One year ago, we launched the AIP History Weekly Edition with a look at an article by Helge Kragh on how physicists and astrophysicists integrated antimatter into their thinking about cosmology. Since then, we have continued to regularly highlight new work in the history of the physical sciences. However, another motivation for publishing on a weekly cadence is to showcase how compelling history can be when we revisit subjects and ask new questions about them.

In that spirit, this week we return to Kragh’s decades-long narrative to focus closely on one piece of it: the extraordinarily rapid acceptance of antimatter after 1933. While some ideas in physics take a long time to find their footing, antimatter became part of mainstream physics so quickly that one tends to overlook just how novel the concept was. In addition, this edition corrects an error in the original piece relating to the adoption of the antimatter concept in science fiction in 1942: Some authors flocked to it much sooner.

Antimatter—a quiet revolution?

That particles can be created and annihilated in interactions with their mirror images is, on its face, a conceptually radical idea. It had no place in physics until 1928, when Paul Dirac published a relativistic quantum mechanical treatment of the electron that implied such behavior, and he did not interpret that treatment as predicting the existence of a new particle until 1931. Then, in 1932 Carl Anderson identified a positively charged, electron-scale particle in photographs of cosmic ray tracks in a cloud chamber. By the end of 1933, it was firmly established that not only do “positrons” exist, but also that they are created and annihilated in conjunction with electrons, as Dirac’s theory supposed. That same year, Dirac won a share of the Nobel Prize in Physics.

How exactly did the new concept of antimatter take root? And why did it happen so suddenly? And why with seemingly little surprise that antimatter’s behavior departed dramatically from the long-held view that matter is composed of immutable fundamental particles? These are questions that tend to be lost in the cracks between two major historical narratives.

It is common in scholarly and especially popular histories of quantum mechanics to portray it as a conceptual revolution in understanding how the physical world works. But that revolutionary narrative typically concludes in 1927 with Werner Heisenberg’s formulation of his uncertainty principle and the debate that year between Niels Bohr and Albert Einstein over whether the indeterminacy of quantum mechanics reflects the fundamental nature of reality. The post-1927 historiography is decidedly less romantic, with conceptual debates cordoned off to the side of a new narrative about a protracted technical slog to extend quantum mechanics, particularly by quantizing electromagnetic fields. Dirac’s incorporation of relativity into quantum mechanics is a key early step in that narrative, with his anticipation of antimatter presented as more of a successful prediction than a stark conceptual break with the past.

Meanwhile, the history of particle physics—the great romance of mid-20th-century physics—begins around this same moment, and from the start it essentially takes for granted that particles can appear, disappear, and transform into other ones. This is certainly understandable, because it quickly became routine to identify positrons, and soon other new particles, and to observe their creation and destruction. Moreover, the discovery of the neutron at almost the same moment offered an explanation of radioactive beta decay that involved neutrons’ transformation into protons while emitting electrons.

Yet, even if we accept that the experimental landscape swiftly made it impossible for physicists to deny phenomena such as particle production and decay, it remains striking that they accepted those phenomena with so little discussion about how novel they were. It is true that physicists had already become accustomed to matter–energy equivalence, to light behaving in particle-like ways, to electrons diffracting like light, and to quantum states spontaneously transitioning into different states. But that should not imply that accepting antimatter was a natural next step.

Dirac’s strange theory and its ultimate acceptance

In fact, we know that Dirac’s theory was not initially embraced. Although he addressed a major shortcoming in quantum mechanics by incorporating relativity, and although he gave the spin and magnetic moment of electrons a firmer theoretical foundation, Dirac’s prediction of antimatter famously arose from a serious problem: In addition to describing ordinary electrons, his theory yielded solutions implying the existence of electrons with negative kinetic energy.

Aside from the unclear physical meaning of a negative-energy state, if such states were real it implied that electrons would be unstable because they would descend inexorably into and downward through these states. To avoid that cataclysmic consequence, Dirac posited an infinite “sea” of negative-energy electrons that blocked the entry of positive-energy electrons, thanks to the Pauli exclusion principle. In that scenario, negative-energy electrons could ascend from the sea to a positive-energy state, creating a detectable electron while leaving a “hole” in the sea, and, occasionally, electrons could also disappear into such holes. Reinterpreting the holes as positively charged particles, this anticipated the creation and annihilation of electron-positron pairs.

This was all deeply strange and there was little sense it was a breakthrough simply awaiting confirmation. Writing in Physics Today in 2019, physicists Mats Larsson and Alexander Balatsky traced Dirac’s path to the 1933 Nobel Prize. This was at a time when the Nobel Committee for Physics faced broader quandaries in deciding how to recognize even the better-established achievements of quantum mechanics in the absence of what the committee considered to be proper experimental tests. Although Werner Heisenberg and Erwin Schrödinger were obvious candidates, the committee’s paralysis on the matter actually prevented it from awarding any prize for physics in both 1931 and 1932.

Larsson and Balatsky make clear that Dirac was never under serious consideration until very late in this period, having been nominated just once in 1928 and then twice at the beginning of 1933. In March 1933, a report by Nobel committee member Carl Wilhelm Oseen rejected Dirac’s nomination, and Dirac was still not in the running that summer. But his star was rising, and finally his name was handwritten into the committee’s September 25 typescript recommendation, which retroactively awarded the 1932 prize to Heisenberg and split the 1933 prize between Dirac and Schrödinger.

To understand what exactly happened during 1933, we can turn to an excellent, fine-grained account published in 1997 by historian Xavier Roqué. Roqué insists that neither Anderson’s discovery claim nor a follow-on cosmic ray study in early 1933 by Patrick Blackett and Giuseppi Occhialini proved convincing to skeptics, who were primed to resist Dirac on account of the oddity of his “hole” theory.

It is in the wake of its apparent experimental confirmation that resistance visibly crystallized to the conceptual shift that antimatter represented, and it is here that we can best understand why that resistance soon melted. Roqué documents a flurry of experimental work throughout 1933 that involved artificially producing positrons using radioactive sources, which allowed a closer study of them. Simultaneously, theoreticians elaborated on Dirac’s theory, affirming that theoretical predictions accorded with the experimental data now coming in.

Roqué argues it was this new work that made Dirac’s framework broadly palatable. Niels Bohr, a key opponent, could dismiss Blackett and Occhialini’s “pathological photographs” in May, but in late November he conceded, “It now really seems as if the hole theory has been brought to a preliminary harmonic conclusion.”

Such histories of conceptual acceptance are worth pinning down because not every conceptual shift was so fortunate as to secure such reasonably swift assent, and it is helpful to understand the reasons why some ideas catch on and others do not. After all, quantum entanglement, articulated in 1935, waited roughly six decades before it moved into the mainstream of research.

Antimatter and science fiction in the 1930s

Once the dust settled, physicists quickly started describing antimatter’s strange behaviors with the nonchalance to which we are accustomed. Meanwhile, some science-fiction authors started to play up the subject’s dramatic side almost immediately.

Bill Higgins emailed following our inaugural Weekly Edition to note that, contrary to our wording, he has never suggested the 1942 story “Collision Orbit” by Will Stewart (real name Jack Williamson) was the first sci-fi story to feature antimatter. He believes that distinction goes to Nat Schachner’s “The Great Thirst,” published in the November 1934 issue of Astounding Stories. In it, a positron beam is discovered to be converting ordinary water into impotable heavy water, and an electron beam is ultimately used to explosively counteract it—bad science, but, per Higgins, “serviceable pulp fiction.”

Other early appearances of antimatter included Frank Belknap Long’s “The Roaring Blot” (March 1936), John D. Clark’s “Minus Planet” (April 1937), Kent Casey’s “Flareback” (March 1938), and Schachner’s “Negative Space” (April 1938).

Higgins further notes that, later in his life, Isaac Asimov specifically cited “Minus Planet” as an example of a work that made him appreciate how science fiction can thrive by drawing on new scientific developments. In 1941, Asimov himself slipped in a reference to antimatter in his second “robot” story, stating that robots had “positronic” brains. He later explained that the positron had a “science fictional flavor about it” and that “positronic” sounded more “exotic and futuristic” than “electronic.”

Higgins’s work on this subject is detailed in his essay, William S. Higgins, “The Road to Seetee,” a version of which was included as an introduction to a small-print-run chapbook edition of Williamson’s story Opposites—React!

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William Thomas
American Institute of Physics
wthomas@aip.org

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