Some sins in physics didactics - comments

[atyy]

It's a bit strange to me to say the Schrödinger waves are in Hilbert space. It's simply a scalar complex valued field which describes waves, but just in the mathematical sense. It's not like the waves of a measurable field quantity like, e.g., the density of air or water (sound waves) or the electromagnetic field, but its physical meaning is given by the Born rule.

What the quantum mechanics is supposed to describe is, of course, given by the choice of the problem. If you have a single particle, you start by defining observables by assuming some operator algebra, heuristically taken from some classical analgous situation. Of course, you cannot derive this in a mathematical sense from anything, but you have to more or less guess it. What helps you, are symmetries and (Lie-)group theory to guess the right operator algebra. The self-adjoint operators live on a Hilbert space, and the rays in this Hilbert space represent the (pure) states. Then, if $|\psi \rangle$ is a normalized representant of such a ray, a wave function is given wrt. a complete basis, related to the determination of a complete set of compatible observables, $|o_1,\ldots,o_n \rangle$, i.e.,
$$\psi(o_1,\ldots,o_n)=\langle o_1,\ldots o_n|\psi \rangle.$$
That's it. In my opinion there's no simpler way to express quantum theory than this. Admittedly it's very abstract und unintuitive, but that's the only way we have found so far to adequately describe (pretty comprehensively) the phenomena in terms of a pretty self-consistent mathematical scheme.

Yes, it's a bit strange, but that it's not wrong shows that there is nothing wrong with wave-particle duality. Again in non-rigourous QFT, the Fock space is still a particle space. Then if we take the Wilsonian viewpoint and accept a lattice regularization, the lattice is again QM, which is a particle space.

 

[atyy]

What if the photoelectric effect could be shown with single photons? Would that vindicate Einstein's analysis?

 

[vanhees71]

I don't understand what you mean by "particle space". Quantum theory is about quanta, not particles nor classical fields, no matter in which of the many equivalent ways you express it. Of course, you can treat the photoeffect also with single photons. For that you have to quantize the electromagnetic field. The only difference at this order is that for the excited bound states there's a transition probability from an excited (bound) state to a lower state under emission of (one or more) photons, spontaneous emission, and that's why Planck's Law shows the necessity for the quantization of the em. field, as Einstein has figured out in 1917 from another semiclassical argument within old quantum theory. To get the correct radiation law, he had to assume spontaneous emission, and that was later explained by Dirac when introducing the formalism for non-conserved "particle numbers" in terms of creation and annihilation operators.

 

[Ken G]

To me, the crucial insight of any "wave theory" is simply the importance of interference. So the problem with "wave/particle duality" is only in how we came to understand waves, historically, in terms of the interference in actual observables (displacements, pressures, etc.). But when Huygens realized that wave propagation was an interference among many different processes going on at the same time, and Feynman discovered how to think about it as a superposition of path integrals, it seems to me we should have generalized what we mean by "waves" to include complex fields that show the same behavior, from which real (observable) fields can just be obtained by matching real initial conditions by use of the complex conjugate (which is how real waves are often analyzed anyway). Take away those real initial conditions, and you have a complex wave theory. Could not such a thing be formalized in terms very similar to "new" quantum field theory? In other words, maybe the problem was not old quantum mechanics, but old wave theory.

Also, I think the main problem with "wave/particle duality" is that it is often explained like "sometimes it acts like a wave, sometimes like a particle." That makes it sound confusing and downright schizophrenic. But there's no need to describe it like that, the wave aspects are consistent, the particle aspects are consistent. What works for me is to say that particles are "told what to do" by wave mechanics. Even trajectories are things that short-wavelength waves do just fine, so there never was anything "different" about what particles do, it was always wave mechanics we just had no reason to see it that way. So to me, the photoelectric effect looks simply like the requirement that if you will get a big response out of an electron (like knocking it out of a metal or forcing a transition in an atom) with a very weak field, you need to find a way to repeat over and over that tiny energy coupling between the field and the electron, in a resonant way, until you accumulate the big response. Like if your house was on springs, and you wanted to raise it an inch, you'd just very gradually bounce it at the resonant frequency until the amplitude was an inch. So that kind of process picks out power from the driving field at the necessary resonant frequency. Behind all that lovely formal mathematics, there is still something quite simple that is physically going on-- the particle is picking out a particular mode from the field, because that is the mode that produces constructive interference in all the possible ways the necessary energy transfer can occur, none of which would independently have sufficient amplitude to be of any consequence, a la Huygens.

So it seems that would have been the greater insight from the photoelectric effect and all types of stimulated emission, and possibly spontaneous emission too (in a kind of radiation reaction force sense). That's what I take from vanhees71's argument-- Einstein thought he was discovering the photon, but that discovery would have to wait-- he was really discovering the quantum mechanics of the electron and he didn't know it! No wonder he never liked quantum mechanics so much...

 

[ZapperZ]

I find the discussion in this thread very confusing and difficult to follow. This is because in some cases, one appeals to the historical context of the derivation, but then one switches to present-day knowledge and criticize the former. I don't get it.

Still, let's get a few things out of the way:

1. Very much like the use of "relativistic mass", is it still news that the basic, simple, historical photoelectric effect is not a "proof" of the existence of photons or quantized electromagnetic field? The paper by J.J. Thorn et al. has been cited many times in this forum (do a check if you don't believe me). In it, the status of the photoelectric effect has been clearly stated as far as the idea of photons is concerned. This paper was published in 2004, and this idea has existed even way before that (see the citation). Are we all just slow to catch on?

2. Is there such a thing as a "proof" in physics? So is the problem here the photoelectric effect description, or overzealous teachers or writers who somehow stated such a word without realizing the fallacy of it?

3. Note that the classical derivation, using modern quantum theory, arrived at the same mathematical expression for the photoelectric effect that Einstein described. So Einstein's insight on the phenomenon gave the same mathematical formalism without any knowledge of the quantum phenomenon of solids and before the existence of modern QM. This leads to his interpretation that this is due to a quantized light based on what was known back then. How is this not any different than our current situation with quantum mechanics itself where we all agree on the formalism, but many of us differ in its interpretation?

4. Because of #3, it is a valid reason to award Einstein with the Nobel Prize, because the mathematical description is still valid (and note that this is coming from someone who had previously written about https://www.physicsforums.com/threads/violating-einsteins-photoelectric-effect-model.765714/ based on newer experiments). Note that the Nobel citation for Einstein's prize read:

.. for his services to theoretical physics, and for his discovery of the law of the photoelectric effect.

i.e. the mathematical description of the photoelectric effect. It says nothing about the quantized light. As far as I can tell, nothing that has been uncovered here contradicts that.

5. I also find it unfair that we apply modern quantum theory to reexamined the naive photoelectric effect, and yet we ignore modern EXPERIMENTS that have expanded the photoelectric effect as a more generalized photoemission phenomenon. If Einstein had access to high-powered laser, the quantum effect of light will be even more apparent via the multiphoton photoemission. I am aware that this is not within the scope of the thread's derivation, but this point should be mentioned.

Zz.

 

[Dadface]

NO, this I don't buy! You must not teach high school students misleading stuff (in fact, we were told "old quantum theory" also before the modern theory was taught in high school, and our (btw. really brillant) teacher said, before starting with the modern part that we should forget the quantum theory taught before, and she was right so.

Of course, in high school, you cannot teach the abstract Dirac/Hilbert-space notation and also not time-dependent perturbation theory, but you can completely omit misleading statements referring to the "old quantum theory". At high school we learned modern quantum theory in terms of wave mechanics. I don't know, how the schedule looks in the UK, but in Germany, usually one has a modul about classical waves before entering the discussion of quantum theory, and thus you can easily argue in the usual heuristic way to introduce first free-particle non-relativistic "Schrödinger waves", but telling right away the correct Born interpretation. This gains you time to teach the true stuff and not waste it for outdated misleading precursor theories that are important for the science historian only (although history of science makes a fascinating subject in itself, and to a certain extent it should also be covered in high school).

I don't understand the 2nd question. Of course, the energy eigenvalue $E$ and the frequency of the corresponding eigenmode of the Schrödinger field are related by $E=\hbar \omega=h f$, where $\omega=2 \pi f$ and $\hbar=h/(2 \pi)$. Usually nowadays one doesn't use the original Planck constant $h$ but $\hbar$, because you don't need to write some factors of $2 \pi$ when using $\omega$ instead of $f$.

Vanhees there really are practical difficulties for any high school teacher in presenting the subject as you suggested. Just look at the relevant section of the Cambridge International AS/A level syllabus. Quantum theory is 25 out of 26 different topics. In addition to covering all of the topics teachers need to teach experimental and practical skills and do numerous other things such as incorporating social, environmental, economic and other aspects into their lesson plans. And, of course, there is the continuing amount of meetings and paperwork to contend with. Taking everything into account, the time teachers have to cover photoelectricity is very limited. Quantum theory is just one small part of a very large syllabus.
Have a look at the syllabus requirements and you will see exactly what it is teachers have to teach. To do otherwise would jeopardise the chances of their students. I don't see anything wrong in teaching a subject as the syllabus demands and then informing the students that the subject is far more developed than what has been taught so far. Most of them know that anyway.

 

[atyy]

I don't understand what you mean by "particle space". Quantum theory is about quanta, not particles nor classical fields, no matter in which of the many equivalent ways you express it.

Well, these things are called "particles" by convention, because in the classical limit the classical particle is recovered.

NO, this I don't buy! You must not teach high school students misleading stuff (in fact, we were told "old quantum theory" also before the modern theory was taught in high school, and our (btw. really brillant) teacher said, before starting with the modern part that we should forget the quantum theory taught before, and she was right so.

OK, but this doesn't mean one should not teach old quantum theory first. It just means we don't say that the photoelectric effect with large numbers of coherent photons cannot be explained without quantization of the EM field.

If we only teach absolutely correct things, then we also cannot teach QM (first quantized language), because it is not relativistic.

But if we teach QFT (second quantized language), we will also find there is only a low energy effective theory. So there has to be some non-perturbatively defined regularization, eg. the lattice, which basically means we go back to QM

But if we start from lattice theory instead, we run into problems with chiral fermions.

So at present we have a theory that is only perturbatively defined by some presumably asymptotic expansion, but we have nothing to which it is asymptotic to, so we have no theory. So we have no laws of physics. Which basically proves Many-Worlds is correct. Because in Many-Worlds, all possibilities happen, so what we observe must happen in at least one world. So we should basically teach MWI and the anthropic principle, since that is the only interpretation that is proven to capture all observations with perfect consistency.

 

[atyy]

5. I also find it unfair that we apply modern quantum theory to reexamined the naive photoelectric effect, and yet we ignore modern EXPERIMENTS that have expanded the photoelectric effect as a more generalized photoemission phenomenon. If Einstein had access to high-powered laser, the quantum effect of light will be even more apparent via the multiphoton photoemission. I am aware that this is not within the scope of the thread's derivation, but this point should be mentioned.

I agree with your general point that old quantum theory should be taught, but aren't multiphoton effects also explained without quantizaton of the electromagnetic field? I think the formalism is similar to that in vanhees71's blog post, except that one has to go to higher orders in the perturbation expansion, eg. http://cua.mit.edu/8.421_S06/Chapter9.pdf.

 

[ZapperZ]

I agree with your general point that old quantum theory should be taught, but aren't multiphoton effects also explained without quantizaton of the electromagnetic field? I think the formalism is similar to that in vanhees71's blog post, except that one has to go to higher orders in the perturbation expansion, eg. http://cua.mit.edu/8.421_S06/Chapter9.pdf.

I don't know. It looks like it is employing the dipole transition matrix for each transition due to photon absorption. That smells very much like it already assumes the photon model.

BTW, here is a reference that I have on an example of multiphoton photoemission. Look at Eq. 1 and how it manifests itself as the slope of the charge with light intensity.

http://qmlab.ubc.ca/ARPES/PUBLICATIONS/Articles/multiphoton.pdf

Zz.

 

[Ken G]

I think a few key points are getting lost here. There are two things that vanhees71 never implied, and I never implied them either: 1) that the photoelectric effect was thought to "prove" light was quanta (we all know science doesn't prove, but we use the word loosely sometimes, that was never the issue), and 2) that Einstein was to be blamed for some incorrect interpretation of his experiment. The whole point, it seems to me, relates to how we teach the significance of the photoelectric effect. Because the Nobel was awarded for it, and because it was awarded because that experiment was initially thought to demonstrate the photon nature of light, that's the way it still gets taught. It seems to me vanhees71 is merely pointing out that we don't need to teach it that way, just because it was once thought about that way, and just because a Nobel committee saw it that way. This isn't about the history of discovery, it is about what are the actual ramifications of that experiment, given what we now know, and how history can follow some ironic turns that need to be ironed out in hindsight. I think that's a valid point, and the objections being raised are somewhat extraneous to that point.

 

[stevendaryl]

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.

 

[stevendaryl]

OK, but this doesn't mean one should not teach old quantum theory first. It just means we don't say that the photoelectric effect with large numbers of coherent photons cannot be explained without quantization of the EM field.

If we only teach absolutely correct things, then we also cannot teach QM (first quantized language), because it is not relativistic.

I agree. I think that it is important to separate empirical results from the theoretical models developed to explain those results. But I think it's okay to teach old models, as long as we make it clear that they are just models, which are at best approximations.

 

[ZapperZ]

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.
.

But there is a problem here because for metals, the conduction bands are not "quantized" states, as if there are no discrete energy levels. The article cited photoemission from atoms and solids.

And yes, there are plenty of other experiments that show the photon's presence, including the Thorn's which-way experiment that I cited. Read the paper.

Zz.

 

[Demystifier]

I would also like to propose some sins in physics didactics:

- The Michelson-Morley experiment is taught to be a proof that aether does not exist. Nevertheless, this experiment by itself does not prove it. The experiment does not exclude the possibility that the aether is dragged by the Earth. (Of course, there are other experiments that exclude this possibility, but not the Michelson-Morley one.)

- The spinorial transformation of the Dirac wave function is often taught as being derived from the Dirac equation. However, the Dirac equation does not really imply the spinorial transformation. The Dirac equation allows also a (physically equivalent) alternative, according to which the wave function transforms as a scalar:
http://lanl.arxiv.org/abs/1309.7070 [Eur. J. Phys. 35, 035003 (2014)]

- In 1930 Einstein proposed the photon-in-the-box thought experiment, which was supposed to demonstrate an inconsistency of the time-energy uncertainty relations. The Bohr's resolution of the problem, based on adopting some principles of general relativity, is often taught to be the correct way to save consistency of the time-energy uncertainty relations. But it is not. The correct resolution of the photon-in-the-box paradox, similarly to the latter EPR paradox, is the non-local nature of quantum correlations:
http://lanl.arxiv.org/abs/1203.1139 [Eur. J. Phys. 33 (2012) 1089-1097]

 

[Ken G]

- The Michelson-Morley experiment is taught to be a proof that aether does not exist. Nevertheless, this experiment by itself does not prove it. The experiment does not exclude the possibility that the aether is dragged by the Earth.

I'd take that a step farther. One might hold that it would be odd for the Earth to drag aether, so in that sense the Michelson-Morley experiment could be viewed as good evidence there is no aether. But isn't the deeper point that it's actually not evidence of that at all, rather, it is evidence that the aether concept is simply not helping us understand the situation? After all, both Poincare and Lorentz himself interpreted that experiment as simply saying that the aether has some physical action on clocks and rulers that covers its tracks. Einstein said, who needs that, just make c a law. So it was a classic example of Occam's Razor, but it was certainly not a no-go theorem, and it is indeed sometimes taught that way. We must all recognize that if some experiment tomorrow shows that we need an aether after all, then no past experiments would need to come out any different, we'd just need to dust off Poincare and Lorentz.

 

[Greg Bernhardt]

I would also like to propose some sins in physics didactics:

- The Michelson-Morley experiment is taught to be a proof that aether does not exist. Nevertheless, this experiment by itself does not prove it. The experiment does not exclude the possibility that the aether is dragged by the Earth. (Of course, there are other experiments that exclude this possibility, but not the Michelson-Morley one.)

- The spinorial transformation of the Dirac wave function is often taught as being derived from the Dirac equation. However, the Dirac equation does not really imply the spinorial transformation. The Dirac equation allows also a (physically equivalent) alternative, according to which the wave function transforms as a scalar:
http://lanl.arxiv.org/abs/1309.7070 [Eur. J. Phys. 35, 035003 (2014)]

- In 1930 Einstein proposed the photon-in-the-box thought experiment, which was supposed to demonstrate an inconsistency of the time-energy uncertainty relations. The Bohr's resolution of the problem, based on adopting some principles of general relativity, is often taught to be the correct way to save consistency of the time-energy uncertainty relations. But it is not. The correct resolution of the photon-in-the-box paradox, similarly to the latter EPR paradox, is the non-local nature of quantum correlations:
http://lanl.arxiv.org/abs/1203.1139 [Eur. J. Phys. 33 (2012) 1089-1097]

Maybe a follow up entry? :)

 

[ZapperZ]

I would also like to propose some sins in physics didactics:

- The Michelson-Morley experiment is taught to be a proof that aether does not exist. Nevertheless, this experiment by itself does not prove it. The experiment does not exclude the possibility that the aether is dragged by the Earth. (Of course, there are other experiments that exclude this possibility, but not the Michelson-Morley one.)

I disagree. That's like saying "OK, so you found that there's no unicorn. But that was because you were looking for 4-legged unicorns. What if there are 2-legged unicorns?"

The MM-experiment was specifically testing a particular characteristic of light, and based on what was described at that time, it tested it perfectly well. Besides, if you bring the same setup to the ISS, the MM-experiment is equally up to the challenge to even test the ether drag. So the experiment in itself is adequate.

Zz.

 

[Ken G]

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.

I think a big issue is the question, what is the "quantum" in "quantum mechanics?" We might say it's first quantization, and then the quantum in "quantum field theory" is second quantization. But first quantization doesn't give us photons, it just gives us the analysis vanhees71 gave. So his remarks can be interpreted as suggesting that we separate what experiments support the theory of first quantization from the experiments that support second quantization, and not simply follow the historical path there. I think we must agree that had Bohr come up with his model of the atom before Einstein did the photoelectric effect experiment, then that experiment is just a way to generalize the concepts of first quantization to other regimes. There might not be any hint that second quantization is needed, so if we teach it the historical way, we are promoting misconceptions about the differences between these two brands of "quanta".

 

[rude man]

" ... the introduction of a velocity-dependent mass in special relativity, which is a relic from the very early years after Einstein’s ground-breaking paper of 1905. "

The statement is incorrect. See below.

I have never liked the elimination of rest mass as a separate parameter. It changes several formulae that were accurate before this change, not the least being E = mc^2 for a moving particle.

If it was good enough for Richard Feynman it's good enough for me. Reminder: the milennial edition of "The Feynman Lectures on Physics" was issued just a year or two ago. It includes significant revised material from earlier editions but the use of rest mass as a separate parameter was retained. And wisely so IMO.

 

[ZapperZ]

" ... the introduction of a velocity-dependent mass in special relativity, which is a relic from the very early years after Einstein’s ground-breaking paper of 1905. "

The statement is incorrect. See below.

I have never liked the elimination of rest mass as a separate parameter. It changes several formulae that were accurate before this change, not the least being E = mc^2 for a moving particle.

If it was good enough for Richard Feynman it's good enough for me. Reminder: the milennial edition of "The Feynman Lectures on Physics" was issued just a year or two ago. It includes significant revised material from earlier editions but the use of rest mass as a separate parameter was retained. And wisely so IMO.

I don't what's "incorrect" about that. In fact, check out one of my earlier posting about this:

https://www.physicsforums.com/threads/relativistic-mass.642188/#post-4106101

Note that even Einstein later on stopped using it.

Zz.

 

[atyy]

I don't know. It looks like it is employing the dipole transition matrix for each transition due to photon absorption. That smells very much like it already assumes the photon model.

I think that although the dipole approximation is used, it is a treatment in which the EM field is not quantized. The Hamiltonian they use for the two-photon process is http://cua.mit.edu/8.421_S06/Chapter9.pdf (Eq 9.3), which looks to me of the same form as http://cua.mit.edu/8.421_S06/Chapter7.pdf (Eq 7.31), which has a classical EM field. In their notation if the EM field is quantized, I would expect to see an expression more like their Eq 7.46.

BTW, here is a reference that I have on an example of multiphoton photoemission. Look at Eq. 1 and how it manifests itself as the slope of the charge with light intensity.

http://qmlab.ubc.ca/ARPES/PUBLICATIONS/Articles/multiphoton.pdf

That is interesting. I had to look up the Fowler-Dubridge theory they mention, on which the BSB theory is based. It basically assumes the E=hf from old quantum theory like Planck and Einstein.

 

[atyy]

Of course, you can treat the photoeffect also with single photons. For that you have to quantize the electromagnetic field. The only difference at this order is that for the excited bound states there's a transition probability from an excited (bound) state to a lower state under emission of (one or more) photons, spontaneous emission, and that's why Planck's Law shows the necessity for the quantization of the em. field, as Einstein has figured out in 1917 from another semiclassical argument within old quantum theory. To get the correct radiation law, he had to assume spontaneous emission, and that was later explained by Dirac when introducing the formalism for non-conserved "particle numbers" in terms of creation and annihilation operators.

Probably the strongest argument for teaching the "old quantum theory" view of E = hf and the photoelectric effect using E = hf is that the photoelectric effect is still how we detect single photons!

 

[stevendaryl]

- The spinorial transformation of the Dirac wave function is often taught as being derived from the Dirac equation. However, the Dirac equation does not really imply the spinorial transformation. The Dirac equation allows also a (physically equivalent) alternative, according to which the wave function transforms as a scalar:
http://lanl.arxiv.org/abs/1309.7070 [Eur. J. Phys. 35, 035003 (2014)]

I seem to recall a somewhat heated discussion the last time this was brought up, but I don't remember what the objections were.

It seems that there are maybe three approaches to understanding the gamma-matrices in the Dirac equation:

1. They are just four constant matrices, and the index does not imply that they form a vector.
2. They are matrix-valued components of a 4-vector.
3. Each gamma matrix is a vector. The index $\mu$ in $\gamma_\mu$ indicates which vector, rather than which component.

I'm not 100% sure whether the third approach is well-worked-out, but it is the approach taken by Hestenes in his "geometric algebra", which is inspired by Clifford algebras. In a Clifford algebra, the anticommutation relation

$e_\mu e_\nu + e_\nu e_\mu = 2 g_{\mu \nu}$

is supposed to hold for basis vectors $e_\mu$; the $\mu$ indicates which basis vector, rather than which component.

 

[carllooper]

Explanations in physics, (be they old or new explanations), are for many people themselves in need of an explanation.

Put an equation in front of many people and it won't explain anything.

There is a case for pointing out to students particular experiments (observations) which can act as inspiration for particular explanations, regardless of whether such explanations are deemed today as correct or otherwise.

Now we can not prove that any particular experiment has historically inspired any particular explanation or understanding. But nor is that the goal. The goal is to identify those experiments which could have inspired a theory, or could re-inspire that same theory ... re-inspire the very understanding we might be otherwise entertaining and expressing in an otherwise difficult equation.

We can explain an explanation in this way.

C

 

[rude man]

" ... the introduction of a velocity-dependent mass in special relativity, which is a relic from the very early years after Einstein’s ground-breaking paper of 1905. "

The statement is incorrect. See below.

I have never liked the elimination of rest mass as a separate parameter. It changes several formulae that were accurate before this change, not the least being E = mc^2 for a moving particle.

If it was good enough for Richard Feynman it's good enough for me. Reminder: the milennial edition of "The Feynman Lectures on Physics" was issued just a year or two ago. It includes significant revised material from earlier editions but the use of rest mass as a separate parameter was retained. And wisely so IMO.

I don't what's "incorrect" about that. In fact, check out one of my earlier posting about this:

https://www.physicsforums.com/threads/relativistic-mass.642188/#post-4106101

Note that even Einstein later on stopped using it.

Zz.

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.

 

[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.