Ewald, Paul Peter (1888–1985), mathematical physicist, was born 23 January 1888 in Berlin, Germany, the only child of Paul Ewald, historian, from a wealthy academic family, and Clara Ewald (née Philippson), artist. His father died of appendicitis three months before he was born, and his mother, who was unconventional in many ways, gave him a rather bohemian upbringing. Educated at first in France, then later in Berlin, he entered the Victoria gymnasium in Potsdam (1900), where he was interested in mathematics, physics, and chemistry. The development of his subsequent career was deeply influenced by reading Leo Königsberger's biography of the German physicist Hermann von Helmholtz (1821–94). This led him to a lifelong interest in the problem of using the properties of light to find the ultimate atomic structure of matter.
He entered Caius College, Cambridge (1905), and studied chemistry, physics, and mathematics. But despite enjoying the social life, he disliked the subjects he had chosen and moved to the University of Göttingen (1906), where he took a course in mathematics by David Hilbert, and decided to make mathematics his main subject. He moved again to the University of Munich (1907), initially drawn to a particular branch of mathematical analysis (epsilontics), where a leading exponent of this new algebraic mathematical method was based. However, after attending a series of lectures on hydrodynamics by Arnold Sommerfeld, he was deeply impressed by the relationship between mathematics and physical phenomena, and devoted his future work to this area. Under Sommerfeld he studied for a Ph.D. (1912). His thesis, which examined the propagation of light of any wavelength in crystals, led to the dynamic theory of X-ray propagation, for which he became internationally famous.
He remained at Munich working with Sommerfeld until the outbreak of the first world war, in which he served as an X-ray technician in a mobile field ambulance in Dwinsk (Djugavpils, Latvia). It was some distance from the main battle areas and he had much time in which to develop his theories about X-ray propagation. After military service, he was made lecturer (privat dozent) at the University of Munich (1917–21). He moved to Stuttgart when he was offered a position as associate professor (extraordinarious) of theoretical physics (1921) at the Technische Hochschule. Around this time he published his seminal work on X-ray crystallography, Kristalle und Röntgenstrahlen (1920), and he was appointed to the board of editors of the Zeitschrift für Kristallographie (1924–39), at the time one of the few journals publishing crystallographic research. He also produced, with C. Hermann, an international reference work for the determination of crystal structures for the period 1913–28, entitled Strukturbericht (1931). He was promoted professor (ordinarius) (1922–36) at Stuttgart, resigning as recktor (president/provost) of the college in protest when, under a new Nazi law, all his Jewish colleagues were dismissed. He left Germany when Sir Lawrence Bragg, an old friend at Cambridge, succeeded in arranging a research fellowship for him at the prestigious Crystallographic Laboratory, Cambridge (1937–9), his family following him to England in April 1938. Obtaining a lectureship in mathematical physics at QUB, and with the promise of a professorship, he moved to Belfast in 1939, aged 51.
In Belfast his daughter Linde attended the university to study medicine, and the family integrated well, living in a large house on Rugby Road. The people they met were very friendly and supported them after the outbreak of the second world war, when they became technically enemy aliens and their youngest son was interned and deported to Australia. They were very involved with the refugee community in Belfast and were a source of great comfort for others in a similar situation. Ewald would have been interned but for the fact that it was considered more important that he continue to lecture engineering students for the war effort. The RUC officer responsible for refugees was strongly supportive of the family, and Ewald's wife ended up painting a portrait of him. The war years were hard for Ewald, and he produced hardly any scientific work during this time. His appointment to a professorship was delayed until the end of the war (1945); the same year he also became a British subject.
Near to retirement, he extended his professional career by accepting (1949) the offer of a position in the Polytechnic Institute of Brooklyn, New York. He remained there as professor and head of the physics department for the remainder of his working life (1949–60). His interest in research was rekindled and he led the development of a research programme in the department, which had previously concentrated on teaching. During the war he had maintained his contacts with other crystallographers and was instrumental in establishing the International Union of Crystallography in 1948, modelled along the lines of the International Union of Pure and Applied Physics, of which he had been secretary general in 1946. After his retirement from the Polytechnic Institute he was president of the International Union of Crystallographers (1960–63). His other major contribution to crystallography was as editor of Acta Crystallographica, a journal he founded in 1946 to replace the defunct Zeitschrift für Kristallographie, and which he edited until his retirement in 1960.
Ewald is chiefly remembered as one of the pioneers in the study of crystal structure by means of X-ray scattering; his speciality was the theory of the propagation of X-rays waves in crystals. He published prolifically. Helmut Juretschke compiled a bibliography of Ewald's work (Juretschke, Cruickshank, and Kato (1992)), based on a manuscript prepared by Ewald himself. In it he lists 100 scientific papers, 39 occasional writings (including obituaries and contributions to general science works), and 135 book reviews. His publications spanned the period from his first publication on radium in a popular science journal as a teenager (1904) to a posthumous scientific publication on corrections to Bragg's Law (1986).
He received many honours and tributes throughout his life for his academic excellence and his service to physics. He was made corresponding member of the Göttingen Academy (1936), fellow of the American Academy of Arts and Science (1955), and FRS (1958). Awarded honorary doctorates from the Technische Hochschule, Stuttgart (1954), the University of Paris (1958), Adelphi University (1966), and the University of Munich (1967), he was later made a fellow of the Akademie Deutscher Naturforscher (Leopoldina) (1966). Having received the Max Planck medal of the German Physical Society (1978) in recognition of his contributions to science, the following year he was awarded the first Gregori Aminoff medal of the Royal Swedish Academy of Sciences for outstanding contributions to crystallography (1979). On his eightieth birthday, the January 1968 issue of Acta Crystallographica was dedicated to him. In 1979 a symposium on dynamical diffraction was organised to honour him on the occasion of his ninetieth birthday, and he presented the first paper.
As a teacher he preferred to stimulate students to thinking rather than spoon-feed them, an approach to teaching that was more popular in Germany than it was in Belfast. As a man he was friendly and unassuming and honest to a fault. He was interested in ideas and people, and was a witty and humorous companion, frequently composing poems to celebrate special occasions for family and friends. He died 22 August 1985 at home in Ithaca, New York, after a long illness, aged 97. His wife survived him.
He married (1913) Ella Philippson, a medical student and a distant relative of his mother. They had four children: Lux (b. 1914), Rose (b. 1917), Linde (b. 1919), and Arnold (b. 1921). The Ewalds were devoted to each other throughout their long life together. She did not pursue her medical degree after her marriage, but devoted herself instead to making a home for their family and his mother, who died in Belfast in 1948, aged 88, during the difficult times before they eventually settled in the US. She entertained friends who came to their house in connection with Paul's work, and she took an active interest in his career.