Thomson, George Paget
THOMSON, GEORGE PAGET
(b. Cambridge, England, 3 May 1892; d. Cambridge, England, 10 September 1975)
physics.
George Paget Thomson, widely known as “G. P.” was born and bred a Cambridge physicist. His father was the famous physicist Sir J.J. Thomson (“J.J.”,) Cavendish Professor of Physics from 1884 to 1919 and subsequently master of Trinity College. His mother was Rose Paget, daughter the of Cambridge Regius Professor of Physic Sir George Paget, and herself a physics student of J.J.’s at the Cavendish Laboratory. G.P. grew up effectively an only child (his sister Joan was not born until he was eleven years old) in the privileged environment of the Cambridge elite. He attended King’s College Choir School and the Perse School, where he worn a scholarship to Trinity College. At Trinity he gained a double first in mathematics and physics, and after a year’s postgraduate work at the Cavendish Laboratory, he was elected a fellow and mathematical lecturer at Corpus Christi College in 1914. During the war, from 1914 to 1918, he served first with the Queen’s Regiment in France and then, from 1915, with the Royal Flying Corps at the Royal Aircraft Factory at Farnborough.
After a brief spell in the aircraft manufacturing company, Thomson returned to Cambridge and to Corpus Christi in 1919. In 1922 he was appointed professor of natural philosophy at Aberdeen University, and while there he married Kathleen Smith, the daughter of the principal of the university. They had four children, the youngest of whom was only two when, in 1941, after a severe illness, Kathleen died. G.P. did not remarry. Meanwhile, in 1931, he had been appointed professor of physics at Imperial College, London, and he remained there apart form wartime assignments until 1952. He then returned to Cambridge as master of Corpus Christi College, retiring from this position in 1962. Apart from a period of illness in the late 1930’s, he enjoyed generally good health and remained active into his eighties.
Thomson was elected a fellow of the Royal Society in 1930, and served as a vice president in 1950 and 1951. He received the Hughes (1939) and Royal (1949) medals of the society, together with the Faraday Medal of the Institution of Electrical Engineers (1960) and a number of other honors and awards. He shared the 1937 Noble Prize in physics with C. J. Davisson and was knighted in 1943. From the beginning of World War II onward he served on a range of government committees. Among many other activities he was president of the Society for Freedom in Science, and a vice president of the Atomic Scientists Association. Outside science and public service his main interest was in the Voluntary Euthanasia Society, of which he was a vice president.
Thomson’s early scientific work fell into two classes. Under his father’s supervision he worked at the Cavendish Laboratory and later at Aberdeen on a range of gas discharge phenomena, in particular those involving positive rays. During and immediately after World War I, he devoted himself to aerodynamics.
Thomson’s work on aerodynamics did not incorporate any great breakthroughs but was of considerable practical value. Combining the skills of mathematician, engineer, and pilot, he concentrated on the application of aerodynamic theory to practical problems of flight, achieving a degree of coherence and completeness that the subject had not previously had. After the war, encouraged by Henry Tizard, he wrote up his own and other people’s work in the form of a comprehensive textbook, Applied Aerodynamics. Having outlined the basic physical theory and experimental methods, he applied these to the aerodynamic performance of each part of an airplane’s structure, and then to the airplane as a whole. In the second part of the book, he gave a thorough mathematical analysis of aircraft stability.
For Thomson himself the work on aerodynamics provided an opportunity to hone his skills in mathematics and develop those in theoretical engineering; in later life his work drew heavily on this combination of abilities. For the interwar period, however, it was his apprenticeship in gas discharge physics, dating from before the war and leading to a series of publications from 1920 onward, that proved most relevant. In one of his earliest publications, he provided experimental confirmation of Johannes Stark’s suggestion that the observed secondary spectrum of hydrogen was to be attributed to hydrogen molecules rather than to the atoms responsible for the primary spectrum. In another investigation he developed a technique for extracting “anode rays” (secondary emissions from the surface of the anode in a discharge tube, consisting of charged metallic atoms) as beams from discharge tubes. Coating the anodes with salts of different metals, he was able, by analyzing these beams, to determine the isotopic content of lithium and other metals. In another investigation, studying the scattering of slow positive rays (protons), he found a significant discrepancy from the established theory, suggesting the existence of a strong field of force within the hydrogen molecule. More precise experiments a few years later (1925–1926) seemed to suggest an inverse cube force law between the protons and electrons in a molecule. Meanwhile, picking up on his father’s interest in the development of mechanical models of light-quanta as structures in the ether, Thomson provided experimental support for the fully localized light-quantum hypothesis as opposed to that of an elon-gated “light-dart.” Then in 1925, prompted by his reading of Louis-Cesar-Victoir-Maurice de Broglie’s theory of matter waves, he worked out a theory of the Bohr atom in which the constituent particles were described by de Broglie’s theory and the emitted and absorbed radiation by J. J. Thomson’s theory of light-quanta.
By 1926 Thomson had accumulated a good body of research, but with nothing spectacular to show for it. In the course of that year, however, his experimental work on positive rays and his theoretical work on the quantum theory began to come together. On the theoretical side he was greatly impressed by the work of de Broglie, and in the summer of 1926 he used de Broglie’s theory to compare the broadening of spectral lines owing to pressure on the classical and quantum theories. (He had not yet encountered Erwin Schrödinger’s wave mechanics, though he did so shortly afterward, and had either not encountered or not taken up the matrix mechanics of Werner Heisenberg and others.) Meanwhile, pursuit of the discrepancy between observation and theory in the scattering of slow positive rays led him to compare the observed phenomena with those noted earlier by Carl Wilhelm Ramsauer, in which slow electrons appeared to pass unhindered straight through an atomic nucleus.
Ramsauer’s results dated from the early 1920’s and were quite widely regarded as problematic, but with the quantum theory of the atom itself in a state of ferment, it had seemed difficult and premature to draw any conclusions form them. A few physicists had however linked these results with others obtained by Davisson and his collaborators, Lester H. Germer and Charles H. Kunsman, at the laboratories of General Electric in America. In work stemming originally from a patent suit, Davisson and Kunsman in 1923 had found distinctive patterns in the scattering of slow electrons by metallic surfaces, principally platinum. And in 1925 Davisson and Germer found a rather different but still more distinctive set of patterns for scattering from single crystals. Meanwhile, in the spring of 1925, in the course of an investigation of de Broglie’s theory in Max Born’s physics seminar at Göttingen, Walter Elsasser had suggested that the Ramsauer and Davisson and Kunsman results might be interpreted as evidence of the wave nature of electrons, the scattering patterns corresponding to those to be expected from wave diffraction.
Thomson did not know, in 1926, of Elsasser’s speculation. But he may have known through his father (who was however skeptical) of the Davisson and Kunsman results. And in September 1926, on his way back from a meeting of the British Association for the Advancement of Science at Oxford (at which Davisson was also present, but at which they apparently did not meet), he discussed with Dymond in Cambridge the latter’s work on the energy distribution of electrons after collisions with helium molecules. In the course of this work, Dymond, who had read Elsasser’s paper, had observed some patterns that suggested to him a wavelike diffraction of the electrons by the helium. Strangely enough Elsasser’s speculation does not seem to have been discussed by Dymond and G. P. Thomson—at least Thomson did not later recall it having been so.
But the idea of a wave theory of the electron based on de Broglie’s theory was discussed, and Thomson, already deeply interested in de Broglie’s work himself, saw how his work with positive rays might be adapted to test the idea. Reasoning that the effect should be easier to analyze with a solid than with a gaseous target, and that some of the positive ray apparatus at Aberdeen could be readily adapted for such an experiment, he asked Alexander Reid to look at the scattering of electrons through thin celluloid films (3.10−6 cms) onto photographic plates. The result was clear pictures of “halos” much as one would expect from a diffraction effect, and in a joint paper with Reid, published in May 1927, Thomson wrote of celluloid as a “diffracting system” and of the electrons as having “wavelengths” given by de Broglie’s theory. Since the structure of celluloid was unknown, the experiment was not conclusive. So Thomson next turned to metallic films of the same thickness, for which the crystal structure was known, and repeated the experiment for scattering from aluminum, gold, and platinum. He found that the dimensions of the observed diffraction patterns agreed in all cases, to within 5 percent, with those predicted by de Broglie’s wave theory of matter. These results were published in brief in December 1927 and in detail two months later. Suggestions that the phenomenon might be owing to classical X-ray effects were refuted in a subsequent paper.
Meanwhile Davisson, whose visit to England had introduced him to be Broglie’s theory, and Germer had pursued their own analysis, and in a paper published in April 1927 (before that by Thomson and Reid), they had interpreted their own results on the scattering of slow electrons by single crystals in terms of de Broglie’s thoery. But the two investigations were essentially independent. Credit for them should probably be shared not only between Thomson and Davisson (who shared the noble prize) but also between de Broglie, Elsasser, and Dymond. Davisson, it would seem had de Broglie’s theory thrust upon him in England, and given Elsasser’s speculation it required no great imagination to test the theory on his own, already existing experiments. Thomson’s work was perhaps more original in this respect, but he too was essentially developing someone else’s suggestion, in his case Dymond’s, which was itself influenced by Elsasser’s speculation.
That G. P. Thomson should have demonstrated the wave nature of the electron was perhaps ironic, in view of the fact that it had been his father who had first demonstrated its particulate nature. But it was also fitting, in that the son’s interests had followed closely those of the father. In the 1920’s they were separated by hundreds of miles, but the two components of G. P.’s electron diffraction work, quantum theory and gas discharge phenomena, were precisely the subjects in which J. J. himself was most interested. And while the electron diffraction work was going on, the two Thomsons were in fact collaborating on a new edition of J. J.’s classic work Conduction of Electricity Through Gases, published in 1928 under both their names.
As an experimental confirmation of the quantum theory, the demonstration of electron diffraction was of the utmost importance. But by the time it occurred, quantum theory itself, in the form of the new quantum mechanics, was already making rapid progress. Thomson followed the more straightforward developments of the new theory, writing the textbook Wave Theory of the Free Electron (1930) and treating Schrödinger’s wave mechanics in his popular book The Atom, published in the same year. But there was no natural development of Thomson’s work at the forefront of that research. Instead he turned to the exploration of the potential of electron diffraction as an investigative tool. First at Aberdeen and then at Imperial College, he studied such phenomena as electron diffraction by single crystals, and used high-voltage diffraction techniques to study the microscopic structure of surfaces. A significant part of the work of the Department of Physics at Imperial College was focused on the practical improvement of diffraction techniques, and although Thomson himself contributed relatively little directly to the enormous strides made in this respect, mainly by his students, he did supervise and direct this work and he did prepare with W. Cochrane the definitive account of the subject, Theory and Practice of Electron Diffraction, published in 1939.
While Thomson maintained his interest in electron diffraction, he also devoted increasing attention to the dramatic developments of nuclear physics in the 1930’s. A significant aspect of the attraction that electron diffraction had held for him had been related to its significance in the context of the developing quantum theory, which aroused his theoretical as well as his experimental interest. With the discovery of the neutron and the positron, and the splitting of the atom, in 1932, followed by the discovery of artificial radioactivity two years later, the leading edge of physics moved from the mathematically obscure realm of quantum mechanics and quantum field theory to the equally taxing but more readily approachable area of nuclear physics, and Thomson’s interest soon moved with it. With J. A. Saxton he looked for artificial radioactivity from positron bombardment. And following Enrico Fermi’s work on the effects of slow neutrons, he led a team of researchers, including P. B. Moon and C. E. Wynn-Williams, in a study of the velocity distribution of slow neutrons. A deuteron beam was used to bombard a target of frozen heavy water, and the neutrons produced in the resulting deuteron-deuteron reactions were slowed down in paraffin. Time-of-flight techniques were then used to measure the velocity distribution and absorption coefficients of the neutrons. Then in 1939, following the discovery of uranium fission, Thomson led another team in a measurement of the neutron yield from fission reactions.
Apart from two postwar papers on effects produced by cosmic ray collisions, Thomson’s fission paper and another joint paper of the same year with a student, Moses Blackman, who was working on the theory of electron diffraction, were his last significant original scientific publications. But there was one major unpublished piece of work to come, and his contributions through science, which were just beginning, were in some ways as significant as his contributions to science.
Thomson’s experiments on the neutron yield from fission reactions gave results roughly the same as those announced a few weeks earlier by Hans von Halban, Frédéric Joliot, and Lew Kowarski in Paris (3.5 ± 0.7, an overestimate), and pointed clearly to the possibility of a self-sustaining chain reaction within a body of fissionable material. And seeing at once the possible implications of this result, Thomson was, more than anyone, instrumental in the establishment of the wartime British atomic energy project. Through Tizard, who was both reactor of Imperial College and chairman of the Committee for the Scientific Study of Air Defence (the principal organization for mobilizing scientific effort for the anticipated war), he persuaded the government to purchase a ton of uranium oxide for his own experiments. With this material he made both a solid pile, with blocks of paraffin as moderator, and a liquid pile of uranium oxide in water. There was no sign of either system even approaching a chain reaction (any neutrons emitted were quickly absorbed); satisfied that a uranium oxide bomb was out of the question and that the separation even of metallic uranium would be a massive and impracticable operation in wartime, Thomson gave up his experiments. In April 1940, however, he read a copy of the O. R. Frisch-R. F. Peierls memorandum, in which these two refugee physicists offered a theoretical analysis leading to the conclusion that a bomb composed purely of the isotope uranium 235 might be possible using only a few pounds of material. Thomson promptly consulted his scientific and government colleagues, and despite continuing skepticism as to the feasibility of a fission bomb—the calculations were far from conclusive and the problem of isotope separation seemed to many people practically insuperable—a uranium subcommittee was set up under Tizard’s committee, with Thomson in the chair. By the summer, his subcommittee had become the MAUD committee of the Ministry of Aircraft Production, and a report on the prospects of a uranium bomb, drafted by Thomson, had reached the prime minister. The wartime project was under way.
Although he had from the first taken the initiative in respect to fission, Thomson soon stepped down. He had been ill himself, and his wife was critically ill in America. In the autumn of 1941 he was appointed to Ottawa as scientific liaison officer to explore the possibility of a team of British and French scientists transferring their nuclear reactor research from Britain to Canada, a move that duly took place. Returning to England in 1942 after his wife’s death, Thomson acted as deputy chairman of the Radio Board (supervising radar research) and scientific adviser to the Air Ministry. Later, in 1946, he also acted as adviser to the British delegation to the United Nations Atomic Energy Commission. But by that time he had become interested in a new topic, the peaceful application of thermonuclear fusion.
The possibility of a controlled thermonuclear fusion device was already in the air by the late 1930’s. There was by then a clear prima facie possibility of net energy production from the fusion of deuterium ions (deuterons), and it was in particular possible to envisage such production taking place in a toroidal gas discharge tube, not unlike a circular particle accelerator, the plasma or ionized gas in which would be isolated from the walls of the tube by the “pinch effect” owing to its own self-magnetic field. A number of people, including Leo Szilard and Peter Thonemann, apparently discussed this possibility before and during the war, and in 1946 a group of leading Los Alamos physicists turned their attention temporarily to an informal study of the problem. But the first detailed, documented proposal for a controlled fusion device appears to be that of Thompson. As an authority on both gas discharge physics (he knew his father’s experiments on toroidal discharge tubes) and nuclear physics (in which he had used the deuteron-deuteron fusion reaction to produce neutrons), Thompson was particularly well placed to see and explore the possibilities of controlled fusion. And early in 1946 he sketched out a proposal in which deuterium gas would be confined in a toroidal solenoid in a strong magnetic field.
He proposed to ionize the gas using an external source and then to heat up the resulting plasma using a high-frequency alternating current applied through pairs of wave guides to feed a unidirectional wave into the torus. This would accelerate the electrons in the plamsa, which would then transfer their energy to the positive deuterons carried around with them. Beginning with a gas density of around 1015 molecules/cm3, Thomson suggested, the deuterons could within a few minutes be heated to about 100keV, at which point fusion energy would be generated. With the help of Moses Blackman, and following helpful criticisms from Peierls, he then developed this idea, with solenoid containment being replaced by reliance on the pinch effect, and made it the subject of a secret provisional patent specification lodged in May 1946. The torus proposed was of 3m diameter (increased in the final specification of 1947 to 4m) and 60cm bore.
Thomson’s proposed torus was never constructed, and the main development of controlled fusion devices in Britain, leading in 1957 to the operation of the ZETA torus, followed instead the ideas of Thonemann. But Thomson’s proposal and subsequent lobbying were instrumental in establishing the British fusion program as the first in the world, and many of Thomson’s ideas were developed by smaller teams at Imperial College and later at the AEI laboratories at Aldermaston Court. Thomson himself remained active in the field through the 1950’s, and maintained a strong interest thereafter.
In his later years Thomson’s scientific activities were dominated by his popular articles and books, including The Inspiration of Science (1961) and The Foreseeble Future (1955). He was a prolific writer and speaker on such subjects as science and religion, the education of scientists, and the place of scientists in society, with all of which he was passionately concerned.
BIBLIOGRAPHY
The principal published account of Thomson’s work is the Royal Society obituary by P.B. Moon, in Biographical Memoirs of Fellows of the Royal Society, 23 (1977), 528–556. Thomson also wrote an unpublished autobiography (see below), and there is an interview with him in the Sources for History of Quantum Physics archive, copies at the American Philosophical Society, the American Institute of Physics, the University of California at Berkeley, the Niels Bohr Institute in Copenhagen, and the Science Museum in London. There are several published reminiscences by Thompson, including Nuclear Energy in Britain during the Last War: The Cherwell-Simon Lecture. . . 1960 (Oxford, 1962), and “The Early History of Electron Diffraction,” in Contemporary Physics, 9 (1968), 1–15. There are no other historical accounts of his work on electron diffraction, attention having been focused more on the work of Davisson, but see R.K. Gehrenbeck, “C.J. Davisson, L.H. Germer, and the Discovery of Electron Diffraction,” Ph.D. dissertation, University of Minnesota, 1973. For Thomson’s work on fusion, see J. Hendry, “The Scientific Origins of Controlled Fusion Technology,” forthcoming.
The Royal Society obituary cited above contains a select bibliography of Thomson’s scientific articles and books, with many omissions and some misprints. His pricipal papers on electron diffraction are “Diffraction of Cathode Rays by a Thin Film,” in Nature, 119 (1927), 890, written with A. Reid; “The Diffraction of Cathode Rays by Thin Films of platinum,” in Nature, 120 (1927), 802; and “Experiments on the Diffraction of Cathode Rays,” in Proceedings of the Royal Society (A). 117 (1928), 600–609. For Thomson’s fusion device see British Patent no. 13963/46.
Thomson’s papers are archived at Trinity College, Cambridge, and have been catalogued by the Contemporary Scientific Archives Centre, catalog no. 75. The collection includes Thomson’s unpublished autobiography, a substantial collection of correspondence, and many drafts of papers, some eventually published, others not.
John Hendry