From "Electrum" to Positronium Helge Kragh Department of History, Cornell University, Ithaca, NY 14853 Positronium chemistry is today an established and promising branch of chemistry, with important applications in such diverse fields as thermodynamics, solid state chemistry, gas reactions, biology, and analytic chemistry. The exotic positronium atoms have been found to be sensitive probes even in the study of pollution resulting from photochemical reactions in air ( I ) . Discovered in 1951, positronium-the unstable "atom" composed of an electron and a positronbecame of interest to chemists in the early 1960's. In 1964 the first monograph on positronium chemistry appeared, to he followed by several later works ( 1 , 2). Few researchers, whether chemists or physicists, are aware that although the name "positronium" first appeared in 1945, the first model of an electron-positron system was introduced in 1934. The present paper outlines the early history of the positronium concept, roughly from 1934 to the discovery in 1951. Special attention is given to the work of Stjepan Mohorovicic, an almost unknown Yugoslavian physicist who first suggested the existence of positronium. Although this work was rather weird bvmodern standards of science (or. for thatmatter.. bv. contemporary standards) and did not have any influence at all on the later develooment, it is interestinn in its own right. Apart from being thefirst paper on positrokum, i t is representative of a large body of speculative literature on the fringe of more conventional science.
ogy with modern scienceand occult wisdom. Like Mohorovicic. Reuterdahl oooosed Einsteinian relativitv. which he declared materialis~~c and unsuited for spirituai &e (5). Electrum and Other Electron Elements In his Nobel Prize lecture of December 1933, Paul Dirac speculated that perhaps remote parts of the universe were made up of antimatter, atoms built up of positrons and negative protons (6). Dirac's idea seems to have been the immediate inspiration for Mohorovicic to consider electronpositron atoms in 1934 (7). However, Mohorovicic's "positrons" were not Dirac's antielectrons, founded in quantum mechanics, hut just mirror particles of the negative electrons. He referred approvingly to the atomic speculations of Reuterdahl, who in 1923 had postulated positively charged mirror electrons called "positons", and claimed that Reuterdahl's system was vindicated by the discovery of the positron. Mohorovicic first considered the simplest electron-positron svstem. two oarticles of eaual mass revolvine around b their ienter'of mass under the influence of ~ o u l o i forces (see fieure). . Essentiallv hvcoovinr Buhr's 1913 oaoer on the hydrogen atom, he fo&dthea&h&l radii and ~ h ~ p e c t r u m of the system. The wavelengths were found to follow the
Mohorovicic
Stjepan Mohorovicic was born in Bakar, Croatia, in 1890, a son of the geophycisist Andrija Mohorovicic. He obtained his PhD in 1918, and from 1923 to 1939 he occupied amodest position as gymnasium professor in scientifically and culturally isolated Zagreb, the capital of Croatia. During this period he published widely on topics in physics, philosophy, chemistry, and astronomy. Most of his writings were puhlished in ohscure Yugoslavianjournals, but he also published in recognized German periodicals such as Annalen der Physik, Zeitschrift fiir Physik, and Astronomische Nachrichten. From an early age he established contact with German physicists belonging to the ultraconservative opposition to relativity and quantum theory. Led by Phillip Lenard, Johannes Stark, and Ernst Gehrcke, reactionary German scientists exerted considerable influence in the early '20's when they strongly attacked Einstein's theory of relativity. Mohorovicic areued in several works that Einstein's theorv was philosophically monstrous and without experimental foundation, and like other antirelativists he believed in the exisan aether (3). As an alternative to relativity, he tence developed a corpuscular aether theory that included particles with negative mass (4). Characteristic of his speculative mind and distance from mainstream physics, Mohorovicic was greatly impressed by the metaphysical system of Arvid Reuterdahl, which he treated as though it were a serious contribution t o physics. An American engineer, philosopher, and mystic, Reuterdahl wrote in the early part of the century on scientific, occult, and religious suhjeets, which he attempted to unite in a synthetic world view. Reuterdahl was a fellow of the American Association for the Advancement of Science and the History of Science Society and also of the Keplerbund, a German organization that aimed a t unifying Christian theol-
of
196
Journal of Chemical Education
Mohomvicic'sdrawings of electrum (above) and nobillurn (below). From ref 7.
generalized Balmer formula with half the Rydherg constant, L.e.,
wavelengths twice as great as the hydrogen lines. He urged experimentalists to look for the spectrum and wrote "I think no physicist will doubt the existence of these unstable hydrogen-like atoms. Direct detection may prove difficult, but the great interest of such observations requires no empha-
where R, is the Rvdherg constant for an infinite nuclear mass. This result fbllows-immediately from Bohr's theory, where the Rydherg "constant" is given by
sis."
but Mohorovicic rederived it in his own version of atomic theory. Far from considering the electron-positron system to he unstable because of annihilation. he sueeested that it was a stable atom of a new rhemical element that he named "electrum" (chemical symbol Ec). The atomic weinht of electrum would he 0.0010863 in units of the hydrogenatom, and because of its large diffusion velocity it would not be part of the Earth's atmosphere. But Mohorovicic speculated that it might well be part of the Sun's corona or exist in the interior of stars. His argument in favor of electrum and other ultralight elements were curiously similar to earlier speculations of aetherial, suhhydrogenic elements such as proposed by Mendeleev and others 30 years earlier (8). Mohorovicic saw no reason why only one positron and one electron should form an atom and went on to consider several electrum isotooes as well as other hmothetical atoms made up of posiiive and negative elect&. He believed these to form a new groupof"abaric"elements that might he detected spectroscopically, but wisely avoided placing them in the periodic system. Apart from electrum, he suggested the existence of "nobilium" IiXbi. . .. nuclear cbaree 2t., and "slavium" (SI) with nuclear charge 3+. For example, the nobilium isotope of atomic weight 0.00436 would consist of a nucleus of two electrons and four positrons, surrounded by two neeative satellites. Mohorovicic calculated the soectral lines of ions of these elements and believed that certain stellar lines could he explained in this way. His position far away from mainstream science is illustrated by his suggestion that the elements "coronium" and "nehulium" might be identical with excited states of slavium. Coronium and nehulium were sometimes assumed to exist in celestial bodies in order to account for unidentified spectral lines, but after Bohr's atomic theory they held little credibility, and by 1934 thev had been discarded for manv vears. ~ o h o r o v i c i promised c to d e v e i 6 his weird hypothesis in further articles, but nothing came of this, and nobody else felt inclined to take it up. Prohahly the hypothesis, more in line with 19th-century chemical speculation than with 20thcentury physics, was-not taken seriously, if noticed a t all. Not only did the antirelativist Mohorovicic have a poor reputation, but his work appeared in an astronomical journal not well known to physicists and chemists. Apart from a skeotical review in The Obseruatorv (9). . .. the sueeestion of "el~ctrum"went unnoticed for more than 15 yea;: u~
~~
.
Posltronlum and Poly-Electrons In the context of standard quantum physics the notion of positron-electron atoms was first entertained by Arthur E. Ruark at the Naval Research Laboratory. He conceived the idea in 1937 but only puhlished i t eight years later when he also suggested the name "positronium" for "an unstable atom com~osedof a ~ o s i t r o nand a neeative electron" (10). The reasdn for the delay was, accordGg to Ruark, that i'n 1937 he realized that the short lifetime would make spectroscopic detection of positronium formed by cosmic ray positrons almost impossible. After the war. when the betatron offered strong hkams of positrons, he believed positronium to be within the range of detection. In his brief letter to Physical Review, Ruark noted, as Mohorovicic had done earlier, that the positronium spectrum would have lines of
Without knowing of Ruark's idea, John Wheeler composed a t the same time a more elaborate scheme for what he called "poly-electrons" (11). These were electron-vositron atoms, thesimplest-called "bi-electron"-heing ibentical with Ruark's positronium and Mohorovicic's electrum. Wheeler also considered higher poly-electrons similar to Mohorovicic's aharic elements. However, unlike his Yugoslavian colleaeue. - . he discussed them as ouantum svstems and suggested that poly-electrons might provide an understanding of the structure of mesons. For the bi-electron he calculated the binding energy against dissociation to 6.8 eV and the lifetime for the singlet state to 1.2 X 10-10 s: the triplet states had much longer lifetimes, about loL6 s. Wheeler suggested various experiments by means of which the poly-electrons might he detected and pointed out that the large Doppler broadening (a result of the small mass) might make direct spectroscnpic detection impossible. He proposed that the Doppler problem might be circumvented hy looking instead for compounds such as e'CI-. The stability of atomic or molecular systems involving positrons was examined by Aadne Ore at Princeton University (12). Ore concluded that in some cases a d)?ramically stable system mirht beobtained by replacinn the proton ina molerule bv a - - - - ~ -. ~
Contrary to the expectations of Mohorovicic, Ruark, and Wheeler, the eventual discovery of positronium did not rely on derection of its spectral lines but on measurements of the annihilation rate as a function of gas pressure. I t was with such measurements that preliminary evidence for positronium was reoorted in 1949 (13). Two vears later Martin Deutsch cokd report to have found "some definite proof of the abundant formation of positronium, the bound electronpositron system analogous to the hydrogen atom" (14). Deutsch and his collaborators were a t first unaware of Mohorovicic's contribution, and i t was only in 1953 that his ~ r i o r i t ywas acknowledged (15). ~ohorovicicwas not a great physicist, but neitherwas hea crank. His publications show that he had a solid knowledge of ohvsics and that be mastered difficult theoretical as well as kxperimental subjects. Opposed to relativity and quantum theorv for ~hilosoohicalreasons. he became an outsider withbut idflueice on mainstream physics. His prediction of positronium was speculative and lacked a proper theoretical foundation. But, as the history of science documents, speculations are often the stuff of which discoveries are made. That this did not happen in the present case was more a result of Mohorovicic's position as an outsider in physics than of the speculative nature of his hypothesis. Literature Cited 1. Ache, H.J.. Ed. Posiironivm and Muonrum Ch~misfry:American Chemical Society: Washington. DC, 1979.Sehrader, D. M.;Jean, V. C.,Eds.PositronondPositronium Chemistry: Elwier: Amsterdam, 1988. 2. Green,J.; Lee, J. Poailmnium Chemislry;Academie: New York, 19M. 3. Mohorovicic. S. In Handbuch dsr physikoliarhm Opfik: Cehreke. E., Ed.; Leiprig, 1928: Val. I. pp 917-1013. 4. Mohorovicic,S. ZPhyr. 1923,18,34-63:Astron.Noehr. 1926,227,230-240. 5. Reuterdahl. A. The Cod of Scisncas; Arys: Minneapoli~.1928. The Physical Boris of the Cr~nstentVdoeily of Li#ht;Arya: Minneapoiis, 1929. 6. Dirac, P. A. M. In Nobsl L e c l u ? ~ .Physics 1922-41: Elsevier: Amsterdam. 1965: pp 32&326. 7. Mohorovicic. S. Asfmn. Nochr 1934.253.93-108. 8. Van Spronrcn. J. W. The P~?iodicSystem of the Chemical Elements: Ekevier: Amsterdam. 1969: oo 229-231. 9. Wooley R . T ~ *dkervolory1934.57.388-390. 10. Ruark,A.E. Phvs ROU.1945.68.278.
11. Wheeler. J . A . A ~ ~ . N . Y&i. . A1946,48,219-238. ~~~. 12 0re.A. Pkya Re". 1948.73,I331-I318, 13. Oeufsch. M.:Shearer. J. Phys Re". 1949.76.462. 14. Deutsch, M. Phys. Re". 1951,82,455-456. 15. Deutsch, M.Progr.Nucl.Phys.1953.3. 131-158.
Volume 67 Number 3 March 1990
197
