6. Dublin

“The scientist only imposes two things, namely truth and sincerity, imposes them upon himself and other scientists.” – Erwin Schrödinger.

In 1941, as Jim put it “some senior scientists” got him a job in the Theoretical Physics school at a new research institute in Dublin, the Institute for Advanced Studies. DIAS had two schools, the other was for Celtic Studies. The physics school was run by two clever scientists “whom Adolf, in a stupid move, had driven away.” These men were Schrödinger and Heitler. Schrödinger had known Ewald from time spent in Stuttgart during 1921 and the two families renewed their friendship in Ireland. It is probable that Ewald recommended Jim for the DIAS fellowship.

While at Zürich, Erwin Schrödinger had established himself among the foremost physicists of the day through his discovery of wave mechanics, and by relating his wave mechanics formulations of quantum mechanics to Heisenberg’s matrix mechanics formulations. Schrödinger had previously fled Europe, shortly after Hitler had become Chancellor of Germany, but had returned to the University of Graz in his native Austria until 1938. Upon Germany’s annexation of Austria he was dismissed from his post for ‘political unreliability’. He contacted Eámon de Valera (who was President of the Council of the League of Nations at that time). De Valera, himself an accomplished mathematician, had been setting up DIAS and straightaway invited Schrödinger to take up a post in the School of Theoretical Physics.

Walter Heitler was German, no longer either welcome or safe in his own country. Just a quarter of a century earlier the Grand Duke of Baden had awarded the ‘Orden vom Zähringer Löwen – Ritter 1. Klasse’ to Heitler’s father for his exceptional merits as professor at a technical high school in Karlsruhe. But as was the case with Paul Ewald, previous service to the nation counted for nothing.

Heitler had studied at Karlsruhe, Berlin and Munich, his Ph.D. study under Sommerfeld and Herzfeld, and had subsequently spent a year at the University Institute of Theoretical Physics in Copenhagen with Niels Bohr. This was followed by a year at Zürich with Erwin Schrödinger and five years with Max Born at Göttingen. As with Ewald and many others, Heitler’s position became untenable in 1933. With the assistance of Max Born, a research fellowship was arranged for him at Bristol and he left Germany. Heitler remained at Bristol for 8 years. He was already well known for his application of quantum mechanics to the molecular bonding of atoms (in 1927 Heitler and London, using quantum theory and the idea of electron spin, had solved a problem which had troubled chemists ever since Avogadro’s hypothesis in 1811- that is why two apparently identical atoms of hydrogen should bond so strongly to form a molecule) and now turned his attention to quantum electrodynamics, publishing ‘The Quantum Theory of Radiation’ in 1936. He also collaborated with Hans Bethe, another émigré. During 1940 he was interned on the Isle of Man for several months on account of his German nationality.

Schrödinger arrived in Dublin during October 1939, and within a year the Council of DIAS had been constituted. The governing board comprised chairman Patrick Browne, Arthur Conway, Francis Hackett, A.J. McConnell, Bill McCrea, D. McGrianna, Schrödinger, Edmund Whittaker and de Valera. At the first meeting of the governing board, Schrödinger was installed as Director of the physics school. Soon after his appointment, Schrödinger arranged for Heitler to join him in Dublin; the move not only reunited the two of them but also ensured that Heitler wouldn’t be detained any further. Heitler took up his appointment in June 1941.

DIAS was operational by early 1941, the buildings chosen for the new institute being two adjacent houses in Merrion Square, just a short distance from Trinity College. One of the houses, no. 65, was ‘Hamilton House’ : de Valera had greatly admired William Rowan Hamilton, the 19th century Dublin mathematician famous for his work on analytic mechanics. William Rowan’s ‘Hamiltonian’ function H was fundamental to quantum mechanics and remains widely used today. Jim was no relation to William Rowan.

Fellowships were awarded for three new scholars in theoretical physics, to Jim; Chinese physicist Hwan-Wu Peng; and Sheila Power. Jim was the first scholar to be admitted to the theoretical physics school, on 19th May 1941, and worked through to the end of September 1941 (he was still giving lectures to undergraduates at Queen’s and was required to return for the new term in October. It had been arranged, presumably between Ewald and Schrödinger, for Jim’s scholarship at DIAS to be on a part-time basis).

Jim worked with both Schrödinger and Heitler during his first year at DIAS. He contributed to a paper by Schrödinger, ‘Exchange and Spin’ which was published in 1941 (Proc. Roy. Irish Acad. 47A, p.39). Schrödinger was highly complimentary about a proof Jim had derived concerning the alternating symmetry character for successive spin values, stating in the paper that this was a very much simpler proof than his own. Together with Heitler and Peng, Jim was also busy working on the newly discovered and important role of radiation damping in the mesonic interaction.

Jim was present at DIAS for the Summer Seminar during June 1941, a gathering of 25 physicists and mathematicians from Irish universities. Heitler delivered 10 lectures on the main topic of the seminar, meson theory. Schrödinger delivered a parallel course of 10 lectures on wave mechanics selected for the purpose of supplementing the main course and of facilitating its understanding.

Peng and Power (Sheila Power was also a part-time scholar) took up work at the school on 1st October 1941. In March 1942 the three original scholars were joined by James McConnell. Denjoe O’Connor indicates that S.T. Ma, who would later collaborate with Heitler (Ma was at the institute in the years 1947-9), might have been a visitor during the early days.

The splitting of his time between Belfast and Dublin was not always easy for Jim. A letter to Schrödinger from early December 1941 explains the situation : “This year my lecture courses have an unusually large attendance resulting in more than usual routine work…I intend spending a week in Dublin just after Christmas. Dr. Ewald asks me to enclose a note of his on the matter.” Jim also talks of “some domestic impacts” – Grandma Mackay had recently died – and refers to work that he had been doing on radiation damping.

The term ‘radiation damping’ might be best understood by analogy to a pendulum. A frictionless pendulum would oscillate with the same frequency and amplitude forever, but in reality the amplitude gradually decreases because of friction : the pendulum is ‘damped’. The classical interpretation of radiation damping was similar – an oscillating electron radiates energy and so, in order to satisfy energy conservation, the oscillation must decrease in proportion to the energy lost. The quantum theory of radiation damping allows for transition from a high energy level to a lower energy level, the lost energy taking the form of radiation, and the probability of finding the oscillator in its original high-energy state gradually decreases with time. The quantum theory of radiation damping had first been proposed by Weisskopf and Wigner (1930, Zs.f.Phys. 63, p.54 and 1930, Zs.f.Phys. 65, p.18). Then Heitler (1941, Proc. Camb. Phil. Soc. 37, p.291), Wilson (1941, Proc. Camb. Phil. Soc. 37, p.301), and Heitler and Peng (1942, Proc. Camb. Phil. Soc. 38, p.296) made further significant contributions.

Considerable importance was attached to understanding this subject because, when examining the scattering of particles, damping theory guaranteed that the sum of all the probabilities; whether the system was in its original high-energy state, in its final low-energy state, or was in an intermediate state, had always to be 1.

At DIAS the physics scholars shared a room and this room also being the tea-room, they got to see a lot of the staff and visitors. Teaching at the physics school comprised both weekly seminars and lecture courses. The scope of the weekly seminar was to unite the staff and scholars of the school with the other physicists or mathematicians living in or near Dublin (mainly the professors and lecturers of the two Dublin colleges, Trinity College and University College). During the first year, seminar topics included field-quantisation, Born’s non-linear electrodynamics, meson theory and radiation damping.

In the first year the lectures were focused primarily on the principles of quantum mechanics, mainly the theories of spin and perturbations (in quantum mechanics ‘perturbation theory’ is an extension of mathematical perturbation, approximation of a solution by using the known solution of a simpler, related problem). In 1942 Walter Heitler started a new lecture course on advanced topics of quantum mechanics, aimed at research students of chemistry and allied branches of science, which proved enormously popular. There would also be lectures from visiting professors, such visits would not be limited to the summer colloquia. For example, during a visit in June 1941, Paul Ewald delivered a lecture on ‘Crystal Optics of X-Rays’.

The first DIAS colloquium (for two weeks in July 1942) was some event, with 43 participating scientists. The term colloquium conveyed that this was a much amplified version of the previous year’s summer seminar. Topics, lectures and lecturers were all more numerous, and the collaboration of two distinguished Cambridge professors, Dirac and Eddington, had been secured. Paul Dirac, on ‘Quantum Electrodynamics’, and Arthur Eddington, on ‘The combination of Relativity Theory and Quantum Theory’, delivered the two principal lecture series of 5 lectures each. Conway, Nevin, O’Ceallaigh and Walton all contributed with other lectures.

Eámon de Valera attended the colloquium. Then in his 60th year, de Valera had a remarkable history. Having reluctantly joined the Irish Republican Brigade prior to the 1916 Rising, he had found himself in command of the Volunteer garrison at Boland’s Mills. He was the most senior commandant to survive the rising : it has been suggested that representations made by the American Consul in Dublin (de Valera had spent the first three years of his life in New York) caused him to be spared the fate of Connolly and Pearse, and the thirteen others who were executed. Imprisoned, released, imprisoned again and this time escaping, de Valera had been involved in the Civil War of 1922 and had then endured a third period of imprisonment. On his release he had founded the Fianna Fáil political party in 1926, and had succeeded Liam Cosgrave as Prime Minister of the Free State in 1932. Jim’s recollection was that de Valera was a quiet, serious man.

Reporting on the colloquium, The Irish Times had it that “…as an indication of their keenness and the interest of the lectures it was noticed that hardly any member of the audience missed a single lecture or discussion.” Following the colloquium, Irish President Douglas Hyde received a number of the scientists at Phoenix Park.

Much of Jim’s work at DIAS was with Heitler. Many years later, in ‘On Mesons and Methods’ (1986, Arkhimedes, 38. vsk, p.118), Jim was to state “From 1941 to 1943 I was fortunate to be able to work, part of the time, under the guidance of Walter Heitler, who was then in Dublin. He was studying the nature of the hard component of cosmic radiation and was trying to find whether it could be linked up with Yukawa-type meson theory. Of course there were difficulties in the treatment of the strong interactions of the mesons, and this led to several interesting developments.” Jim goes on to describe problems with perturbation expansions (working before the days of renormalization) and difficulties with unitarity which were overcome by using ‘radiation damped’ scattering equations (“what the modern dispersion theorist would call rescattering terms”). “The method was called ‘radiation damping’ partly because it showed how the reaction on the scatterer could restore unitarity, and partly because the derivation of the equations is somewhat similar to Weisskopf and Wigner’s derivation of atomic line width behaviour, where the emitted radiation reacts on the initial state causing its exponential decay.”

From the work of Rutherford and Bohr in the 1910s, the understanding of atom structure was that an atom comprised a dense, positively charged nucleus of heavy particles which was orbited by much lighter, negatively charged electrons. The heavy particles of the nucleus were seen as comprising positively charged protons and (following Chadwick’s discovery of the neutron in 1932) at least as many uncharged neutrons – the hydrogen nucleus being an exception, comprising only a single proton. The charge carried by protons and electrons was understood to be equal and under normal conditions the atom would be electrically neutral, thus the number of protons in the nucleus would be equal to the number of orbiting electrons. That the electrons remained in orbit around the nucleus was explained by the attraction of opposite electrical charge. However, in the atomic nucleus of every chemical element (apart from hydrogen) there would be more than one positively charged proton. If the combined electrical force of the protons was sufficient to govern the behaviour of orbiting electrons, why by the same token did the same charge protons remain in such close proximity to each other within the nucleus ? There had to be some explanation as to why the nucleus was cohesive and the term ‘strong interaction’ or ‘strong force’ was used in order to describe a force sufficient to overcome the repulsion between the positively charged protons.

Hideki Yukawa had in 1935 (Proc. Math. Soc. Japan 17, p.48) predicted the existence of the ‘meson’ (‘intermediate’ in mass between a proton and electron), this was his term for the particle then thought to be the carrier of the strong force. Heitler’s recent work had been on cosmic-rays, he had been examining the collisions caused as cosmic-ray particles enter the atmosphere. In 1937 he had, together with Homi J. Bhabha, presented the ‘Cascade Theory of Electron Showers’. The ‘heavy electrons’ observed were thought to be the same as the predicted mesons. Bhabha, by the way, had returned in 1939 to Bombay, where he later became the first director of the Tata Institute of Fundamental Research and established the Atomic Energy Commission of India in 1948. He died in 1966, a passenger aboard a scheduled flight which crashed into the mountainside of Mont Blanc during a blizzard.

In 1943 Jim’s collaboration with Heitler and Peng on ‘Theory of Cosmic-Ray Mesons’ (Phys. Rev. 64, p.78) applied the quantum theory of damping developed by Heitler and Peng to the production of mesons by proton-proton collisions. One of the conclusions reached was that cosmic-ray mesons were in fact identical to the quanta predicted by Yukawa. The work uses a modified form of the meson theory published in 1940 by Christian Møller and Léon Rosenfeld, who were working in the Institute for Theoretical Physics at the University of Copenhagen (Kgl. Dansk. Vid. Sels. Math.-Fys. Medd., 17, p.8). Theoretical values are compared against experimental measurements obtained by Patrick Blackett, who had succeeded Lawrence Bragg at Manchester University in 1937. Additionally, the paper carries an acknowledgement to Dr. L. Jánossy “for a most useful comment and also for communicating some of his experimental results to us before publication.” The Hungarian Lajos Jánossy, fostered son of Marxist philosopher Georg Lukács and himself a committed Marxist, was a gifted physicist who had arrived at Manchester University in 1936 and who would later join DIAS, in 1945.

Jim later said of this work : “Heitler and his colleagues applied the damping equations to the elastic scattering of pseudoscalar and vector mesons on nucleons, and used the results to set up a model for the production of the hard component of cosmic radiation by very high energy protons colliding with the nuclei in the top of the atmosphere. The model fitted the available experimental data in the upper atmosphere fairly well…”

‘Theory of Cosmic-Ray Mesons’ was followed by the publication of Jim’s paper with Peng in 1944, ‘The Production of Mesons by Light Quanta and Related Processes’ (Proc. Roy. Irish Acad. 49A, p.197). They showed for the first time that the principle of detailed balance is not always valid in quantum mechanics (Boltzmann had shown much earlier that it is not always valid in classical mechanics). Of this Jim later said : “In the cosmic-ray work it was necessary to estimate the rate of photoproduction of pseudoscalar and vector mesons on nucleons. Because of the strong interaction in the final state, it was necessary to use the damping equations. H.W. Peng and I calculated the production amplitudes and obtained results of a reasonable size, but there was a surprise.” The process in :

y + N₁ -> m + N₂

did not in general have the same rate as the reverse process :

m + N₂-> y + N₁

The principle of detailed balance, as expressed in the textbooks on statistical mechanics at that time (Fowler, R.H., Statistical Mechanics, Cambridge 1936; Mayer, J.E., and Mayer, M.G., Statistical Mechanics, Wiley 1940; Tolman, R.C., The Principles of Statistical Mechanics, Oxford 1938) stated that the rates for a process and its reverse should be equal. However the statistical mechanics experts had only used first-order perturbation theory. Jim and Peng’s use of rescattering terms had exposed the violation. Günther Rasche mentions that Heitler later wrote an article about ‘The Production of Mesons by Light Quanta and Related Processes’, citing the pioneering contributions of Hamilton and Peng.

The paper with Peng again refers to work which had been carried out in Manchester, in this instance by Bernard Lovell, who also had arrived there in 1936. One has the impression that the meson study in Copenhagen, Dublin and Manchester was a cause for regular dialogue between the three places during the late 1930s. The German occupation of Denmark in April 1940 would cause Copenhagen effectively to be isolated from this dialogue, but it continued between Dublin and Manchester.

DIAS blossomed, attracting much interest. Wolfgang Pauli, from Princeton and Rudolf Peierls, from Birmingham were among the early frequent visitors. Pauli had attended the University of Munich under Sommerfeld during 1918-21. Sommerfeld had said of him : “I can’t teach him anything” and suggested that instead the young Pauli write a summary of Einstein’s theory of relativity. He did so, brilliantly. Later, in 1932 and by then at Zürich, Pauli put forward his ‘exclusion principle’ which proposed that no more than two electrons could ‘live’ on the same orbit. Otto Frisch recalled that this earned Pauli the nickname ‘Atomic Housing Officer’.

Jim’s photograph from the 3rd DIAS Summer School (2nd Colloquium), in July 1943, shows a front row comprising Heitler, McConnell, Schrödinger, de Valera, Conway, Born, K. Lonsdale, Ewald and Hackett. Jim identified two other men in the photograph, W.H. Ramsey and Eric Lindsay (Lindsay, as Director of Armagh Observatory, played a significant part in persuading de Valera to save Dunsink Observatory). Max Born, invited from Edinburgh, lectured on ‘The Dynamics of Crystal Lattices’, and Paul Ewald, from Queen’s, delivered two lectures on ‘The Dynamical Theory of X-ray Interference’. Kathleen Lonsdale, invited from London, gave an interesting account of the latest research work with X-ray photography (she lectured on the experimental study of ‘X-ray Scattering due to Thermal Motion in Crystals’).

He found Dublin a pleasant place, even though there were considerable shortages of commodities in those days, and the institute was a good place to work. It might have been tempting, with the emerging prospect of a promising career, for Jim to become utterly absorbed in his research. As regards World War II, the position in the newly established Irish Free State was one of neutrality throughout the period of the ‘emergency’ as the war was termed, though many did volunteer their services to the Allies. It is no doubt also true that many of the population were concerned at the possibility of British re-occupation : the Free State had only finally achieved full control of its own ports in 1938. Cabinet papers later released by the British Government revealed that re-occupation of Irish ports was indeed an option considered, but that the government had eventually decided not to use force unless the U-boat situation worsened considerably.

As for Jim’s own position during the emergency, it should be remembered that he was splitting his time between Dublin and Belfast, and the latter place remained under British control and had been bombed. Also it should be remembered that Jim’s mother was Scottish. From Paul Ewald, if not earlier, Jim had become aware of the problem of fascism. He was well informed as to the racial theme underpinning Hitler’s regime. He would later mention that he had received first-hand accounts of conditions in pre-war Germany, and would leave one with the impression that this had contributed to his own motivation to become involved in the war effort, though he never went so far as to say precisely that.

Jim departed for London in 1943. He later wrote : “some senior scientists moved me to the Admiralty to study how to make naval war in a more efficient way”, which somehow made it seem like it was everyone else’s decision except his. The DIAS Annual Report for the financial year 1943-4 states : “The work by Hamilton, Heitler and Peng on production of mesons in cosmic radiation and on related problems was brought to a successful conclusion. It turned out that the theory led to a satisfactory understanding of a large number of phenomena of cosmic radiation.” Jim did not need to feel that he was leaving loose ends behind him.

Not everyone was in accordance about his move. Günther Rasche has the impression that Heitler had not approved of Jim’s departure for the Royal Navy (without being pressed) and that Heitler’s view was that Jim was a very clever Ph.D. student who should have continued with his physics at DIAS. Though Jim and Heitler parted ways in 1943, Heitler would assist him with the resumption of his Ph.D. work after the war and they retained a great respect for each other over the years.

Similarly, between Schrödinger and Jim there would be a continuing respect, and further contacts after the war. A letter from Jim to Schrödinger in 1946 mentions the interest by Egil Hylleraas, a Norwegian scientist who had visited Manchester, in Schrödinger’s work on unified field theory (a theory to encompass both gravitation and electromagnetism).

Today, the City of Dublin remains a largely pleasant place. The half mile walk south from Parnell Street along O’Connell Street – past the General Post Office, centre of operations during the Rising in 1916 – and over O’Connell Bridge to Trinity College, fills one with a sense of the history of the city. Merrion Square is only a quarter mile from here, taking Nassau Street and Leinster Street, though DIAS is now located about half a mile further south, on Burlington Road.

Bibliography.

Frisch, Otto R., What Little I Remember, Cambridge University Press 1979
Moore, W., Schrödinger, Life and Thought, Cambridge University Press, 1989