a quantum collapse, I would pass. How many gazillions of universes have to be created in my little office every time a photon hits a wall? Are these universes comparable to our universe? Is there a "big bang" every time there is a quantum collapse? How much energy would this require? But, at some point I will look for clarification about these issues. I have to guess I am missing something(s), but I have to understand it better to know if my current skepticism (to put it mildly) of MWI is warranted.

 

[sbrothy]

As you all know I like philosophy (end especially history) of physics more than most. But to be honest that's because I have difficulty understanding the math. Some philosophy papers are heavy on math but they are rare. If you want to study physics to really grasp the subject you're not gonna get around the math. I wish I had more discipline when I was young. The philosophy can come later, and in my opnion, be important in pointing out new avenues of exploration. But to be brutally honest, I'd advice you to study the hardcore "bracket" of the real deal.

 

[gentzen]

I'll dig into the book more. (I'm reading it occasionally and slowly). I suspect it doesn't make the math easier. ..., particle-based QM makes it difficult to "conceptually" understand what is going with the a quantum system's interaction with a detector ("collapse") and entanglement. At least that's my understanding of Hobson's argument. I had a similar impression (based on FAR less knowledge) when I first learned (conceptually) about Quantum Field Theory. I'll report back.

I sincerely hope you will learn what you hoped for, when you decided to start reading that book. At least you already know some QFT, so at least you have a fair chance of judging Hobson's arguments for yourself.

 

[PeterDonis]

To the extent it really requires universes to come into existence every time there is a quantum collapse

It doesn't. The term "many worlds" is really a misnomer. The MWI is just unitary evolution, all the time, nothing else, and unitary evolution can't create or destroy anything. There's just one wave function, and that's all there is. The term "worlds" comes from the wave function having multiple decoherent branches, but nowhere in the process is anything created (or destroyed).

 

[gentzen]

For me, an extra variables interpretation like Bohmian mechanics (about as "particle-based QM" as it gets) is conceptually easy to understand (there's particles, and they are guided by the wave function).

Is it really easy to understand? What do you think of the following quote

It is widespread to call any variables that are not functions of $\Psi$ “hidden variables”; in Bohmian mechanics, the configuration Q is a variable that is not a function of $\Psi$, so it is often called a hidden variable although the particle positions are not hidden at all in Bohmian mechanics, as they can be measured any time to any desired accuracy.

from Tumulka's book “Foundations of Quantum Mechanics”?
My personal opinion is that this is a mistake which originated with John Bell. Not in his early years, but only very late when he was already so famous that ... everybody forgot for a second that QM is still non-intuitive:

It is thus from the xs, rather than from $\Psi$, that in this theory we suppose 'observables' to be constructed. It is in terms of the xs that we would define a 'psycho-physical parallelism' - if we were pressed to go so far. Thus it would be appropriate to refer to the xs as 'exposed variables' and to $\Psi$ as a 'hidden variable'. It is ironic that the traditional terminology is the reverse of this.

Although $\Psi$ is a real field it does not show up immediately in the result of a single 'measurement,' but only in the statistics of many such results. It is the de Broglie-Bohm variable X that shows up immediately each time. That X rather than $\Psi$ is historically called a 'hidden' variable is a piece of historical silliness.

Why do I think that this is a mistake? First, this would be an assumption in addition to the assumption of the existence of the wavefunction and the particle trajectories. And second, this assumption does not fit well with what we actually experience. I think, a much better approach would be to first investigate what follows from the actual math of BM, and if then it should turn out that some additional assumptions are still missing for reproducing what we actually experience, try to connect those additional assumptions with similar assumptions in MWI and other interpretations.

 

[gentzen]

I need to understand MWI better. To the extent it really requires universes to come into existence every time there is a quantum collapse, I would pass. ... But, at some point I will look for clarification about these issues. I have to guess I am missing something(s), but I have to understand it better to know if my current skepticism (to put it mildly) of MWI is warranted.

Different MWI proponents interpret it in different ways, and have different motivations and goals. A perspective which is "easy to connect to BM" was shared by Sam Kuypers here:

I discussed with Sam Kuypers, who works with David Deutsch, ...

Sam said this “Before the measurement, there already exist uncountably infinitely many copies, in the same way that there are uncountably infinitely many points on a finite line. Suppose there are two possible outcomes of a measurement. Then one part of those already existing copies will differentiate in one way, while the remaining copies will differentiate in another way. Each of these two parts of the multiverse is still uncountably infinite, but each is smaller than the original whole—just as you can cut a line into two segments, each of which still contains uncountably infinitely many points, yet each has a smaller length than the original line.”

If you try to interpret BM in the way I suggested above

I think, a much better approach would be to first investigate what follows from the actual math of BM

then Sam Kuypers elaboration can be interpreted as "this interpretation" of BM, where the trajectories have been removed, and became worlds instead.
This is not as "stupid" as it may seem initially: If you just have a single trajectory and cannot repeat the experiment, then in what sense can you say that the quantum equilibrium hypethesis ("Born rule") is satisfied. But if you instead keep "all trajectories" and give them their "measure of existence" according to the "quantum equilibrium hypethesis"/"Born rule", then it suddenly makes perfect sense mathematically.

 

[PeterDonis]

A perspective which is "easy to connect to BM" was shared by Sam Kuypers here:

A reference to an actual published peer-reviewed paper would be much better than someone's comment on a blog post. Even well-known physicists will say a lot of things in a informal context that they know they would never get away with in a peer-reviewed paper.

 

[berkeman]

Thread is closed temporarily for Mentor review...

 

[Doc Al]

After Mentor review, the thread is reopened with the following caveat: As described in the rules, purely philosophical discussions are not suitable for PF. However, the discussion here has focused on the contrasts between particle vs wave interpretations of QM, which is OK in this subforum. The thread title has been revised accordingly.

 

[Matterwave]

Why do I think that this is a mistake? First, this would be an assumption in addition to the assumption of the existence of the wavefunction and the particle trajectories. And second, this assumption does not fit well with what we actually experience.

Hello~

I tried reading your post a few times, but I wasn't able to understand your objection in a very precise and clear way. When you say "this" is a mistake, and "this" would be an assumption. What exactly are you referring to with "this"?

I see that you bolded "any time" and "to any desired accuracy" in the earlier quote. Am I to understand that those two are your objections?

If so, could you clarify why you object to "any time"?

For "any desired accuracy" this is true of the standard QM formalism. Ignoring practical experimental considerations and just looking at the pure formalism, you may measure q to any desired accuracy. The Heisenberg uncertainty relations only come into effect when you consider q and p simultaneously.

I would like to understand the lack of conceptual clarity that you see. I see nothing wrong in any of the quotes you provided. I agree with both Tumulka and Bell that calling Bohmian mechanics a hidden variables theory is a quirk of history. And that the particle positions are in fact the observed quantities so they are not hidden in that sense.

 

[Demystifier]

Bohmian mechanics (BM) is a hidden variable theory in the following senses:
(1) It claims that something observable (the particle position) has a value even when it is not observed.
(2) In practice you never directly observe a position of a macroscopic particle such as electron. All you directly observe is an aggregate position of a macroscopic object, typically the position of the pointer of a macroscopic measuring apparatus.

 

[Matterwave]

Bohmian mechanics (BM) is a hidden variable theory in the following senses:
(1) It claims that something observable (the particle position) has a value even when it is not observed.

So the moon is indeed there even when I'm not looking at it.

 

[PeroK]

So the moon is indeed there even when I'm not looking at it.

The idea that things do not exist if not observed is one of the greatest misunderstandings of QM. The Moon exists continuously, but technically the position of each of its constituent particles is not well-defined, in a classical sense. Nevertheless, the wave function associated with the Moon's constituent particles evolves according to the relevant Hamiltonian. That unitary evolution happens whether you observe the Moon or not.

The uncertainty of where you will detect the Moon if you observe it is negligible. Even leaving aside that the Moon is continually interacting with the rest of the Solar system, there is no practical doubt that the Moon is there, precisely enough where the laws of gravity predict it to be.

The electron in a hydrogen atom exists, even if you do not measure its position. The difference is that here the uncertainty is significant. That the electron has no well-defined position is important.

There is no workable analogy between the orbital of an electron in an atom and the Moon's orbit around the Earth. It would make no sense to impose a classical orbit on the electron. Likewise, it makes no sense to impose an atom-like orbital model on the Moon's classical orbit.

Even if the only justification were the law of large numbers, you cannot imagine that the Moon is a simple QM system where it makes no sense to talk about its classical trajectory.

QM does not override what we already know and observe about the macroscopic world. Instead, classical physics has to be accommodated by invoking the law of large numbers, decoherence etc.

 

[Matterwave]

Yeah I was just making a tongue in cheek reference.

 

[Demystifier]

The idea that things do not exist if not observed is one of the greatest misunderstandings of QM. The Moon exists continuously, but technically the position of each of its constituent particles is not well-defined, in a classical sense. Nevertheless, the wave function associated with the Moon's constituent particles evolves according to the relevant Hamiltonian. That unitary evolution happens whether you observe the Moon or not.

Is the Moon itself the same thing as its wave function? If not, then what exactly the Moon is? Can the Moon itself be described by a mathematical object? How about the same questions for electron instead of the Moon?

It is precisely the absence of coherent answers to such questions why some physicists find the standard interpretation of QM unsatisfying.

 

[martinbn]

Is the Moon itself the same thing as its wave function? If not, then what exactly the Moon is? Can the Moon itself be described by a mathematical object? How about the same questions for electron instead of the Moon?

It is precisely the absence of coherent answers to such questions why some physicists find the standard interpretation of QM unsatisfying.

I think that asking these questuons is what brings the confusion.

 

[Demystifier]

I think that asking these questuons is what brings the confusion.

Yes, but that is exactly the point. Some of the most important scientific insights were inspired by questions that brought confusion.

 

[martinbn]

Yes, but that is exactly the point. Some of the most important scientific insights were inspired by questions that brought confusion.

But those questions were not meaningless like these.

 

[PeroK]

Is the Moon itself the same thing as its wave function? If not, then what exactly the Moon is? Can the Moon itself be described by a mathematical object? How about the same questions for electron instead of the Moon?

We've had similar discussions in the past. It depends on the way we use our intelligence to think about things. The Moon can be described in macroscopic terms. The same way that, say, the continent of Africa can be described. The fact that it's essentially impossible to describe these things in purely QM terms does not invalidate the macroscopic definitions. I see this as a form of abstraction that is essential to make any sense of the universe and the things in it.

In any case, the Moon is to an electron what an ant colony is to an ant; or, what the human species is to an individual human being. The larger things have emergent properties and characteristic that are not inherent in the constituent parts.

It is precisely the absence of coherent answers to such questions why some physicists find the standard interpretation of QM unsatisfying.

i don't find any interpretation of QM satisfactory. I see that, however, as a separate issue from trying to treat the Moon as a giant electron and imagining that it has no classical trajectory. And that in some sense it doesn't exist unless someone is looking at it.