Quantum electrodynamics in the 1920s

Quantum electrodynamics is often shortened to QED. As for what it is exactly, I find it difficult to say. Wikipedia says it’s the relativistic quantum field theory of electrodynamics, and gives “a complete account of matter and light interaction”. But that’s not enough. The Encyclopaedia Britannica says it’s a quantum field theory which “describes mathematically not only all interactions of light with matter but also those of charged particles with one another”. That’s not enough either. Particularly since it’s defining QED in terms of other things that aren’t defined. For example, what’s a quantum field theory? When you follow the link you can read that it’s “a body of physical principles combining the elements of quantum mechanics with those of relativity to explain the behaviour of subatomic particles and their interactions”. That doesn’t actually tell you much, especially since the rest of the definition is somewhat circular. Instead of giving more information on quantum field theory, Encyclopaedia Britannica says “the prototype of quantum field theories is quantum electrodynamics”.

Pascual Jordan quantizes the electromagnetic field

And as for how it all started, I find that difficult to say too. There’s Arthur Miller’s 1994 book Early Quantum Electrodynamics, but there’s also mixed messages. The Wikipedia history of quantum field theory suggests it started with Paul Dirac in 1927. But Steven Weinberg says it started earlier. See his 1997 essay what is quantum field theory, and what did we think it is? He says quantum field theory goes back to the “Dreimännerarbeit” three-man paper on quantum mechanics II, which was written in 1925. He says “in one of the very first papers on quantum mechanics, Born, Heisenberg and Jordan presented the quantum theory of the electromagnetic field”. See Pascual Jordan’s resolution of the conundrum of the wave-particle duality of light by Anthony Duncan and Michel Janssen dating from 2007. On page 8 they quote Jordan saying this: “what the Dreimännerarbeit says about energy fluctuations in a field of quantized waves is, in my opinion, almost the most important contribution I ever made to quantum mechanics”.

Pascual Jordan solves the problem of wave-particle duality

On page 47 they quote Jordan saying this: “Einstein drew the conclusion that the wave theory would necessarily have to be replaced or at least supplemented by the corpuscular picture. With our findings, however, the problem has taken a completely different turn. We see that it is not necessary after all to abandon or restrict the wave theory in favour of other models; instead it just comes down to reformulating the wave theory in quantum mechanics. The fluctuation effects, which prove the presence of corpuscular light quanta in the radiation field, then arise automatically as consequences of the wave theory. The old and famous problem [of] how one can understand waves and particles in radiation in a unified manner can thus in principle be considered as solved”. Jordan’s final section had taken care of wave-particle duality. Sadly even his contemporaries didn’t appreciate it. See page 12 of Duncan and Janssen’s paper where they quote Jordan saying this: “other theorists in Göttingen, Frenkel for instance, considered my opinion… that the electromagnetic field and the Schrödinger field had to be quantized… as a somewhat fanciful exaggeration or as lunacy. This changed only when Dirac also quantized both the electromagnetic field and the field of matter-waves. I still remember how Born, who had been the first to receive an offprint of the relevant paper of Dirac, showed it to me and initially looked at it shaking his head. When I then pointed out to him that I had been preaching the same idea all along ever since our Dreimännerarbeit, he first acted surprised but then agreed. Heisenberg then also set aside his temporary skepticism, though it was not until considerably later that he himself started to work toward a quantum theory of fields (or “quantum electro-dynamics”) in the paper he then published with Pauli, which followed up on my three joint papers with Pauli, Klein, and Wigner”.

The vibrations of an elastic body idealized to a continuum

See page 375 of Bartel van der Waerden’s 1967 book sources of quantum mechanics for the section concerned. Jordan said the same will apply if we consider the vibrations of an elastic body idealized to a continuum, or the vibrations of an electromagnetic cavity. He also talked about vibrating membranes or strings:

Image from the physics classroom see  Physics of Musical Instruments, note how the waves are the same height

He said this on page 376: “So strong an association between the eigenvibrations of a cavity and the light quanta postulated formerly can nonetheless be drawn that every statistic of cavity eigenvibrations corresponds to a definite statistic of light quanta, and conversely”. He was saying light quanta are waves. We might describe them as self-contained waves with an E=hf nature that propagate through space without dispersing. We might also say photons are solitons, and that Pascual Jordan nailed wave-particle duality over ninety years ago.

Einstein was having none of it

However Duncan and Janssen say Jordan received a less-than-enthusiastic reaction. And that this “no doubt partly explains why Jordan’s result has not become a staple of the historical literature on the wave-particle duality of light”. They say another factor “may have been that Jordan’s result was too many things at once. It was the resolution of the conundrum of the wave-particle duality of light, but it was also a striking piece of evidence for matrix mechanics”. It was a striking piece of evidence for matrix mechanics, so Einstein was having none of it. That’s from the AHQP interview with Jordan, session 3, page 9. It’s half in German but you can access the text and use Google translate to catch the drift. Also see chapter 1 of QED and the men who made it by Silvan Schweber. He says Jordan was influenced by Gustav Mie, and “became committed to a unitary view of nature in which both radiation and matter were described by wave fields”. He also says “in a series of seminal papers, Jordan, with Klein and Wigner as his collaborators, indicated how to quantize this ‘classical’ field theory”. But Jordan’s approach “did not become the accepted interpretation of non-relativistic quantum mechanics during the 1930s”. Schweber describes Jordan as the unsung hero who laid the foundations of quantum field theory. He was the only major contributor who didn’t attend the 1927 Solvay conference. His stutter didn’t help. Nor did his Nazi sympathies. See Jordan’s biography on the St Andrew’s website by John O’Connor and Edmund Robertson. It includes the story of how fermions are called fermions because Max Born mislaid one of Jordan’s papers for six months. Also see Bert Schroer’s 2003 essay Pascual Jordan, his contributions to quantum mechanics and his legacy in contemporary local quantum physics. It’s well worth a read. Ditto for the physical nature of wave/particle duality by Marcello Cini. He says from Jordan’s point of view “the wavelike behaviour of any field’s state with any number of discrete quanta simply reflects the property of a physical nonlocal entity which exists objectively in ordinary three dimensional space”.

The rotating electron and other papers

Jordan wrote other papers such as the application of quantum mechanics to the problem of the anomalous Zeeman effect, co-authored with Heisenberg in 1926. It was recently translated into English by David Delphenich. It starts by saying “Uhlenbeck and Goudsmit invoked Compton’s hypothesis of a rotating electron in order to explain the anomalous Zeeman effect”. It ends by saying their result gives important support for the Compton-Uhlenbeck-Goudsmit hypothesis and for quantum mechanics. Jordan also wrote a paper on the polarization of light quanta in 1927. Again it was recently translated by David Delphenich. Other papers by Jordan in 1926 and 1927 include Über eine neue Begründung der Quantenmechanik I und II (On a New Grounding in Quantum Mechanics I and II), Über quanten-mechanische Darstellung von Quantensprüngen (On the quantum mechanical representation of quantum jumps), and Über Wellen und Korpuskeln in der Quantenmechanik (On waves and corpuscles in quantum mechanics). Papers in 1928 include Über das Paulische Äquivalenzverbot (On Pauli’s equivalence prohibition) co-authored with Eugene Wigner, and Zum Mehrkörperproblem in der Quantentheorie (The multi-body problem in quantum theory) co-authored with Oskar Klein. But there’s also a paper on the quantum electrodynamics of charge-free fields co-authored with Wolfgang Pauli. Yet again it was recently translated by David Delphenich. It talked about the decomposition of fields into transverse partial waves that propagate at the speed of light, and about spatially-isotropic spherical-shell waves that contract and expand at the speed of light. It feels like something from Milo Wolff. It isn’t what I think of as particle physics. Silvan Schweber spoke of seminal papers, but the dearth of 1920s English translations is rather odd.

Wolfgang Pauli and the spinors

Talking of Pauli, see the Wolfgang Pauli Physics Today article by Karl von Meyenn and Engelbert Schucking. They quote Max Born saying Pauli was a genius. They don’t say he was famously arrogant too. But they do say his first publication “dealt with Hermann Weyl’s gauge theory of gravity and electromagnetism” and according to Weyl “dealt it a pernicious blow’”. Schucking and von Meyenn also say Pauli’s principal concern was to clarify the greater picture. But In 1924 he wrote his two-valuedness paper on the influence of the velocity dependence of the electron mass on the Zeeman effect, and there was no picture at all. Then in 1925 he wrote his Pauli exclusion paper on the connexion between the completion of electron groups in an atom with the complex structure of spectra. Again there was no picture:

Pauli exclusion image by Marco Fedi, see a superfluid theory of everything?

Indeed it seems like Pauli wanted to shoot down anybody who did try to come up with a picture. Like he shot down Ralph Kronig’s electron spin in 1925, using the straw-man claim that the electron’s hypothetical surface would have to be moving faster than light. Yes, as Schucking and von Meyenn say, Pauli beat Schrödinger to the theory of the hydrogen atom in his 1926 paper on the hydrogen spectrum from the standpoint of the new quantum mechanics. But perhaps it would have been better if he hadn’t. In 1927 Pauli wrote a paper on the quantum mechanics of magnetic electrons. That’s where he spoke of a two-component spinor wavefunction, but still gave no picture. Pauli did refer to Yakov Frenkel’s 1926 paper on the electrodynamics of rotating electrons, which said the electron will thus be treated simply as a point. Pauli wondered “whether such a formulation of the theory is even possible at all as long as one retains the idealization of the electron by an infinitely small magnetic dipole”. And “whether a more precise model of the electron is required for such a theory”. But he didn’t pursue it, more’s the pity. Especially since he also referred to Charles Galton Darwin’s 1927 Nature paper on the electron as a vector wave, which talked about a spherical harmonic for the two directions of spin. In similar vein Pauli said “the function ψ_E(q,δ) thus defined cannot return to its starting value as δ continually advances from the value 0 to 2π, but must change its sign”. But it’s Dirac’s belt, not Pauli’s belt. It’s the Dirac string trick, not the Pauli string trick.

Dirac makes a contribution

Talking of Dirac, see Royal Society Publishing for some of his papers. Also see the Wikipedia history of quantum field theory which says the first reasonably complete theory of quantum electrodynamics was created by Dirac in 1927. That’s when he wrote the quantum theory of the emission and absorption of radiation. He said “hardly anything has been done up to the present on quantum electrodynamics. The questions of the correct treatment of a system in which the forces are propagated with the velocity of light instead of instantaneously, of the production of an electro-magnetic field by a moving electron, and of the reaction of this field on the electron have not yet been touched”. That sounds reasonable enough if you’ve never read Gustav Mie’s 1913 foundations of a theory of matter. That’s where Mie said electrons “are not, as has been believed for twenty years, foreign particles in the ether, but they are only places at which the ether takes on a particular state”. Mie’s chapter 2 is Knot Singularities in the Field, which reminds me of Thomson and Tait and  TQFT. Dirac clearly never read it, because in his 1927 paper he talked about “plane waves representing the incident electron approaching the atomic system, which are scattered or diffracted in all directions”. In section §4 he said “the square of the amplitude of the waves scattered in any direction with any frequency is then assumed by Born to be the probability of the electron being scattered in that direction”. In section §6 he said “light quanta obey the Einstein-Bose statistics and have no mutual interaction”. He also said a light-quantum is in a stationary state when it’s moving with constant momentum in a straight line. And that “it apparently ceases to exist when it is in one of its stationary states, namely, the zero state, in which its momentum, and therefore also its energy, are zero”. On top of that he said “we must suppose that there are an infinite number of light-quanta in the zero state”. This is the seminal paper where Dirac is said to have introduced second quantization to model photon creation. But in Dirac’s beautiful mathematical world, light doesn’t interact with light, light moving in a straight line is stationary, stationary waves have no energy, and there’s an infinite number of non-existent zero-state photons waiting to jump into existence. The claim that this was the first reasonably complete theory of quantum electrodynamics is not justified.

Darwin and his vector-wave electron

Particularly since it came just before Charles Galton Darwin’s February 1927 Nature paper on the electron as a vector wave. Darwin started by saying “the spinning electron of Uhlenbeck and Goudsmit has brilliantly filled up a serious gap in atomic physics”. He also said “it is only fair to consider certain defects from which it suffers”, and “what is required is to double the number of states of the electron”. He later said “It is however, possible to construct by a much more inductive process a system of waves, of a vector character though not transverse, which completely reproduces the doublet spectra, and then to generalise from this”. He then said “The above equations were found by solving the complete problem of the spinning electron then adjusting the constants by trial; a_m and b_m are then the coefficients of a spherical harmonic for the two directions of spin”. So we’ve got a wave spinning in two directions, presumably like a Möbius strip. As per a spin ½ particle, you need to go round twice to get back to your original position and orientation. What’s not to like?

Anapole image credited to Michael Smeltzer/Vanderbilt

Especially seeing as Pauli referred to Darwin’s paper, and expressed “my deepest thanks to Darwin at this point for his encouragement”. Darwin later wrote a 27-page PRSA paper on the electron as a vector wave, which was received in July 1927 and printed in December. That feels like quite a delay for back then, especially seeing as it’s good stuff. Darwin said this: “Pauli has published a paper on the same subject, and arrived at the same mathematical results, but owing to the fact that he is more disposed to regard the wave theory as a mathematical convenience and less as a physical reality, he stops short of the point which was the guiding principle to me”. He also said this: “the Zeeman effect determines precisely what the spinning electron must be like. It must have angular momentum ½h/2π and it must have magnetic moment eh/4πmc. The positive and negative values of the momentum give the doubling. On account of the magnetic moment it is sometimes called the “magnetic electron”, but that suggests that it is like a little bar-magnet tied to an electric charge and so tends to make us forget the mechanical angular momentum, which is just as essentially one of its qualities”. He didn’t say at how absurd it gets. Instead he said we must regard the electron as a wave, and its motion in free space or weak fields can be treated by the ordinary theory of waves. He said “it is possible to regard the wave of the electron as in ordinary space”. Yep, sounds good to me.

Assimilated by the Bohr

Dirac referred to Darwin’s later paper in his 1928 paper on the quantum theory of the electron, but not to the earlier paper that Pauli referred to. Dirac said “Goudsmit and Uhlenbeck have introduced the idea of an electron with a spin angular momentum of half a quantum and a magnetic moment of one Bohr magneton. This model for the electron has been fitted into the new mechanics by Pauli,* and Darwin,† working with an equivalent theory, has shown that it gives results in agreement with experiment for hydrogen-like spectra to the first order of accuracy”. Dirac talked about the Pauli-Darwin theory, even though Darwin’s earlier paper preceded Pauli’s paper. Then in Dirac’s March 1928 paper on the quantum theory of the electron part II, Darwin isn’t mentioned at all. It’s now “Pauli’s theory of the spinning electron”. This is same Pauli who hated Ralph Kronig‘s idea of electron spin. Who, according to Samuel Goudsmit, was the man who never cared to believe in spin. The man who rejected visualizable models. In 1928 Darwin wrote a paper on the wave equations of the electron and a paper on the magnetic moment of the electron, then a paper on the diffraction of the magnetic electron. See his papers on the Royal Society website. But in Dirac’s papers, there are no further references to Darwin. There is no mention of Darwin in Dirac’s 1930 negative-energy “hole theory” paper a theory of electrons and protons. It is said that Dirac was “not bothered by issues of interpretation in quantum theory”. But perhaps there was more to it. Perhaps he’d spent too much time in Copenhagen, and had been assimilated by the Bohr. As Kurt Gottfried said in his 2010 essay P.A.M. Dirac and the Discovery of Quantum Mechanics, “Heisenberg, Dirac et al were hostile to wave mechanics because they thought it gave the misleading impression that the classical concepts of continuity and visualizability had survived the revolution, whereas they believed that it was a central virtue of their abstract theory that it did not evoke such delusions”. Since when was understanding some kind of delusion?

Heisenberg and Pauli join forces

Talking of Heisenberg, Kurt Gottfried also said his “ground-breaking paper is notoriously difficult to follow because it contains what seem to be ‘magical’ steps. Even van der Waerden, a powerful mathematician who made significant contributions to quantum mechanics, admits to not being able to follow at critical points although he had the benefit of interviews with Heisenberg”. Gottfried also refers to understanding Heisenberg’s ‘magical’ paper of July 1925 written by Ian Aitchison, David MacManus, and Thomas Snyder in 2004. They quote Steven Weinberg saying this: “If the reader is mystified at what Heisenberg was doing, he or she is not alone”. And this: “the papers of magician-physicists are often incomprehensible. In this sense, Heisenberg’s 1925 paper was pure magic”. With this in mind, in the Wikipedia article on Heisenberg you can read that “in early 1929, Heisenberg and Pauli submitted the first of two papers laying the foundation for relativistic quantum field theory”. The first paper was on the quantum dynamics of wave fields:

Fair use excerpt from on the quantum dynamics of wave fields

In his 1977 essay the search for unity Weinberg said this is where Heisenberg and Pauli showed that material particles could be understood as the quanta of various fields, with one field for each type of elementary particle. I didn’t notice that, but I did notice that there was no mention of Darwin. They talked of dealing with electrostatic, magnetostatic, and radiation-mediated interactions from a unified standpoint. They said “it will be necessary to give a relativistically-invariant formalism that will allow one to treat the interaction between matter and the electromagnetic field, and thus also the one between matter and matter”. They seemed to be saying the photon-electron interaction of Compton scattering is the same as the interaction between the electron and the proton, when it isn’t. They also talked about a Lagrangian, which is kinetic energy minus potential energy. That’s perhaps not ideal, because repeated Compton scattering suggests the photon is all kinetic energy. Then they launch into an absolute blizzard of mathematics.

Lost in maths

It would be tempting to suggest that something got lost in translation, but instead I think they got lost in maths. All too often when they do give some physics, they do not shine. Like on page 26 when they say both fields move through the space-time manifold, not realising that there is no motion in spacetime because it models space at all times. On page 29 they airbrush away the need to explain the exclusion principle saying “a satisfactory explanation for the preference of the second possibility by nature can therefore not be given”. On page 54 they liken an electron in an atomic orbital to a wave in a cavity, but they don’t think about the electron itself. On page 59 they finally mention the Compton effect, but at the end, not at the beginning. All in all the paper feels like it lacks logical development. It feels like it’s based on zero understanding of the photon and the electron. It’s said to be a foundational paper, but it lacks foundation, and it’s as clear as mud. Others share my sentiments. In the peculiar notion of exchange forces part II Cathryn Carson said “with its inconvenient notation, frequent changes of variables, and reliance on formal manipulations, there was very little that was clear”. It’s similar for Heisenberg and Pauli’s paper on the quantum theory of wave fields II. The title doesn’t even match the previous paper, and they throw in quantization of the gravitational field, gauge theory, and annihilation processes which “have no place in particle theory”. It’s as if they’re throwing in everything plus the kitchen sink hoping to score a lucky hit.

Plagued with troublesome divergences

No wonder the two papers by Heisenberg and Pauli aren’t included in Julian Schwinger’s 1958 book selected papers of quantum electrodynamics. They’re supposed to be seminal papers, but again they’ve only been translated into English recently. They also contained the physics equivalent of a birth defect. As Silvan Schweber said on page 86 of QED and the men who made it, the theory was plagued with troublesome divergences that led to infinities. Take another look at on the quantum dynamics of wave fields. See the bottom of page 2 where Heisenberg and Pauli said this: “the formulas of the theory lead to an infinite zero-point energy for the radiation, and thus include the interaction of an electron with itself as an infinite additive constant”. Infinities in the mathematics are a killer. However Heisenberg and Pauli swept them under the carpet, saying “these difficulties are of a sort that they do not interfere with the application of the theory to many physical problems”. This was a mere year after Oskar Klein and Yoshio Nishina’s paper on the scattering of free electron radiation according to the new relativistic quantum dynamics of Dirac. Their Klein-Nishina formula modelled Thomson scattering and Compton scattering. It is said to be “one of the first results obtained from the study of quantum electrodynamics”.

The theory was wrong

But shortly thereafter Julius Robert Oppenheimer said the theory was wrong. He’d spent time in Göttingen, he’d written a paper on the quantum theory of molecules with Max Born in 1927, plus other papers. He knew the key players, and he knew what he was talking about. See page 3 of his 1930 note on the theory of the interaction of field and matter. He said this: “The theory, is, however, wrong, since it gives a displacement of the spectral lines from the frequency predicted on the basis of the nonrelativistic theory which is in general infinite. This displacement arises from the infinite interaction of the electron with itself”. So in essence, quantum field theory aka quantum electrodynamics was wrong from the off because it employed point charges. Regardless of the hard scientific evidence that proved the wave nature of matter. Even though the foundational paper was called the quantum dynamics of wave fields. No wonder Schweber said the occurrence of the divergences pointed to a deep inconsistency in the conceptual structure of QFT. The same could be said for Dirac’s negative-energy “hole” theory. Oppenheimer shot that down too, in his 1930 paper on the theory of electrons and protons. He said Dirac’s derivation of the Thomson formula was invalid, and there are several grave difficulties which arise when “one tries to maintain the suggestion that protons are the gaps of negative energy, and that there are no distinctive particles of positive charge”. He also said if we return to the assumption of two independent elementary particles of opposite charge and dissimilar mass, we can resolve all the difficulties raised in this note. Well said Oppie. Let’s come back to that another day.

The infinity puzzle

This was the beginning of what Frank Close called the infinity puzzle. But I have to say it was only ever a puzzle because a Copenhagen cabal of quantum quacks had a dogged opposition to the wave nature of matter, and to any attempt to provide intuitive understanding. As a result they painted themselves into a corner with the point particles that resulted in infinities and failure. Whatever happened to Pascual Jordan? Or Charles Galton Darwin? Or Louis de Broglie, whose 1929 Nobel lecture was on the wave nature of the electron? What happened to Erwin Schrödinger and his wave group in a small closed path? It’s as if the Einstein-de Haas effect never happened. That’s the effect which “demonstrated that spin angular momentum is indeed of the same nature as the angular momentum of rotating bodies as conceived in classical mechanics”. It’s as if Arthur Compton’s 1921 paper the magnetic electron had never been written. That’s where he referred to the Parson electron which featured a rotation with a “peripheral velocity of the order of that of light”. It’s as if the 1922 Stern-Gerlach experiment never happened either. That’s the experiment that demonstrated that “particles possess an intrinsic angular momentum that is closely analogous to the angular momentum of a classically spinning object”. It’s as if the 1927 Davisson-Germer experiment and the Thomson and Reid diffraction experiments never happened too.

Where did it all go wrong?

There’s a book written in 1991 by Walter Grandy junior called the Relativistic Quantum Mechanics of Leptons and Fields. Chapter 2 is the Dirac equation. Grandy says “for almost 100 years a major goal of physics has been to understand the electron”. I would dispute that. See Hans Ohanian’s 1984 paper what is spin? He says Pauli pontificated that spin is a quantum-mechanical property, and that the lack of a concrete picture was a satisfactory state of affairs. He quotes from Pauli’s 1955 essay Exclusion Principle, Lorentz Group and Reflection of Space-time and Charge. Pauli said this: “After a brief period of spiritual and human confusion caused by a provisional restriction to ‘Anschaulichkeit’, a general agreement was reached following the substitution of abstract mathematical symbols, as for instance psi, for concrete pictures. Especially the concrete picture of rotation has been replaced by mathematical characteristics of the representations of rotations in three-dimensional space”. Have you ever heard such arrogant ignorant garbage? Anschaulichkeit means visualizability. Clarity. Pauli was saying quantum physics surpasseth all human understanding. Take another look at Bert Schroer’s 2003 essay. On page 9 he says this: “in times of stagnation and crisis as the one we presently face in the post standard model era of particle physics, it is helpful to look back at how the protagonists of quantum field theory viewed the future and what became of their ideas and expectations. Perhaps the past, if looked upon with care and hindsight, may teach us where we possibly took a wrong turn and what alternative path was available”. You bet Bert. Especially since QED was about to take another wrong turn.

There was no mention of exchange particles

I don’t know if you noticed, but in all those seminal early papers there was no mention of exchange particles. Such as the photons that are said to mediate the linear and rotational electromagnetic forces between the electron and the proton in the hydrogen atom. Or the rotational electromagnetic force that makes an electron go round and round in circles in a uniform magnetic field. Or makes the moving electron take a curved path near the current in the wire, but does not affect the stationary electron:

Image from Special Relativity in 14 Easy (Hyper)steps by George Gollin, wording abbreviated by me

Take another look at the peculiar notion of exchange forces part I and part II by Cathryn Carson dating from 1996. This is forensic physics at its finest. She says “for such a central concept, the historical origins of exchange forces are distinctly unclear”. She refers to a 1927 chemistry paper by Walter Heitler and Fritz London on the interaction between neutral atoms and homopolar binding according to quantum mechanics. And to a 1928 paper by Heisenberg on the theory of ferromagnetism. She says the exchange-particle idea began to work its way into QED from the mid-to-late 1930s. And that Heisenberg’s importation of exchange forces into nuclear physics depended essentially on a model of the neutron that was later retracted. She says “the idea now taken as definitional of the concept of force – for quantum field theory, the all-important idea of particle exchange – was not in fact there from the start, but rather worked its way in from somewhere outside”. Yes, that’s interesting all right. Especially since hydrogen atoms don’t twinkle, and magnets don’t shine. And especially because of what Schrödinger said on page 27 of his 1926 paper quantization as a problem of proper values, part II. About light rays influencing one another and showing remarkable curvature.

On the rotating electron

It’s all the more interesting because the Wikipedia exchange force article talks about boomerangs. Which ought to make you think if you’ve ever read Franco Raseti and Enrico Fermi’s 1926 paper on the rotating electron. They said “the electron has almost always been considered to be a material point up to now”. They also said this: “it was only in recent years that Uhlenbeck and Goudsmit made the hypothesis that the reason for some spectroscopic phenomena – in particular, the anomalous Zeeman effect – was to be found in a structural element of the electron. Those authors assumed precisely that the electron is animated with a rotational motion around itself, in such a way that it possesses a quantity of a real motion, namely, a magnetic moment”. Raseti and Fermi also said “despite the grave energetic difficulties that have been pointed out, one can conclude that the hypothesis of the rotating electron must not be abandoned”. Unfortunately for quantum electrodynamics, it was.

Bubble chamber image from the CERN courier 

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