Why matter dominates our universe while antimatter is scarce ranks high among the great questions of our time in both particle physics and cosmology. Finding an answer could well lead to fundamental physics beyond the long-prevailing Standard Model as well as illuminate processes during the Big Bang that shaped the entire subsequent evolution of the universe.

But how did physicists arrive at this question? Helge Kragh of the Niels Bohr Institute in Copenhagen explores this history in his newly posted article in Annals of Science, “Antimatter in Astronomy and Cosmology: The Early History” (open access).

The origins and allure of antimatter

Physical understanding of antimatter traces to Paul Dirac’s relativistic application of quantum mechanics to the electron in 1928, yielding seemingly unphysical negative-energy states that he interpreted in 1931 as implying the existence of a positively charged counterpart to the electron. Shortly thereafter, such particles were photographed in cloud chamber experiments and identified with Dirac’s prediction. Winning a share of the 1933 Nobel Prize in Physics, Dirac surmised in the conclusion to his Nobel Lecture that it was accidental that the Earth, and presumably the solar system, were composed of matter as we know it, and he supposed other stars might be made of oppositely charged particles.

Kragh points out a precedent to the idea that extraterrestrial objects might be made of mirror-image matter in a “holiday dream” published in Nature in 1898 by physicist Arthur Schuster. What Schuster termed “anti-matter” would experience a gravitational repulsion from ordinary matter that might be detected in astronomical observations. In a follow-up note , Schuster acknowledged a precedent in mathematician Karl Pearson’s 1891 notion of “negative matter” that was grounded in Pearson’s own ether theory, and which he, too, suggested might prevail in places outside the solar system.

Schuster’s and Pearson’s ideas had fallen by the wayside by the 1930s, but Dirac’s framework, and its experimental confirmation, spurred a more thriving tradition of speculation. Varying significantly in their particulars, these new suppositions all had to accept the local, and perhaps universal, rarity of antiparticles due to the fact that, when they encounter their ordinary-matter equivalents, they annihilate each other, releasing highly energetic photons in accord with Albert Einstein’s equation E=mc².

Most of these new suppositions also sought, in one way or another, to extend Dirac’s theory to predict new kinds of particles and processes. For example, Kragh points to Reinhold Fürth, a physicist at the German University in Prague, who hypothesized that the then-newly discovered neutron might undergo decay and absorption processes akin to neutrons’ ordinary beta decay, but involving oppositely charged protons and electrons. George Gamow denied a negatively charged proton could exist with characteristics like the positively charged electron, but he defended the idea of a negatively charged proton that could coexist with its ordinary counterpart. Gleb Wataghin accepted the idea of a Dirac-type antiproton and predicted neutrons might also have such counterparts.

Kragh identifies University of Allahabad mathematician and astrophysicist Amiya Banerji as the first, in 1934, to posit a cosmological significance for antimatter. Building on the Lemaître-Eddington model of a universe expanding from the static state advocated by Einstein, Banerji supposed that proton–antiproton pair creation could have triggered an instability leading to expansion.

Astronomical speculations concerning what would later be called antimatter began to circulate by the end of the 1930s. Vladimir Rojansky, a Russian-American physicist at Union College in New York, coined the term “contraterrene matter” in 1935 to describe it in opposition to familiar, terrestrial forms of matter. In a 1940 paper in the Astrophysical Journal, he supposed contraterrene matter might be detected by photons produced when it was annihilated in collisions with ordinary matter—and that meteoroids and comets might carry such matter into our solar system. This latter suggestion was taken up by Ohio University astronomer Lincoln LaPaz, who supposed certain craters on Earth might have been produced by contraterrene meteors.

Kragh further details the history of speculation about antimatter in “near space” in a recent article in the Journal of Astronomical History and Heritage. As an aside: in 2008 Fermilab engineering physicist and science-fiction connoisseur Bill Higgins pointed out that public controversy surrounding Rojansky and LaPaz’s ideas about meteors inspired the first sci-fi story about antimatter. Writing under the pen name Will Stewart, writer Jack Williamson published “Collision Orbit” in Astounding Science Fiction in July 1942. It portrayed a maverick engineer—scornfully nicknamed “Seetee” after his quixotic interest in contraterrene, or CT, matter—struggling to maneuver a contraterrene meteoroid so that its energy could be safely tapped. He ultimately settled on the use of magnets (which is how antimatter came to be handled in particle experiments, albeit in much smaller quantities).

According to Kragh, physicists’ interest in cosmic antimatter began to grow in earnest after the production of antiprotons and antineutrons in particle accelerators in the mid-1950s. Sometimes these were fanciful popular expositions on the nature of antimatter. Kragh notes an item in the New Yorker in 1956, in which Edward Teller drolly described a “Teller” meeting an “Anti-Teller” in the void of space. “They can see each other without trouble, because light and anti-light are the same. Upon contact, however, a violent explosion will occur,” Teller wrote, describing a spasm of particles created in a moment “faster than anti-thought, which is probably the same as thought.” Teller again imagined his own obliteration at the 1960 IBM Junior Science Symposium, supposing he paid a visit to Andromeda, which turns out to be an “antigalaxy.”

The rise of antimatter as a cosmological problem

Brookhaven National Laboratory nuclear physicist Maurice Goldhaber was, per Kragh, the first to connect antimatter to the controversy brewing in the 1950s between the proponents of the “hot big-bang” and “steady-stage” cosmological theories (a topic Kragh has written about at length). Observing in Science in 1956 that neither theory addressed why matter should be created but not antimatter, he imagined a variation on Georges Lemaître’s 1931 “primeval atom” theory in which a single “universon” particle divided into a “cosmon” and “anticosmon” that ultimately themselves decayed, giving rise to our own cosmos as well as a separate anticosmos. Goldhaber left open the question of whether the anticosmos might still be observable, receding away from us.

As some physicists entertained variations on Goldhaber’s idea, steady-state proponents considered the prospect that the continuous matter-creation demanded by their model might create antimatter symmetrically. However, there was little sign of the high-energy photons to be expected from the annihilation of particles and antiparticles thus created. This absence became clear , Kragh states, with the launch of the Explorer 11 satellite in 1961, which carried instruments necessary to detect them.

Kragh further surveys several cosmological ideas on both sides of the Iron Curtain that dealt variously with the prospect of an anti-universe, the scientific viability of symmetric creation, and the possibility of mechanisms that might keep matter and antimatter separated. On this last point, Kragh discusses an unusual cosmology advocated by Hannes Alfvén and Oskar Klein in the 1960s in which cosmological expansion arises not from a big bang but from annihilation within a matter-antimatter plasma, and in which electromagnetic fields otherwise segregate the two forms of matter, preserving them to the present.

According to Kragh, the rapid acceptance of the hot big bang following the 1965 discovery of the cosmic microwave background decisively transformed how antimatter was thought about in cosmology, raising as a crucial issue why annihilation in the very early universe did not result in a photon-dominated cosmos. This specter of a “radiation catastrophe” swiftly arose out of calculations published by Yakov Zeldovich in the Soviet Union in 1965 and by Hong-Yee Chiu in the US in 1966 .

In his PhD thesis, summarized in Nature in 1969, American astronomer Gary Steigman considered the radiation catastrophe argument to be so clear that the triumph of big-bang cosmology sealed the case for a universe asymmetrically dominated by ordinary matter from the start. British astronomer Edward Harrison took another view, arguing in 1967 that inhomogeneities in the early universe could lead to pockets of each kind of matter. Andrei Sakharov suggested around the same time that asymmetries in fundamental physics might be responsible for the excess of matter. Then, the early 1970s saw a series of theories, similarly grounded in the physics of fundamental symmetries, that revived Goldhaber’s concept of a pair of universes created at the Big Bang.

It was, per Kragh, in the late 1970s that a variation on Sakharov’s approach began to prevail, positing a symmetric creation at the Big Bang that was quickly tilted in favor of matter by exotic physics that manifested in the conditions of the earliest moments of the universe. As examples, Kragh cites efforts by Motohiko Yoshimura and Steven Weinberg that were grounded in then-emerging Grand Unified Theory physics, and he concludes with a nod to a 1980 review in Scientific American by Frank Wilczek that optimistically suggested the antimatter problem might be solved by the century’s end.

Of course, the mystery has endured much longer.

Will Thomas
American Institute of Physics
wthomas@aip.org

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