A FORUM OF THE AMERICAN PHYSICAL SOCIETY • VOLUME XIV • NO. 3 • FALL 2019

“Francis Perrin’s Early-1939 Analysis of Uranium Criticality: A Look Back”

By Cameron Reed

Introduction the outset of his paper that he was interested in the possibility In the May 1, 1939, edition of Comptes Rendus, the weekly of explosively liberating large amounts of energy (“libérant journal of the French Academy of Sciences, physicist Francis une énergie énorme”). In February, 1939, Niels Bohr posited Perrin (1901-1992; Fig. 1) of the Collège de published that it was the rare 235 isotope of uranium that was respon- a 3-page paper containing the first numerical estimate of sible for slow-neutron fission; perhaps Perrin was thinking the critical mass necessary to achieve a fast-neutron chain that the much more abundant 238 isotope might suffer fast- reaction in a natural-abundance uranium compound [1]. neutron fission if there was some energy threshold involved. His result was about 40 metric tonnes (40,000 kg), which he Bohr’s hypothesis had been made in response to the differing estimated could be reduced to about 12 tonnes with a suit- fission properties of thorium and uranium uncovered by able tamper. He did not seem particularly concerned with Otto Frisch and others [3, 4]; Perrin referenced neither Frisch the enormous scale of this result – which would later prove nor Bohr, but it is hard to imagine that he would have been to be about three orders of magnitude too large – but he did unaware of their work. Further research over the following remark that since a fast-neutron reaction would not be easy to year by Bohr and John Wheeler (among others) would reveal control, a better hope might be to use hydrogenic compounds the poisoning role of inelastic scattering and capture of neu- to slow neutrons and so achieve a controllable reaction. trons by U-238. Perrin’s calculation was soon eclipsed by deeper under- Perrin considered uranium oxide (U3O8) of natural standing of the mechanism of fission; it had no real relevance isotopic abundance, and treated a chain reaction as an for a Hiroshima-type nuclear weapon, which requires essen- tially pure U-235. He was, however, apparently the first to Continues on page 4 publish a diffusion-theory-based quantitative estimate of the critical mass (“masse critique”), so it is interesting to take a retrospective look at his work to see both how his numbers arose and to explore the impact of his paper. As I will show, In This Issue Perrin’s physics was largely correct: Had he considered using pure U-235 and been armed with even rough numbers Francis Perrin’s Early-1939 Analysis for the relevant cross-sections, he could have made a quite of Uranium Criticality: A Look Back 1 respectable estimate of the critical mass for a nuclear weapon nearly a year before Otto Frisch and Rudolf Peierls alerted FHP Essay Contest Winners 2 British authorities to the possibility of “super-bombs” in their famous memorandum of March, 1940 [2]. In this article I examine Perrin’s model, make some speculations as to the provenance of his parameter values, and consider how his March and April 2020 FHP Sessions 2 work compared to later efforts.

Perrin’s criticality model April Session Reports 3 To set the context of Perrin’s work, it is helpful to briefly summarize what was known of fission in the spring of 1939. Otto Hahn and Fritz Strassmann had discovered fission via Book Review 8 slow (moderated) neutron bombardment of uranium, but Per- rin considered fast neutrons, a choice which may have been conditioned by the fact that he would not have to deal with Officers and Commiittees 9 the complicating effects of a moderator; also, it is clear from

Revised 12/19/19 FHP Essay Contest Winners

By Paul Cadden-Zimansky

he FHP Student Essay Contest received a record number of entrants in 2019, with submis- Tsions from four continents and essay writers ranging from high school stu- dents to graduate students. The win- ning essay “A Changing Dichotomy: The Conception of the ‘Macroscopic’ and ‘Microscopic’ Worlds in the Histo- ry of Physics” was submitted by Zhixin Wang, a graduate student in applied physics at Yale University. Wang's essay examines scientists' shifting views on the what distinguishes micro and macro over four centuries, from seventeenth century hypotheses of hid- Zhixin Wang, a graduate student in applied Melia Bonomo, a Ph.D. candidate in applied den mechanical mechanisms as expla- physics at Yale University physics at Rice University nations for visible phenomena to more contemporary distinctions where it is Rice University. Bonomo's essay charts Bonomo will receive a $500 cash award not size, but quantum metrics that are the career of the largely unknown Stone for her work. Both essays are posted often appealed to. For winning the con- who, under the supervision of Albert on the FHP's website at aps.org/units/ test Mr. Wang will receive a cash award Michelson, became the first American fhp/essay/index.cfm of $1,000, plus support for travel to an woman to be awarded a physics doc- APS annual meeting to deliver a talk toral degree in 1897 and went on to be based on his essay. This year's runner- a co-founder of the American Physical up essay "Isabelle Stone: breaking the Society while making significant con- glass ceiling with thin films and teach- tributions to multiple schools of higher ing" was submitted by Melia Bonomo, education that served women. Ms. a Ph.D. candidate in applied physics at

The Forum on History of Physics of the American Physical Society pub- March and April 2020 FHP Sessions lishes this Newsletter biannually at http://www.aps.org/units/fhp/newslet- ters/index.cfm. Each 3-year volume consists of six issues. March 2020 FHP Sessions April 2020 FHP Sessions The articles in this issue represent the views of their authors and are not necessarily those of the Forum Monday 3/2 2:30 pm Sunday 4/19 8:30 am or APS. History of Materials Science Science Policy and Science’s History Editor Tuesday 3/3 8:00 am Sunday 4/19 1:30 pm Robert P. Crease History of Physics and Computation History of Cosmology Department of Philosophy (cosponsored with DCOMP) Stony Brook University Sunday 4/19 3:30 pm Stony Brook, NY 11794 Friday 3/6 8:00 am Pais Prize Session [email protected] History of Physics in India (631) 632-7570 Monday 4/20 1:30 pm Author in Dialogue: Jeff Bub, Assistant Editor Bananaworld: Quantum Mechanics for Don Salisbury Primates Austin College [email protected]

2 Volume XIV, No. 3 • Fall 2019 • History of Physics Newsletter april session reports: “Secrecy and Espionage in Science”

By Dan Kennefick

t the April 2019 APS meet- vision of intelligence gatherers central- Acquire German Science and Technol- ing the FHP and the Forum izing the latest scientific knowledge ogy during and after the Second World on Physics and Society co- in government hands never became a War.” He is the author of the forthcom- Asponsored a session of invited talks reality, but considerable penetration of ing book Taking Nazi Technology: Allied on Secrecy and Espionage in Science. the scientific community by the intelli- Exploitation of German Science after the There was considerable interest in the gence apparatus was achieved. In her Second World War. His talk was focused topic, with the room being packed. view “The ’ commitment on the spoils of German science and Quite a few people were standing at the to international scientific cooperation technology, which were greatly coveted back. The large and engaged audience was never primarily about scientific by the wartime Allies, who hoped to was treated to three excellent talks. values; it was scientific international- be recompensed for their enormous The first was given by Audra Wolfe, a ism for the sake of anti-Communism.” wartime losses by benefiting from historian of science who is a member Alex Wellerstein is a historian of technology and scientific transfer from of the FHP’s Executive Committee as science at the Stevens Institute of Tech- Germany to their countries. He demon- well as being the author of Freedom’s nology and the creator of NUKEMAP strated strikingly that their understand- Laboratory: The Cold War Struggle for the (nuclearsecrecy.com/nukemap/) which ing of the problems involved in such a Soul of Science. Her talk, entitled “Scien- permits the ordinary citizen to find out transfer was often on a very high level. tific Internationalism, Scientific Intelli- how their city would fare in a nuclear There was considerable appreciation of gence, Or Both?” concerned the efforts attack. His talk was entitled “The legacy the difficulties in simply transferring made by the United States government of nuclear secrecy in the United States” science and technology. Allied powers to wrestle with competing impulses and it discussed the scientific area in often recognized that taking material in post-war science policy. On the one which the tensions addressed in Wolfe’s and personnel out of their own environ- hand they wished to maintain secrecy, talk were at their sharpest, namely ment would probably merely result in especially in the area of nuclear phys- nuclear research. In this subject even the loss of the very “know-how” they ics, and on the other hand they hoped most of the scientists agreed about the sought to acquire. The very phrase to use science as an aid to Diplomacy need for secrecy and he discussed the “know-how”, he confides, emerged in in the cold war. Quite early on they rec- early self-censorship of the small group this period. Often they preferred to send ognized that secrecy was not palatable working on fission chain-reaction in their own people to work with German to the scientists themselves and that it the early days of World War II and the research groups within Germany, hop- would be unhelpful in efforts to attract role of major figures like Oppenheimer ing to thereby transfer the knowledge scientists to the US and to encourage in formulating secrecy policy after the thus absorbed back home. The French, positive interactions between them war. One particularly interesting point he argues, saw the dangers in sending internationally. She showed how the that he made was the extent to which older scientists (too set in their ways) postwar emergence of the Central Intel- secrecy efforts during the period of the or younger people (too susceptible to ligence Agency naturally suggested the Manhattan project itself were probably putting down roots in Germany) and idea that interactions with and between mostly aimed at keeping Congress in preferred to send mid-career scientists. scientists should fundamentally be an the dark, since they could have shut Of course simply grabbing whatever intelligence matter, but that this con- the project down. Leaks as such were wasn’t nailed down (and some of what ception was, to a considerable extent, unlikely to reach the Germans in a form was) and transporting it home still had trumped by the desire to let the State which would be helpful to them and the its attractions, especially to the Soviets, department handle science as an arm of Russians were still allies at that point. who felt the most in need of an infu- international Diplomacy. Most interest- As with all of the talks, Wellerstein sion of technical equipment. It was a ing of all she shows how the Diplomats was confronted with a long queue of fascinating talk with which to end an and the Spooks could find common listeners at the microphone eager to ask engaging session. ground on the idea of the science atta- questions when he concluded. All three session contributors are ché as a scientific intelligence gatherer The final speaker was Doug- social media stars and can be followed at American embassies abroad. Internal las O’Reagan, whose talk was titled on twitter at @ColdWarScience (Audra government documents reveal that a “Stealing Nazi Science: Allied Efforts to Wolfe), @wellerstein and @D_OReagan.

Volume XIV, No. 3 • Fall 2019 • History of Physics Newsletter 3 Francis Perrin’s Early-1939 Analysis of Uranium Criticality: A Look Back

Continued from page 1

cross-section. Details of solving the at which appeared diffusion equation can be found in [6]. in the March 1, 1939, edition of the Doing so for a spherical system along Physical Review, where they report pre- with the boundary condition that criti- cisely the figure of 0.1 barns for natural- cality occurs when the neutron density abundance uranium [8]. Perrin does not at the edge of the sphere falls to zero, he reference Anderson et al., nor does he arrived at an explicit expression for the give any source for his transport cross- critical radius: sections. A paper published in the April 15, 1939 Physical Review by Anderson, , (1) Fermi, and Hanstein did report a total capture cross-section for uranium of where nU is the number density of about 5 barns, but this was for slow neu- uranium atoms, ν is the number of trons [9]. Perrin does refer to a paper neutrons emitted per fission, ands fiss the published by Hans von Halban and col- fission-cross-section for uranium under laborators in the April 22, 1939, edition u fast-neutron bombardment. With s trans of Nature wherein they reported ν ~ 3.5 o and s trans respectively representing the ± 0.7 neutrons per fission [10]. In view transport cross-sections for uranium of the still-unfolding understanding the and oxygen and nO the number density roles of U-235 and U-238 in the fission of oxygen atoms, lcore is given by process, we cannot be critical of Per- Francis Perrin (1902-1992) in 1951, Source: rin’s small fission cross-section. Curi- AIP Emilio Segrè Visual Archives, Physics ously, Frisch and Peierls would err in . (2) Today Collection the opposite direction by assuming too nU and nO are computed by first com- large a fission cross-section for U-235, 10 puting the number density of U3O8 barns, and significantly underestimat- application of diffusion theory, exactly molecules from its known bulk density ing the critical mass. Since the critical the appropriate physics. He would have and atomic weight, and multiplying by radius behaves as R~1/ρ√sfisss trans, small come to diffusion physics from years of factors of three and eight. The true den- changes in parameter values can have -3 expertise: his father, Jean Perrin, was sity of U3O8 is about 8.4 gr cm ; Perrin big effects on the mass. awarded the 1926 Nobel Prize for Phys- assumed 4.2 gr cm-3. ics for his experimental determination Equation (1) appeared in Robert Ser- Effect of tamper of Avogadro’s number, and Perrin fils’ ber’s Los Alamos Primer four years later, Perrin then turns to the idea of sur- 1928 doctoral thesis was on the topic of in April, 1943 [7]. As Serber explained, rounding the fissile core with a spheri- Brownian motion [5]. demanding that the neutron density cal metallic neutron-reflective tamper Perrin considered the simplifying goes to zero at the edge of the bomb of thickness L. This has the effect of case where the mean free path of the core is too restrictive a condition, and lowering the critical mass by returning neutrons is small in comparison to the ends up leading to overestimating the neutrons that would otherwise escape characteristic size of the sample. While critical mass. However, it does have back into the core and giving them fresh not strictly true for nuclear weapons the advantage that it leads to equation opportunities to induce fissions. He materials, this is a commonly-made and (1) for the critical radius; otherwise, then presented the following transcen- reasonably accurate assumption. This one has to deal with a transcendental dental equation for the critical radius R: allowed him to write a simple expres- equation. u sion for the mean free path in terms of Perrin adopted s trans = 6 barns (mod- . (3) u the number densities of uranium and ern value for U-235 = 5.8), s trans = 2.0 oxygen atoms and their cross-sections barns (2.7; Perrin neglected any neutron Here, lcore is as in equation (2). ltamp for neutron transport and fission; this capture by oxygen), sfiss = 0.1 barns (1.2), depends on the nuclear number den- is equation (2) below. His terminol- and ν = 3 (2.6). These numbers give lcore sity and transport cross-section of the ogy is somewhat confusing in that he = 9.8 cm (Perrin claims 10 cm), and R = tamper material: uses the term “absorption” to refer to 134 cm, corresponding to a mass of just both the fission cross-section of ura- over 42,000 kg. . (4) nium and the capture cross-section But for his density and fission cross- of a tamper material or the oxygen in section, Perrin’s numbers are respect- a and b both have units of inverse U3O8, and defines what we now call the ably close to what we would use today length, and depend on the properties transport cross-section, the sum of the for U-235. The source of his fission of the core and tamper materials. But cross-sections for fission (if possible) cross-section may have been a paper by for a factor of p, a is the reciprocal of the and elastic scattering, as the “diffusion” Herbert Anderson and his collaborators critical radius of equation (2):

4 Volume XIV, No. 3 • Fall 2019 • History of Physics Newsletter , (5) differ drastically, and taking an average As for Perrin, he enjoyed a distin- of his results gives a critical mass only guished career after the war, remaining about 14% high when compared to the as a professor at the Collège de France and results of later, more sophisticated Los until 1972, serving as the High-Com- Alamos diffusion theory [13]. Peierls’s missioner of the French Atomic Energy , (6) paper contains no numbers: He did not Commission from 1951 to 1970 (during tamp where scapt is the capture cross-section estimate a critical mass. which time that country developed for the tamper material. In his memoirs, Peierls related that its own nuclear weapons), and being By considering the analysis of after refining Perrin’s calculation, he involved with the discovery of the Oklo tamped cores presented in ref. [6], I have had some misgivings about publishing, natural reactor in 1972. He passed away determined that equation (3) results and claims that he consulted with Otto in on July 4, 1992. from a standard solution to the neutron- Frisch on the advisability of doing so. I am grateful to former Alma College diffusion equation in conjunction with Frisch was confident that Niels Bohr student Cory Townes for translating Per- assuming the continuity of the neutron had shown that an atomic bomb was rin’s paper. density and flux at the core/tamper not a realistic proposition, and advised interface and that the neutron density Peierls that there was no reason not to References goes to zero at the outer edge of the publish [14]. A few months later they [1] F. Perrin, “Calcul relative aux conditions éventu- tamper, again a more restrictive but would find themselves in a very differ- elles de transmutation en chaîne de l’uranium,” simpler condition than what applies in ent circumstance. Comptes Rendus 208, 1394-1396 (1939). actuality. Perrin chose iron as the tam- In the Los Alamos Primer, improved [2] J. Bernstein, “A memorandum that changed per, taking a thickness of 35 centimeters, cross-section data allowed Robert Ser- the world,” Am. J. Phys. 79(5), 440-446 (2011). tamp tamp and adopting strans = 3 barns, and scapt = ber to estimate a critical mass of ~ 200 [3] N. Bohr, “Resonance in Uranium and Tho- 0.05 barns. He may simply have made a kg, which he reduced to ~ 60 kg upon rium Disintegrations and the Phenomenon of lucky guess here: the modern value for considering a more refined boundary ,” Phys. Rev. 55, 418-419 (1939). the fission-spectrum averaged elastic- condition. Serber also considered a [4] O. R. Frisch, “Physical Evidence for the Divi- scattering cross-section for neutrons on tamper, although he did not incor- sion of Heavy Nuclei under Neutron Bom- bardment,” Nature 143(3616), 276 (1939). iron is in fact almost exactly 3 barns; the porate any capture-cross section for radiative-capture cross-section is about the tamper material, which changes [5] A copy of Perrin’s thesis can be found at tel.archives-ouvertes.fr/tel-00278195/ 3.4 millibarns. In any event, the situa- the form of the solution for the diffu- document tion is still impractical: with the modern sion equation from what Perrin used. value for the density of iron (Perrin did Since the Primer was not (originally) [6] B. C. Reed, The Physics of the Manhattan Project (3rd ed., Springer, Berlin, 2015). See Chapter 2. not specify what value he used), this intended for publication, it contains reduces the critical mass to just over no references; we can have no idea as [7] R. Serber, The Los Alamos Primer (Berkeley, University of California Press, 1992), Sect. 10. 14,000 kg, but the tamper itself would to what extent Serber was aware of or have a mass of over 42,000 kg. influenced by the work of Perrin or oth- [8] H. L. Anderson, E. T. Booth, J. R. Dunning, E. ers. That the same problem would be Fermi, G. N. Glasoe, and F. G. Slack, “The Fis- sion of Uranium,” Phys. Rev. 55, 511-512 (1939). Subsequent developments considered in largely the same way by Two other papers on critical condi- two very competent theoretical physi- [9] H. L. Anderson, E. Fermi, and H. B. Hanstein, “Production of Neutrons in Uranium Bom- tions soon followed Perrin’s. In the June cists is not surprising, but it is striking barded by Neutrons,” Phys. Rev. 55, 797-798 6, 1939, edition of Naturwissenschaften, to see the connection from 1939 to 1943. (1939). Seigfried Flügge presented a much The overall conclusion is that Per- [10] H. von Halban, F. Joliot, and L. Kowarski, lengthier analysis of the prospects for rin conceived very clearly the concepts “Number of Neutrons Liberated in the Nucle- realizing nuclear energy [11]. He was of both critical mass and tampering, ar Fission of Uranium,” Nature 143 (3625), much more concerned with neutron and formulated the physics correctly 680 (1939). resonance-capture phenomena; he but for the boundary-condition issue. [11] S. Flügge, “Kann der Energieinhalt der Atom- did not reference Perrin, and does not The details would evolve, but by the kerne technisch nutzbar gemacht werden?” appear to have estimated a critical mass. opening days of World War II the key Naturwissenschaften 27, 402-410 (1939). For English-language readers, the most formulations had appeared in the [12] R. Peierls, “Critical Conditions in Neutron interesting impact of Perrin’s work open literature. In view of this, Werner Multiplication,” Proc. Camb. Philos. Soc. 35, can be found in a paper published in Heisenberg’s initially very muddled 610-615 (1939). October, 1939, by Rudolf Peierls [12]. presentation of the issue during his [13] B. C. Reed, “Rudolf Peierls’ 1939 Analysis of Peierls, who explicitly referenced Per- postwar captivity at Farm Hall remains Critical Conditions in Neutron Multiplica- rin, simplified the situation by work- a mystery; his claim that he never stud- tion.” Physics & Society 37(4), 10-11 (2008). ing with pure uranium and no tamper, ied the question of critical mass because [14] R. Peierls, Bird of Passage: Recollections of a took an integral-equation approach, he did not think it possible to separate Physicist (Princeton: Princeton University Press, 1985), 153. and developed approximate expres- U-235 does not seem fully convincing. sions for the critical radius for when The mystery is only deepened by the [15] Bernstein, “Heisenberg and the Critical mass,” Am. J. Phys. 70(9), 911-916 (2002). the number of secondary neutrons per fact that an unsigned 1942 report to fission is either very close to unity or German Army Ordnance seems to indi- very large. In the regime of practical cate that somebody must have looked interest, the two approximations do not at the issue very closely [15].

Volume XIV, No. 3 • Fall 2019 • History of Physics Newsletter 5 april session reports: “Remembering Julian Schwinger” (DPF co-sponsor)

By Donald Salisbury

elebrating somewhat tardily the centennial birthdate of Julian Schwinger on February C12, 1918, roughly forty attendees were treated to personal reminiscences on one of the leading theoretical physi- cists of the twentieth century. Touch- ing a theme that would be repeated by all three presenters, Michael Lieber, whose title was “Divergence”, was the first to recall the mesmerizing effect of Schwinger's lectures, characteriz- ing them as like virtuoso violin per- formances. He asked Schwinger two years after having commenced his graduate studies at Harvard in 1957 Michael Kiefer, Kimball Milton, and Howard Georgi if he would oversee a dissertation, and he agreed. Lieber experienced a for UCLA. Milton subsequently joined and there was also the failed chemistry couple of ``misstarts" involving first him at UCLA for the rest of the 1970's course at Columbia. the 1955 renormalization approach of as his post-doctoral assistant. He has Perhaps less known of Schwinger's Bogoliubov and Shirkov, and then the co-authored two textbooks based on myriad accomplishments is the paper Euclidean TCP theorem. After a respite Schwinger's lectures. In addition he co- he wrote at age 16 at City College in in industry Lieber returned to Har- wrote, with Jagdish Mehra, Climbing the which he effectively introduced the vard and to Schwinger. The latter had Mountain. A Scientific Biography of Julian field theoretic interaction representa- famously exploited the link between Schwinger. The authors interviewed tion. (This unpublished paper appears Lorentz transformations and Euclid- several of Schwinger's friends and in a collection edited by Milton.). The ean rotations in his 1957 paper. He had associates, and Milton recounted many central role of nuclear physics in his of course also very early emphasized of their recollections in his remarks. Par- early career has also tended to be unap- an almost universal role for Green ticularly striking to me was a tale I had preciated. Seven papers written under functions, and the link between these heard from my own teacher and friend Rabi's guidance constituted his disserta- themes formed a basis of Lieber's dis- at Syracuse University, Joe Weinberg. tion at Columbia. Most noteworthy was sertation, completed in 1967, in which While a student at the City College of his correct prediction that the deuteron he constructed a Coulomb Green func- New York the fifteen year old Weinberg possessed an electric dipole moment tion by exploiting the O(4) symmetry had found himself competing in the arising through a broader isospin inter- of the non-relativistic hydrogen atom library with another youngster of the action. Already during his prewar stay as analyzed by Schwinger in 1964. The same age as they retrieved a mathemat- with Oppenheimer and his wartime result was an alternative derivation of ics book on real variables. Schwinger appointment to the MIT Radiation Lab- Schwinger's original Lamb shift cal- was the faster reader, but Weinberg oratory he developed insights into the culation. Not incidentally, this project was no slouch. They ended up as life- renormalization program in quantum also involved the use of variational long friends, actually overlapping for electrodynamics that would earn him a techniques he had learned from his a time with Oppenheimer at Berkeley share of the 1965 Nobel prize in physics. advisor, and it was Schwinger himself before the war. It was soon after that His work with synchrotron radiation who recommended that since Larry Schwinger came to the attention of not only taught him the importance of Spruce at NYU was working on varia- Rabi who arranged a transfer as an Green functions, but also suggested to tional techniques related to scattering undergraduate to Columbia Univer- him that electromagnetic interactions theory, he should join him there with a sity. The stories of the gentle prodigy at could be interpreted as altering both post-doctoral appointment. both institutions are legion and Milton the mass and the charge of accelerating The title of Kimball Milton’s could not begin to cover them. But he particles. talk was “A Remembrance of Julian did include the episode at CCNY when Howard Georgi spoke on “Julian Schwinger”. He was a PhD student Schwinger stopped abruptly before the Schwinger at Harvard before June, of Schwinger at Harvard beginning in hurdle in the compulsory gym exam, `67 – remembered by a star-struck 1968. He received his doctorate in 1971, lamenting that there was ``not enough the year that Schwinger left Harvard time to solve the equations of motion", Continues on page 9

6 Volume XIV, No. 3 • Fall 2019 • History of Physics Newsletter april session reports: “Centennial of the Eddington Eclipse Expedition” (DGRAV co-sponsor)

By Donald Salisbury

ver a hundred conference participants gathered almost exactly one hundred years after theO date in March 1919 when two Eng- lish expeditions embarked from Liv- erpool to test Einstein's theory during the solar eclipse of May 29. The posi- tive results reported by Arthur Stanley Eddington and Sir Frank Watson Dys- on at a combined London meeting of the Royal Society and the Royal Astro- nomical Society on November 6, 1919, as reported in the New York Times with the headline LIGHTS ALL ASKEW IN THE HEAVENS, brought Einstein instant worldwide fame. As the first presenter, Daniel Kennefick’s title was Daniel Kennfick [Photo by Donald Salisbury] “No Shadow of a doubt: Eddington, Einstein and the 1919 Eclipse”. He had Famous”. He reported on a remarkably Einstein's Jury. The Race to Test Relativity. recently thoroughly researched the successful optical measurement of He offered a complementary perspec- expeditions of Dyson to Sobral, Brazil starlight deflection that he performed tive on historical eclipse observations. and Eddington to the island of Principe in Wyoming during the August 21, A central player in his saga was the and recently published his studies in 2017, solar eclipse. The experiment was astronomer William Wallace Campbell the book No Shadow of a Doubt. The 1919 carefully planned over many months who was based at the Lick Observatory Eclipse That Confirmed Einstein's Theory – with the use of optimal amateur on Mount Hamilton in California. He of Relativity. Looking at correspondence astronomical equipment and analysis had planned to observe the August 21, between Dyson and Eddington he was software. The site selection itself was 1914, full eclipse in Russia, but although able to expose as a myth the claim that not accidentally fortuitous. He chose his equipment did reach his intended since Eddington believed in the correct- an altitude that reduced atmospheric site he was forced to abandon his trip ness of Einstein's theory he had made turbulence and offered high confi- because of the outbreak of World War the data conform with general relativ- dence in good seeing conditions. The I. This was doubly disappointing since ity. It is indeed true that the data from chief innovation was the use of a CCD he was then forced to employ inferior the Greenwich astrographic lens taken camera that permitted much shorter equipment in observations he actually in Brazil was excluded, but it was the exposure time and reliance on the 2016 undertook in Goldendale Washington skeptic Dyson who made this decision Gaia data release both to calibrate the in June, 1916. He and his assistant were without any input from Eddington. plate scale and to determine the stellar further hampered by their inability to The data, which seemed to indicate a image displacements during the eclipse. obtain timely comparison plates that deflection of starlight passing the rim His reported value was precisely 1.75” would enable them to compute the stel- of the sun of roughly twice the value with an extraordinary uncertainty in the lar deflections. They never did success- predicted by Einstein, was discounted deflection of only 0.05”. This is by far fully analyze and publish their results because of probable errors in analyz- the best earth-based optical measure- from 1916 though rumors occasion- ing the expansion of the photographic ment. Radio telescope quasar measure- ally surfaced, even after 1919, that their plates. The four-inch telescopic data ments are of course now able to confirm observations conflicted with Einstein's yielded 1.98” compared to Einstein's Einstein's theory with a precision of prediction. Finally in 1922 seven dif- prediction of 1.75”. Because of the par- 0.01%. This work was considered so ferent attempts were made to measure tial cloud cover in Principe only five important from an historical perspec- light deflection during the 1922 total stars were imaged as opposed to the tive that it was published in Classical eclipse in Australia. Campbell's group twelve in Sobral, and Kennefick argues and Quantum Gravity. made observations on the west coast that Eddington was justified in assign- Jeffrey Crelinsten’s title was “Ein- that were finally announced in 1924 as ing a larger uncertainty of 0.30” to the stein’s Jury: Trial by Telescope.” He being in accord with Einstein. quoted result of 1.61”. is a science writer who has analyzed Donald Bruns’ spoke on “Repeat- the early resistance of US astrono- ing the Experiment that Made Einstein mers to Einstein's theory in his text

Volume XIV, No. 3 • Fall 2019 • History of Physics Newsletter 7

Continues on page 8 book review: The Oxford Handbook of the History of Modern Cosmology by Helge Kragh and Malcolm Longair

By Cormac O’Raifeartaigh

A valuable addition to the cosmologies (by Kragh). Other chapters history of cosmology include an article on observational and astrophysical cosmology in the period t is sometimes claimed that cosmol- 1940-1980 (by Longair) and a paper on ogy, the study of the universe, is the the discovery of the cosmic microwave oldest of the sciences. background (by Bruce Partridge). Of IHowever, there is little question that particular interest to many students, cosmology only became a quantitative practising cosmologists and historians science during the 20th century. Fol- will be the chapter (by Longair and lowing the advent of Einstein’s general Chris Smeenk) on inflation, dark matter, theory of relativity and the observation dark energy and other little-understood of the first evidence of a universe in components of modern cosmology. expansion, the stage was set for cosmol- Some of the above may sound a little ogy to emerge as a mature science based familiar to readers and this is perhaps on mathematical models combined with one slight problem with the handbook. astronomical observations and deduc- Much of the material in the first half of tions from physical theory. the volume has already been presented However, the path from static to in classic books such as Kragh’s Cosmol- The Oxford Handbook of the expanding cosmologies, and from the ogy and Controversy (Princeton Univer- History of Modern Cosmology, discovery of the cosmic microwave sity Press 1996), Longair’s The Cosmic Helge Kragh and Malcolm Longair background to today’s concordance Century (Cambridge University Press (Oxford University Press 2019) model of the universe was a route with 2006) and the anthology Finding the Big many twists and turns, false trails and Bang (Cambridge University Press 2009) cul de sacs. Thus, the publication by edited by Jim Peebles, Lyman Page and theoretical cosmology from the 1960s Oxford University Press of a handbook Bruce Partridge. onwards, with only passing descrip- on the history of modern cosmology is However, it’s hard to see how a tions of the key contributions of figures a most welcome development. As edi- handbook that seeks to provide a rea- such as Roger Penrose, George Ellis, tors Helge Kragh and Malcolm Longair sonably comprehensive history of mod- Robert Dicke, Yakov Zeldovitch and point out in the preface, there are hun- ern cosmology in one place can avoid the late Stephen Hawking. It is prob- dreds of books on the emergence of the some re-working of this material. ably fair to say that the book does not Copernican model of the solar system, With this in mind, I thought it was convey a strong sense of the historical yet few scholarly accounts of the emer- a pity that the book did not feature a development of theoretical cosmology gence of today’s model of the universe. chapter on the sociological history of in the second half of the 20 th century, The book comprises thirteen dis- cosmology, i.e., on the emergence of or of the manner in which advances in tinct chapters written by an impressive modern cosmology as a distinct scien- general relativity impacted on cosmol- array of astronomers, astrophysicists, tific discipline with its own journals, ogy. Indeed, it could be argued that cosmologists and historians of science. associations, degree programs and the second half of the book presents a The chapters can be read as stand- reward system. As the editors point history of cosmology that is quite phe- alone articles but are arranged in rough out in the preface, the social history of nomenonological; this may be due to a chronological order, from Kragh’s open- modern cosmology has received almost surprising lack of theoretical cosmolo- ing article on pre-relativistic models of no scholarly attention and this would gists amongst the contributors. the universe in the period 1850-1910 to have been a valuable original addition However, these are minor quibbles. Longair’s paper on the observational to the literature. All in all, this new handbook provides and theoretical foundations of today’s A related but perhaps more serious a comprehensive and highly useful his- ‘Lambda-CDM’ model towards the end criticism concerns the description of tory of modern cosmology that many of the volume. Along the way, one finds the development of modern theoretical physicists and science historians alike articles on the development of extra- cosmology. While the story of the emer- will want on their shelf for reference. galactic astronomy and the discovery gence of dynamic cosmologies in the Cormac O’Raifeartaigh lectures in of Hubble’s law (by Robert Smith), on first decades of the 20 th century is well physics at Waterford Institute of Technology the rise of dynamic relativistic models covered in early chapters, this narrative and University College Dublin in Ireland. of the universe in the 1920s (by Mat- is quite well known. By contrast, there He is a Fellow of the Royal Astronomical teo Realdi), and on the long debate is surprisingly little in later chapters Society and a Fellow of the Institute of between expanding and steady-state on the chronological development of Physics.

8 Volume XIV, No. 3 • Fall 2019 • History of Physics Newsletter “Remembering Julian Schwinger”

Continued from page 6 and frequently confused undergradu- Schwinger can rightly be identified as own contributions, in particular with ate”. As an undergraduate Georgi took a grandfather of electroweak unifica- the Schwinger student Glashow. But he Schwinger's course in source theory tion. In 1956 he introduced the notion did stress Schwinger's early mastery of at Harvard in 1966. After graduation of hypercharge, and in 1957 he alluded group theory and his suggestion, con- in 1967 he went on to obtain his Ph. to a notion of electroweak unification - comitant with Gell-Mann in 1957, that D. from Yale in 1971 where his advi- with a V-A interaction, two neutrinos, there exists a larger global symmetry sor was Charles Sommerfeld, another and a charged intermediate boson. group for which the known baryons of Schwinger's 73 dissertation stu- These are building blocks of the eventu- formed a multiplet. The suggestion was dents. He returned to Harvard in 1971 al Glashow, Weinberg and Salam theory. that this symmetry be broken through after Schwinger had left for UCLA. Georgi modestly omitted mention of his interaction with the known mesons.

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Forum Officers Program Committee Pais Prize Committee Chair: Paul Cadden-Zimansky Chair: Joseph Martin Chair: Peter Zimmerman Chair-Elect: Joseph D. Martin Paul Cadden-Zimansky Peter Galison Vice Chair: Michel H. P. Janssen Michel Janssen Gregory Good Past Chair: Daniel Kennefick Helge Kragh Secretary-Treasurer: Nominating Committee Jinyan Liu Dwight E. Neuenschwander Chair: Daniel Kennefick Bruce Hunt Forum Councilor Allan Needell Virginia Trimble Katemari Rosa Rebecca Ullrich Other Executive Board Members Melinda Baldwin Fellowship Committee Robert P. Crease (newsletter) Chair: Michel Janssen Chanda Prescod-Weinstein Peggy Kidwell Katemeri D. Rosa A. Douglas Stone Alfred Douglas Stone Rebecca Ullrich Volume XIV, No. 3Audra Wolfe • Fall 2019 • History of Physics Newsletter 9