Born London 21 June 1924; Research Assistant, Department of Physics, University College London 1948-50, Assistant Lecturer 1950-54, Lecturer l955-57, Professor of Physics 1967-89 (Emeritus); Lecturer in Mathematical Physics, Birmingham University 1957-63; Lecturer in Physics and Fellow, Wadham College, Oxford l963-67; married 1950 Cecilia Jezzi (marriage dissolved 1973), 1973 Susan Engledow (two daughters); died Oxford 11 May 1995. To students brought up in an age of publish or perish, Leonardo Castillejo must seem like a glaring anomaly. He was somebody who loved discussing new ideas in depth with students, colleagues, and visitors and his ability to get to the crux of the problem, no matter what the field, made him widely respected and admired, despite the fact that his name has been on fewer papers than many current postdoctoral fellows!
Born of a Spanish educationalist father and an English psychotherapist mother, the family had to leave Spain during the Civil War and eventually settled in England. In the forties it was possible to complete a degree in two years and so by the age of 18 he had obtained a BSc in Engineering from Imperial College. Following four years of “land work”, Leonardo then obtained a BA in Mathematics from King's College, Cambridge, also in two years.
In 1948 Sir Harrie Massey took him on as a research assistant in the Mathematics Department at University College London, working on what was then high energy (83 MeV) nucleon--nucleon scattering, and he transferred with Massey to the Physics Department in 1950. In 1954-55 he went on leave to Cornell, a university where his brother-in-law Michael Fisher would later settle. He there collaborated with Dalitz and Dyson on what is now called the CDD ambiguity, which had a major impact on the application of analyticity and bootstrap ideas in particle physics in the sixties. They realised that it was possible to add arbitrary poles to the denominator function of a scattering amplitude f, without creating extra singularities in f. In a sense such poles express the physical possibility that stable particles are present even when the interactions are switched off. Back in London, Leonardo worked in atomic physics with Percival and Seaton on exchange effects in the elastic scattering of electrons from hydrogen where he formulated the problem in the more consistent time--dependent approach.
When Matthews left Birmingham in 1957 to join Salam at Imperial College, Leonardo went up and made a very good impression on Sir Professor Rudolph Peierls. However, after looking at the c.v., Peierls commented to Gerry Brown “I knew that Castillejo hadn't published much but you didn't tell me that he hadn't even written a thesis!” His most significant nuclear physics paper, The Dipole State in Nuclei, was written with Brown and Evans in Birmingham in 1960. Leonardo laid out all of the angular momentum algebra in a very clever fashion. This converted the Brown and Bolsteri schematic model, where matrix elements were chosen proportional to the degeneracy, into a real shell model calculation.
In 1966 Leonardo was approached by Massey to return to UCL, to take the chair vacated by Hamilton. It was noted at the time that “he does not publish very much – partly because of his high standards, which make it very hard for him to be completely satisfied with his own results, and partly because he is always so willing to help other people with their problems. However, this is a disadvantage to him personally and not to the Department to which he belongs”. It was this participation in other people's research, putting away all his own work while you spent a couple of hours explaining your ill-thought-out ideas, was to remain a lasting characteristic. His preferred way of working was through conversation and you generally emerged with either a consistent model or ideas for a completely different problem. We only worked together once, on multiple scattering theory, and I don't think that he was too disappointed to be scooped by others just as we got to the end. For him the pride lay in posing a good question and in understanding the answer.
More recently Leonardo was interested in effective Lagrangian models of hadrons and hadronic matter, such as the Skyrme model, and much of this work was carried out with Andy Jackson from Stony Brook. He initiated studies of the nucleon-nucleon interaction in the Skyrme models and played an important role in establishing its connection to more familiar meson exchange interaction. His interest in the phase structure of the Skyrme model and its connection to chiral symmetry restoration led him to a more general investigation of collective coordinates in effective field theories. Possible connections of Skyrmions to high-Tc superconductors also fascinated him.
Leonardo stayed a staunch liberal and young in spirit to the end, ever receptive to new ideas and challenges and in recent years much of his energy was taken up by helping to organise the family foundation for the Arts in Madrid.
None of the other physicists that I have known has been so universally loved as a man.
Colin Wilkin, UCL 1995
Leonardo Castillejo's father, Jose Castillejo, was a leading reformer of Spain's educational system who helped to free it of state and church controls and encouraged co-education. He was also the intellectual most responsible for linking Spain's culture and science with Europe after centuries of isolation. In 1907 he founded the Junta para Ampliacion de Estudios, which sent hundreds of graduates to study abroad. His junta set up the Residencia de Estudiantes, where Lorca, Dalf and Buñuel invented Spain's Surrealist art.
Leonardo was educated at his father's experimental multilingual Escuela Interna1onal where the children learnt four languages from the age of four. In 1936, the family fled Spain after Jose Castillejo was blacklisted by both extreme right and extreme left.
The family settled in England. Leo's English mother, Irene Claremont, became one of the leading Jungian lecturers and psychotherapists in England. Her autobiography, I Married a Stranger, has now been translated into Spanish and is being published this month under the title Respaldad Pol El Viento.
Under her influence, Leonardo registered as a conscientious objector and in 1944 was directed to work at a chicken farm in Surrey, which he later managed. His sister remembers that even at the age of eight it was impossible to get his attention if he was engaged in thought. At the chicken farm he had no time to read books, but plenty of time for abstract thought. It was here that he learnt to enjoy fresh eggs: later he always consumed two before sitting an exam. He remained deeply committed to the cause of civil liberties throughout his life, always fighting on the side of his students.
After the war Castillejo completed a degree in electrical engineering at Imperial College and went on to King's College, Cambridge, to do a degree in mathematics. Then he assumed his research and teaching career. His recreation was to spend long periods camping in wild and isolated sites around Spain with his first wife, Cecilia. Susan, his second wife, whom he married in 1973, is a painter and was a student at the Slade when they first met. His two daughters, Alice and Clare, were the delight of his later middle age.
Leo Castillejo and his family often visited his parents' former house set in the last large olive orchard surviving in the centre of Madrid. Eventually part of the land had to be sold, but an area containing the house and 132 old olive trees was dedicated by Leo and his brother and sisters to become the Fundacion Cultural Olivar de Castillejo.
Leonardo devoted his last years to establishing this cultural centre in pursuit of his father's objectives: in particular to bring to the younger generation of Spanish artists and writers awareness of and links with contemporary artistic and intellectual developments abroad.
The foundation requires outside funding and at the time of Leo Castillejo's death it seems that its unique achievements in stimulating and disseminating Spanish culture are receiving government and other recognition, so that the funding can be found for it to continue.