Talk:History of quantum mechanics

Electrons before their discovery

I will be editing a sentence that is referenced:

In 1887, Heinrich Hertz observed that when light with sufficient frequency hits a metallic surface, the surface emits electrons.

The reference Taylor, J. R.; Zafiratos, C. D.; Dubson, M. A. (2004). Modern Physics for Scientists and Engineers, does indeed say something like this but the context was not history but rather an explanation of the photoelectric effect.

Since the electron was not discovered until 1897, what Hertz observed would be called "cathode rays". Johnjbarton (talk) 00:32, 16 July 2023 (UTC)

Used Whittaker's account.

Resolved

Johnjbarton (talk) 01:02, 16 July 2023 (UTC)

Under "Electrons", we read: "In the final days of the 1800s, J. J. Thomson established that electrons carry a negative charge opposite but the same size as that of a hydrogen ion while having a mass over one thousand times less. Many such electrons were known to be associated with every atom."

I propose adding: "In 1897, Thomson discovered the electron, making it the first subatomic particle to be detected. Charlie Faust (talk) 23:13, 18 March 2025 (UTC)

References

Original Papers

I see Einstein's "On a Heuristic Point of View Concerning the Production and Transformation of Light" (1905) is linked to. Good. Why not link to some more original papers, like Planck's "On the Theory of the Energy Distribution Law of the Normal Spectrum" (1900) and Bohr's "On the Constitution of Atoms and Molecules" (1913)? That would lead readers to the originals, which are historic. Often the originator of an idea is its clearest explicator. Charlie Faust (talk) 23:24, 18 March 2025 (UTC)

There is nothing wrong with adding historic primary sources if the content is sourced to secondary history. Developing the content based on the historic primary source is problematic in a history because the point of the article is to express the historical significance of the work. Such significance can't be in the original paper.

Ironically this specific case is one where there is a secondary source, Folsing, in ref to the Einstein quote:

• This has been called the most revolutionary sentence written by a twentieth century physicist.

Based on everything I've read this is far from a consensus view. In fact the nature of photons is a matter of controversy to this day. Johnjbarton (talk) 01:48, 19 March 2025 (UTC)

Fosling's discussion is a lot more nuanced. He spends many pages explaining "revolutionary" in terms of the context of the times rather than the ultimate significance of the idea. Classical EM was so successful that the quanta really upset that apple cart. Johnjbarton (talk) 02:16, 19 March 2025 (UTC)

I continue to think original papers are useful, for readers to see how the ideas developed. Einstein leads us through the thinking that led to his theories. His paper on light quanta was his most revolutionary, according to Abraham Pais. It may, as you say, be the most revolutionary paper by a twentieth century physicist. I recommend Pais's Subtle is the Lord: The Science and the Life of Albert Einstein, but odds are you've already read it. Charlie Faust (talk) 03:45, 19 March 2025 (UTC)

So citing Pais and then Einstein is fine. But it is Wikipedia policy to rely on secondary references. Reporting on a primary source amounts to analysis which should be based on historians. Johnjbarton (talk) 16:14, 19 March 2025 (UTC)

Annus mirabilis

We read: "In 1913, Niels Bohr proposed a new model of the atom that included quantized electron orbits: electrons still orbit the nucleus much as planets orbit around the Sun, but they are permitted to inhabit only certain orbits, not to orbit at any arbitrary distance. When an atom emitted (or absorbed) energy, the electron did not move in a continuous trajectory from one orbit around the nucleus to another, as might be expected classically. Instead, the electron would jump instantaneously from one orbit to another, giving off the emitted light in the form of a photon. The possible energies of photons given off by each element were determined by the differences in energy between the orbits, and so the emission spectrum for each element would contain a number of lines."

A few paragraphs later, we read:

"In 1913, Bohr explained the spectral lines of the hydrogen atom, again by using quantization, in his paper "On the Constitution of Atoms and Molecules" in which he discussed and cited the Nicholson model. In the Bohr model, the hydrogen atom is pictured as a heavy, positively charged nucleus orbited by a light, negatively charged electron. The electron can only exist in certain, discretely separated orbits, labeled by their angular momentum, which is restricted to be an integer multiple of the reduced Planck constant. The model's key success lay in explaining the Rydberg formula for the spectral emission lines of atomic hydrogen by using the transitions of electrons between orbits. While the Rydberg formula had been known experimentally, it did not gain a theoretical underpinning until the Bohr model was introduced. Not only did the Bohr model explain the reasons for the structure of the Rydberg formula, it also provided a justification for the fundamental physical constants that make up the formula's empirical results.:

Wasn't Bohr's model given in the 1913 paper? Shouldn't these paragraphs be merged? Charlie Faust (talk) 00:17, 25 March 2025 (UTC)

Well Bohr wrote at least four papers in 1913, but yes these sections should be merged. The content of the section of this article on Bohr model should resemble the intro paragraph of Bohr model at least in level of detail.

The Nicholson model deserves one sentence at most in this article, it is covered in the Bohr model. So the intervening paragraph "An important step..." should be merged into Bohr model. I believe I already have the content and refs Bohr model, so just check it.

The section "A more detailed explanation of the Bohr model" should be deleted as unsourced or if you love it merged into Bohr model. It's not appropriate here even in hide/show. Johnjbarton (talk) 00:49, 25 March 2025 (UTC)

I also think that presenting the emission spectra as an existing outstanding problems, as the paragraph "A second, related puzzle was the emission spectrum of atoms." implies is not correct history. Rather as far as I know, Bohr learned about Balmer series as he was building an atom model and realized he could explain it. No one was clamoring for atom model to explain Balmer. Bohr was motivated by working with Thomson and Rutherford, not the spectrum problem. Johnjbarton (talk) 01:15, 25 March 2025 (UTC)

The paragraphs beginning "In 1913" should be merged. Shouldn't the paragraph beginning "An important step was taken in the evolution of quantum theory at the first Solvay Congress of 1911..." be before the paragraphs (should be paragraph) beginning "In 1913"? "An important step was taken in the evolution of quantum theory" sounds awkward. Charlie Faust (talk) 14:43, 25 March 2025 (UTC)

I deleted that paragraph about Solvay. Johnjbarton (talk) 16:31, 25 March 2025 (UTC)

In The Discoveries, Alan Lightman writes: "Surprisingly, Bohr had never seen Balmer's formula at the time that he visited Rutherford in Manchester." But "in February 1913, Copenhagen's expert on spectroscopy, H. M. Hansen, asked Bohr to explain Balmer's formula." He quotes Bohr as saying "As soon as I saw Blamer's formula, the whole thing was immediately clear to me." It sounds like he was initially unaware of the Balmer series, but that when he was informed of it, it gave him the insight he needed. It was an outstanding problem, one that Bohr's model solved. Lightman mentions Mendeleyev's law of periodicity as another. Charlie Faust (talk) 16:20, 25 March 2025 (UTC)

Yes, that is exactly what I was saying earlier. Except the part of "an outstanding problem". It was not an outstanding problem in the usual sense of an issue that physicists discussed. Rather it was an unknown connection between spectroscopy and atom models. Bohr knew that atomic spectra was important to explain, Nicholson had tried in his model of 1912. But the Balmer series was a regular convergent collection of values known to spectroscopy experts but not other physicists. When he heard about the series Bohr realized matched the spacing of the orbits. So the incorrect history in the article is to imply that Bohr should have known about Balmer before he started. Hope this makes sense. Johnjbarton (talk) 16:42, 25 March 2025 (UTC)

Yes, I think so. (Making sense is relative when it comes to quantum theory!)

On the WP Niels Bohr (a Featured Article) I read: "His three papers, which later became famous as "the trilogy", were published in Philosophical Magazine in July, September and November of that year.

So "On the Constitution of Atoms and Molecules" was in three parts. That complicates matters. But my understanding is that he wanted to make Rutherford's model of the atom work, and thought the answer might lie in quantum theory. Per Lightman, "As Bohr stood before Professor Rutherford that first time in the winter of 1911, he was haunted by two major problems. Both had to do with the nature of atoms. The first was a paradox. According to the well-established theory of electromagnetism, laid out sixty years earlier, any electrically charged particle that is diverted from a straight-line path should radiate electromagnetic waves and thereby lose energy. In particular, the electrons orbiting the central nucleus in Rutherford's atom should continuously lose energy, causing them to spiral into the tiny nucleus. A simple calculation, based on numbers known at the time, indicated that all atoms should collapse by this process in less than a second." They don't. The other mystery, according to Lightman, was Mendeleyev's law of periodicity. Balmer's series provided a missing link. Charlie Faust (talk) 15:26, 26 March 2025 (UTC)

Lightman is creating a story for readers of his book. It's not wrong but just "mythologized": made in to a simple easy to follow story. Rutherford had no atomic model. "Rutherford's atom" is an invention of pop-science which confuses conjectures over beer with theories. See the sources in Rutherford model. So there were no orbits in Rutherford's model because he had none.

The application of Maxwells equations to orbits of electrons in the presence of positive charge does not immediately result in radiation as demonstrated by JJ Thomson's early work on electrons circulating in positive jelly. See Heilbron, John L. (1968). "The Scattering of α and β Particles and Rutherford's Atom". Archive for History of Exact Sciences. 4 (4): 247–307. doi:10.1007/BF00411591. ISSN 0003-9519. JSTOR 41133273..

Bohr knew about Thomson's work. Thomson had tried many "mechanical" models going back to before he discovered the electron. Mechanical planetary models had been tried for a couple of decades, all failed. So "spiral into the nucleus" is in my opinion a short hand for "mechanical models had been tried and all failed".

Thomson's main success had been in attempts to explain chemical trends, but van den Broek's proposal of atomic number must be seen as vitally important to the periodic table. I'm surprised that Lightman does not talk about spectra.

I encourage you to read Kaugh's and Heilbron's work on the history of this era. Johnjbarton (talk) 16:50, 26 March 2025 (UTC)

Lightman does talk about spectra: "While the observed features of atomic spectra did not exactly violate any known laws of physics, neither could they be explained. For example, why are there gaps between the emitted frequencies of each atom, as if a singer could singly only the notes C-sharp, G, and A, with no notes in between? The precision with which spectra could be measured, the stubborn insistence of an atom to emit the same frequencies of light over and over, always identical to the emission of other atoms of its kind but different from all other atoms, only served to emphasize the degree of ignorance of science at the time."

Didn't Henry Moseley theorize atomic number? I remember asking my high school chemistry teacher who she'd name an element for. "Moseley" she said. Good choice. Charlie Faust (talk) 19:50, 26 March 2025 (UTC)

Please see Atomic number § Moseley's 1913 experiment. Johnjbarton (talk) 22:23, 26 March 2025 (UTC)

Thank you, did not know van den Broek's name. A pity they don't award Nobel Prizes posthumously, or Moseley would have won one.

Shouldn't there be something about Arthur Compton in the article? His eponymous scattering experimentally demonstrated the existence of photons, didn't it? Charlie Faust (talk) 23:16, 26 March 2025 (UTC)

Yes, Compton should be covered. He demonstrated particle behavior of electromagnetism in scattering, and it was highly influential at the time. "Existence of photons" might imply more local realism than what was observed, in light of later developments in QM. Johnjbarton (talk) 02:54, 27 March 2025 (UTC)

Added Compton. Charlie Faust (talk) 02:56, 27 March 2025 (UTC)

Eventually we should move Compton to the intro of "Development of modern quantum mechanics" section as one of the triggers for the new QM. We also need a secondary ref for his role, not just his primary source. I'll look for one. Johnjbarton (talk) 03:01, 27 March 2025 (UTC)

Well, as Feynman said, "It has not yet become obvious to me that there's no real problem. I cannot define the real problem, therefore I suspect there's no real problem, but I'm not sure there's no real problem." You think that quote would be worth adding to his page? I made a page for Genius: The Life and Science of Richard Feynman. Please take a look. Charlie Faust (talk) 23:23, 27 March 2025 (UTC)

No, actually I don't think such a quote is appropriate. On the other hand a section on Feynman's path integral formulation of quantum mechanics should definitely be added. Johnjbarton (talk) 23:44, 27 March 2025 (UTC)

I was (mostly) kidding. But, in all seriousness, I would like it if you'd take a look at my page.

Path integral is worth adding. Charlie Faust (talk) 23:46, 27 March 2025 (UTC)

References

The Franck–Hertz experiment

I read (on the Main Page of Wikipedia, under "On this day"): "The Franck–Hertz experiment, the first electrical measurement to clearly demonstrate quantum mechanics, was presented to the German Physical Society." There should probably be more about it in this article, yes? Charlie Faust (talk) 18:21, 24 April 2025 (UTC)

And perhaps in Bohr model Johnjbarton (talk) 18:32, 24 April 2025 (UTC)