Some sins in physics didactics - comments

[rude man]

Well, you said it was a relic from the early years of 1905. Feynman taught the course in question at Caltech in the '60's.

I'm aware Einstein later changed his mind but Feynman certainly did not.

 

[ZapperZ]

Well, you said it was a relic from the early years of 1905. Feynman taught the course in question at Caltech in the '60's.

I'm aware Einstein later changed his mind but Feynman certainly did not.

I didn't say anything about a relic.

So how do you decide who to listen to? The one better looking and with less messy hair?

Zz.

 

[carllooper]

The idea that previous explanations for something are "relics" is typical of the belief that the past is no longer relevant - that contemporary theory is the only theory that should be entertained. As if theory as a whole should be whatever is currently fashionable - that anything older than this morning should be put out with the rubbish.

I've read criticisms of some theories (even on this forum) where the critique is literally no more than: "that's old fashioned".

Well, Einstein's Relativity Theory is old fashioned. It's more than a 100 years old.

The age of a theory has no bearing whatsoever on it's value. If a theory is wanting it won't be necessarily due to it's age. And there are plenty of freshly minted theories which could be framed as wanting.

Another critical angle is this notion of "correctness" or "truth value" - that the value of a theory is in terms of how correct or true it is.

No theory is correct. No theory is true.

Theories are particular ways of understanding the way in which nature works. Nature herself doesn't care. She behaves the way she behaves regardless of whatever theory we might develop. How we understand her behaviour is an entirely different thing - be it on a simple approximate level or in enormous detail. The value of a theory is to be found in what it might allow - on various levels - technology being the most powerful driver of theories (of the fashionable variety) but by no means the only driver.

The history of a theory is important in how a theory is to be understood. The genesis of Einstein's Theory of Relativity didn't just spring out of thin air. It has a context in which ideas such as the aether fed into such, and the Michelson-Morley experiment, and so on, each of which help to understand Einstein's theory and why it emerged at that time and why it is the way it is.

Getting in the way of this are myths about personal genius (Einstein as a genius). They distract from understanding why theories (fashionable or otherwise) are the way they are. When cleansed of all historical context they appear either silly or genius, neither of which are true.

C

 

[bhobba]

I know first hand in answering questions on this forum the many misconceptions people have because of popularisations and beginner texts.

Feynman was aware of it and, if I recall correctly, has a section devoted to it somewhere in his lectures. He laments you can't always tell the students the truth from the start, but doesn't know any other way of resolving the issue - they simply do not have the background for the complete story.

I see no reason we can't keep doing the same thing, but simply, like Feynman, have the occasional lecture explaining some of the stuff they are learning will need to be unlearned later, that's simply the way physics is, nothing much can be done about it, but just be aware that's the case.

That doesn't mean of course we shouldn't look at physics curriculum to ensure students need to unlearn as little as possible.

Thanks
Bill

 

[atyy]

No, of course one should never teach anything which may require unlearning later. That does not mean one should not teach old quantum theory.

 

[bhobba]

I'm aware Einstein later changed his mind but Feynman certainly did not.

I find that Feynman didn't understand the issues of relativistic mass a little difficult to fathom. What happens when you apply a force in the direction of motion? What mass do you use then? And such being the case how does that gell with the usual concept of mass being a scalar?

Thanks
Bill

 

[atyy]

I find that Feynman didn't understand the issues of relativistic mass a little difficult to fathom. What happens when you apply a force at right angles to the direction of motion? What mass do you use then? And such being the case how does that gell with the usual concept of mass being a scalar?

The way he did it gives the right results. He used the relativistic mass not in F=ma, but in F=dp/dt.

 

[vanhees71]

This article is suggesting that the photo-electric effect doesn't actually prove anything about the quantization of the electromagnetic field; the quantization of energy levels of matter is sufficient to explain it. So does ANYTHING prove the quantization of the E&M field? I guess not, because Feynman's "absorber theory" reformulates QED so that there are no additional degrees of freedom in the E&M field.

On the other hand, it seems strange to treat matter (fermions) completely different than gauge particles, when their physics is so similar.

Yes, as stressed in the article and many times in this discussion, e.g., the Planck Black Body radiation law proves the quantization of the electromagnetic field, because you need spontaneous emission to derive it from kinetics, as was found by Einstein already in 1917, but there he had to introduce spontaneous emission ad hoc, while in QFT it's derived from the bosonic nature of the em. field (symmetry under exchange of identical bosonic quanta). The analogue for fermions is Pauli blocking, which was introduced by Pauli (as the name correctly suggest) in an ad hoc way also before the discovery of modern quantum theory and is nowadays implied by the fermion many-body space (antisymmetry under exchange of identical fermionic quanta).

The Feynman-Wheeler absorber theory, to my knowledge, has never been put into a (semi-)consistent quantum theory, as was famously predicted by Pauli after listening to Feynman's talk at Princeton. It's a funny to read story in one of Feynman's autobiographical (story) books (I guess "Surely you are joking").

 

[bhobba]

No, of course one should never teach anything which may require unlearning later. That does not mean one should not teach old quantum theory.

Ok - then Feynman's QED - The Strange Story of Light And Matter needs to be banned eg its explanation of why light moves slower in glass is wrong:
https://www.physicsforums.com/threads/do-photons-move-slower-in-a-solid-medium.511177/

Should one expose beginning students to Zappers correct explanation and forget the intuitive incorrect one? Would beginning students even understand what Zapper said?

Like I said - Feynman was a teacher of some renown, and grappled with the issue. He decided students, correctly IMHO, need to be eased into the correct understanding.

Thanks
Bill

 

[atyy]

Ok - then Feynman's QED - The Strange Story of Light And Matter needs to be banned eg its explanation of why light moves slower in glass is wrong:
https://www.physicsforums.com/threads/do-photons-move-slower-in-a-solid-medium.511177/

Should one expose beginning students to Zappers correct explanation and forget the intuitive incorrect one? Would beginning students even understand what Zapper said?

Like I said - Feynman was a teacher of some renown, and grappled with the issue. He decided students, correctly IMHO, need to be eased into the correct understanding.

I'm not convinced Feynman's explanation was wrong. But yes, if it is wrong, we should not teach it. Of course there will be errors from time to time, but we should not teach things that are deliberately wrong. In this case, if Feynman is wrong, I'm pretty sure he made an unintended error.

 

[bhobba]

The way he did it gives the right results. He used the relativistic mass not in F=ma, but in F=dp/dt.

I am not sure that resolves the issue - but I would need to check my copy of the lectures.

Thanks
Bill

 

[bhobba]

I'm not convinced Feynman's explanation was wrong. But yes, if it is wrong, we should not teach it. Of course there will be errors from time to time, but we should not teach things that are deliberately wrong. In this case, if Feynman is wrong, I'm pretty sure he made an unintended error.

Ok - at least you are consistent about it.

Thanks
Bill

 

[bhobba]

The Feynman-Wheeler absorber theory, to my knowledge, has never been put into a (semi-)consistent quantum theory, as was famously predicted by Pauli after listening to Feynman's talk at Princeton. It's a funny to read story in one of Feynman's autobiographical (story) books (I guess "Surely you are joking").

That's my understanding as well.

But some claim Paul Davies fixed that issue:
http://journals.cambridge.org/action/displayAbstract?fromPage=online&aid=2103380

Thanks
Bill

 

[atyy]

BTW, the reason I don't know whether Feynman's explanation is wrong is that I don't think it is the one ZapperZ argues against. ZapperZ argues against the slowing down being due to the delay of absorption and re-emission by atoms. If I remember correctly, Feynman's argument involved superposition and a change in phase. Heuristically, this seems to be correct, since it is more or less an attempt to apply QED to a material. It also seems similar to ZapperZ's phonon explanation, since a phonon is a superposition of localized atomic wave functions, so perhaps the explanations are "Fourier transform" pairs of each other. Of course it can't be so simple, but this is why I don't think Feynman's argument is obviously wrong.

Feynman did make mistakes in his lectures. A famous one is an error in the application of Gauss's law. http://www.feynmanlectures.info/flp_errata.html (See the story right at the bottom)

 

[bhobba]

BTW, the reason I don't know whether Feynman's explanation is wrong is that I don't think it is the one ZapperZ argues against.

I jusr checked it.

It's in chapter 3. He explains it due to the extra time its takes to traverse the medium from scattering by the electrons. He doesn't assume its absorbed and re-emitted - but scattered in an unknown direction.

I think its better than the usual explanation of absorption and remission - but its not entirely correct either.

Thanks
Bill

 

[stevendaryl]

I'm not convinced Feynman's explanation was wrong. But yes, if it is wrong, we should not teach it. Of course there will be errors from time to time, but we should not teach things that are deliberately wrong. In this case, if Feynman is wrong, I'm pretty sure he made an unintended error.

I think it depends on how you teach it. If you teach something as a model, rather than as the "truth", then there is nothing wrong (in my opinion) with using models that are known to have limited applicability.

 

[atyy]

I think it depends on how you teach it. If you teach something as a model, rather than as the "truth", then there is nothing wrong (in my opinion) with using models that are known to have limited applicability.

Almost everything is a model with limited applicability, so this is not any real criterion.

 

[stevendaryl]

Almost everything is a model with limited applicability, so this is not any real criterion.

I'm just saying that I disagree with your rule that you should never teach something that you know is false. That's true with everything.

As far as what models should be taught, I think that it's kind of subjective. Some models are definitely dead ends--nothing learned from them is of any use in more advanced treatments (the phlogiston model might be an example). Other models teach concepts that get refined by later models, and it's a matter of opinion whether knowing the model is a hindrance or help in understanding better models.

 

[atyy]

I'm just saying that I disagree with your rule that you should never teach something that you know is false. That's true with everything.

As far as what models should be taught, I think that it's kind of subjective. Some models are definitely dead ends--nothing learned from them is of any use in more advanced treatments (the phlogiston model might be an example). Other models teach concepts that get refined by later models, and it's a matter of opinion whether knowing the model is a hindrance or help in understanding better models.

But if you read my comment in context, that is not what I said at all. For example, I argued that you should not teach things that are wrong in the sense that they are misleading. But I immediately said that did not mean the old quantum theory photon explanation of the photoelectric should not be taught. In fact, I said exactly what you are saying as a away to advocate teaching the old quantum theory explanation of the photoelectric effect.

 

[stevendaryl]

But if you read my comment in context, that is not what I said at all. For example, I argued that you should not teach things that are wrong in the sense that they are misleading. But I immediately said that did not mean the old quantum theory photon explanation of the photoelectric should not be taught. In fact, I said exactly what you are saying.

Okay, I misunderstood. But I wouldn't use the word "wrong" here, because every model is wrong, in some sense. Misleading is more relevant, if we can objectively say what it means to be misleading. I guess I would say that an explanation, based on one model, is misleading if it is contradicted (as opposed to tweaked/refined?) by more accurate models?

 

[atyy]

Okay, I misunderstood. But I wouldn't use the word "wrong" here, because every model is wrong, in some sense. Misleading is more relevant, if we can objectively say what it means to be misleading. I guess I would say that an explanation, based on one model, is misleading if it is contradicted (as opposed to tweaked/refined?) by more accurate models?

Yes, which is why my comment really had to be read in context. There you can see I argued for teaching two wrong models - the photoelectric effect and possibly Feynman's explanation of the slow speed of light in a medium - because they capture ways of thinking that are powerful, even by the standards of our current best theories. I argued both that the wrong models should be taught, and that they should not be taught in a way that anything had to be unlearnt later.

Also, one doesn't have to use the idea of "not being contradicted" as the idea of not being misleading. We still teach Newtonian physics, yet it is contradicted and not just tweaked by general relativity and quantum mechanics. But teaching Newtonian mechanics is usually not considered misleading.

What is misleading is to teach the photoelectric effect as "proving" the necessity of photons. That was vanhees71's point. I agree with that. However, I don't agree that one should not to teach it as very powerful picture, aspects of which are formalized in quantum field theory, and that is still an efficient way of deriving Planck's blackbody formula, the Fowler-Dubridge theory still used in modern papers like the one pointed out by ZapperZ, and its use in modern devices for detecting single photons.

In the same way, I don't agree that "wave-particle duality" is a myth or misleading, since it is formalized into the particle nature of the quantum mechanical Hilbert space and the Fock space of non-rigourous quantum field theory and the wave nature of the equation of motion in the Schroedinger and Heisenberg pictures.

 

[martinbn]

@atyy: Sorry for the off topic questions, but what do you mean by the particle nature of the quantum Hilbert space and the wave nature of the equations of motion in the Heisenberg picture?

 

[atyy]

@atyy: Sorry for the off topic questions, but what do you mean by the particle nature of the quantum Hilbert space and the wave nature of the equations of motion in the Heisenberg picture?

Let's work in QM. There we have the Schroedinger equation which is a "wave" equation. For 1 particle, the Hilbert space basis is some set of wave functions. For two particles, the Hilbert space basis is made from the tensor products of the 1 particle basis functions. So particles define the Hilbert space. The only difference to a classical particle is that a quantum particle does not have simultaneous position and momentum at all times. However, in the classical limit, we do recover the classical equation of motion for classical particles, justifying the term "particle" for the quantum object.

Non-rigourous QFT is the same, except we use a second quantized language and work in Fock space, and the number of particles is not necessarily conserved in relativistic theory.

The other way that wave-particle duality is formlized in QM are the commutation relations. Position is particle and momentum is wave, and they do not commute.

So rather than saying wave-particle duality is a myth, I would rather say wave-particle duality is a vague notion that is formalized deep in QM in several ways.

It is like the equivalence principle. It started vaguely, with some idea that it is only "locally" true, but we don't have a definition of "local" before we have the mathematical theory. After we have the full theory, we find that the equivalence principle can be formalized, and local means "first order derivative".

 

[martinbn]

This is still not clear to me. You have to keep in mind that I am not a physicist and need things said explicitly. Perhaps this is too far from the topic to discuss it here.

 

[atyy]

This is still not clear to me. You have to keep in mind that I am not a physicist and need things said explicitly. Perhaps this is too far from the topic to discuss it here.

How do we know how to describe the Hilbert space?

1 particle basis functions: ψ_m(x)

2 particle basis functions: ψ_m(x₁)ψ_n(x₂)

So we define the Hilbert space by using particles.

 

[martinbn]

And what are these functions?

 

[atyy]

And what are these functions?

Let's take the particle in an infinite well. These are energy eigenfunctions of the Schroedinger equation.

 

[martinbn]

Ok, but you are already considering a space of functions (smooth, complex valued, solutions of the equation ect.). Then you build a Hilbert space out of them, which is just $L^2(\mathbb R^3)$. You can just start with it. What is its particle nature?

 

[atyy]

Ok, but you are already considering a space of functions (smooth, complex valued, solutions of the equation ect.). Then you build a Hilbert space out of them, which is just $L^2(\mathbb R^3)$. You can just start with it. What is its particle nature?

For one particle, the classical limit recovers the classical particle.

 

[martinbn]

What is a classical limit of a Hilbert space? And these Hilbert spaces, for one or two or many particles, are all isomorphic.