For most physicists, finishing a problem presents a new opportunity to write a paper, to enhance the reputation. For Leonardo Castillejo, the pride lay in posing a good question and understanding the answer. It was difficult, sometimes impossible, to convince him that others would enjoy sharing his insights.
He wrote infrequently and with difficulty, in part because of his unconventional spelling, but with enormous care and clarity. Perhaps getting away with not writing his PhD thesis was another reason for his publishing little in later life.
Castillejo's primary research activities were in a branch of elementary particle theory, which, over the years, evolved into intermediate energy physics. His interests covered large areas in nuclear physics, condensed matter physics and statistical physics. It was this breadth which allowed him to take the topological models used to describe elementary particles and apply them in superconductivity.
His best-remembered work will be that with the particle physicist Dick Dalitz and the mathematical physicist Freeman Dyson, at Cornell in 1954- 55, which led to what the textbooks call the CDD ambiguity. This had important consequences for the application of analyticity ideas in particle physics. Even here the other authors had to persuade him that it was worth publishing. The collaboration with the nuclear physicist Gerry Brown at Birmingham in 1960 was also very fruitful, culminating in a famous paper describing a model for electric dipole states in nuclei.
Given the opportunity, Castillejo preferred his working style to be conversational. He needed colleagues with whom he could discuss and dismember his concepts, and he found them at innumerable international seminars and at the various universities at which he held appointments - at University College London (where he started his career as research assistant to Sir Harrie Massey and ended it as professor), with Sir Rudolf Peierls at Birmingham and Wadham College, Oxford, and in the United States (at Cornell, where his fellow theorist and brother-in-law Michael Fisher settled, at Princeton and Stoneybrook).
Castillejo had an extraordinary perceptivity and infinite patience when explaining difficult concepts to students and colleagues. A man of great personal charm, with an infectious twinkle, he would begin by asking you what you were doing and then listen in a way that reminded you that good listening is not passive. Gentle, general questions would inevitably lead to more specific and penetrating probes which cut right to the heart of the inadequacies of the argument. At this point he would take the chalk and suggest, "It must be like this."
His insights were offered with a complete absence of ego or arrogance uncommon amongst physicists. He believed that good friends make good problems and his collaborations were filled with the strongest sense of collegial pleasure and goodwill.
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, writes Stephen Keynes.
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 Bunuel invented Spain's Surrealist art.
Leonardo was educated at his father's experimental multi- lingual Escuela International 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.
Leonardo Castillejo, physicist: born London 21 June 1924; Research Assistant, Department of Physics, University College London 1948-50, Assistant Lecturer 1950-54, Lecturer 1955-57, Professor of Physics 1967-89 (Emeritus); Lecturer in Mathematical Physics, Birmingham University 1957-63; Lecturer in Physics and Fellow, Wadham College, Oxford 1963-67; married 1950 Cecilia Jezzi (marriage dissolved 1973), 1973 Susan Engledow (two daughters); died Oxford 11 May 1995.
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