EWALD, PAUL PETER

(b. Berlin, Germany, 23 january 1888 d Ithaca, New York, 22 August 1985)

physics, crystallography.

Ewald was the son of Paul Ewald, a Privatdozent in history at the University of Berlin, and of Clara Philippson Ewald, an internationally known portrait painter. His father died of appendicitis shortly before Paul was born. His mother raised Ewald, and as a result of their travels he learned to speak English and French at a very early age. He was educated at the Königliches Wilhelmsgymnasium in Berlin and the Königliches Victoriagymnasium in Potsdam. Ewald graduated from the latter in 1905 and then began to study chemistry at Gonville and Caius College, Cambridge. In 1906, after on e semester, he entered the University of Göttingen to contingen to continue studying chemistry. He was, however, disappointed by the lack of consistent theoretical connection of the various facts of chemistry and soon changed to mathematics, which he studied for three semesters (1906–1907) Although he found his first mathematics courses at Göttingen not very helpful, he was compensated during the following semesters, when he worked with David Hilbert and Ernst Hellinger on differential and integral calculus.

Ewald transferred in 1907 to the University of Munich, where his two semesters of mathematical studies included Alfred Pringsheim’s lectures on functional analysis. He also attended Arnold Sommerfeld’s lectures on hydrodynamics. Fascinated by the interplay of theory and experiment in physics, he became Sommerfeld’s student.

In 1910 Ewald chose as the subject of his doctoral dissertation the problem of how to find the optical properties of an anistropic arrangement of isotropic resonators, an area in which Sommerfeld could offer little help. Ewald’s approach was quite original.ǀInstead if investigating the reaction of the dipoles on the incident light, he focused on the electromagnetic wave field that is “dynamically possible” in the interior of a lattice arrangement of oscillators. With the help of boundary considerations and especially by explicit calculations, in the reprint of his dissertation (1916) he could explain the “compensation [extinction] of the incident wave,” the “dynamically closed” state of the refracted waves (the oscillation modes in the crystal), and the existence of reflected waves outside the crystal. Among the difficulties Ewald had to overcome in working out his thesis was the calculation of the electromagnetic field exciting the dipole oscillations of any one atom. The problem was how to subtract from the entire field the one that originated from the atom itself. This amounted to subtracting infinity from infinity.

Fruitful in another way was Ewald’s attempt to discuss some details of his dissertation with Max von Laue in February 1912. His dissertation, with its underlying assumption of a regular spatial arrangement of particles in a crystal, stimulated Laue to think of the phenomena produced by light of very short wavelength (comparable with atomic distances) in the space lattice of a crystal. The personal recollections of Laue and Ewald stress the unique situation in Munich, whe re Leonhard Sohncke and Paul von Groth allegedly had kept alive the space lattice theory of crystals and Sommerfeld advocated the wave theory of X rays.

At Laue’s initiative, Sommerfeld’s assistant Walter Friedrich and Röontgen’s doctoral student Paul Knipping in April 1912 succeeded in taking photographs of what was soon recognized as X-ray diffraction in a copper sulfate crystal. After receiving his doctorate in that year, Ewald applied its “dynamical theory” to the diffraction of X rays. In this context he invented the “sphere of reflection”(1913). The possible intersections of the “sphere of reflection”with certain points in the reciprocal lattice furnish the directions of constructive interference.

Ewald did postdoctoral work at the University of Göttingen, where he was assistant to David Hilbert. In 1913, soon after his marriage to Elisa Berta (Ella) Philippson, he returned to Munich, at first sharing the post of assistant to Sommerfeld with Wilhelm Lenz. He had learned to operate X-ray equipment for medical purposes, so during World War I he was a field X-ray technician on the northern Russian front. By the autumn of 1915 fighting had practically ceased there, so he had time to continue his work on the crystal optics of X rays, which contains the elaborate dynamical theory of X-ray diffraction in perfect crystals.

Ewald’s theory predicted a deviation from the simple Bragg equation, which explained the X-ray diffraction geometrically by reflections of the incident rays on the internal atomic net-planes and their subsequent interference. Indications of this deviation could be seen in experimental investigations performed in Manne Siegbahn’s institute at Lund. A confirmation of Ewald’s theory was, however, possible only with the perfect crystals produced by the semiconductor industry many decades later. Today the perfection of crystals can be tested with the help of Ewald’s theory. His dynamical theory stimǀulated Hans Bethe (who later became his assistant and his son-in-law), who in 1928, in his doctoral dissertation (under Sommerfeld), dealt with electron diffraction by crystals. Another application was in the field of neutron diffraction.

Ewald’s lifelong research in the field of X-ray diffraction caused him to develop the ideas of his dissertation and of his Habilitationsschrift. The dissertation furnished exact solutions only for two beams. Ewald hoped eventually to solve the n -beam problem. According to Bethe’s recollections, Ewald wanted thus to finish his dissertation.

In 1918 Ewald became Privatdozent at the University of Munich. He was named extraordinary professor of theoretical physics at the Technische Hochschule (now the University) of Stuttgart in 1921, and was appointed professor in 1922. While there, following the work of Richard Glocker, who as early as 1919 had done intensive experimental work on X-ray analysis of metal structures. Ewald helped to create a center of X-ray research and solid state physics.

Ewald was appointed rector at Stuttgart in 1932. In his inaugural address, he urged his audience to strive for social and political harmony, and he ended pathetically with the famous verse of the national anthem “Deutschland über alles.” The following year, however, the National Socialists’ “law for the restoration of the civil service” caused him to resign the rectorship because his wife was Jewish and he was part Jewish. However, his service at the front in World War I and the Nuremberg redefinition of “part Jewish,” and a succession of nominal National Socialists as rectors, allowed him to continue his work as professor. In 1936. when a young Nazi teaching corps leader read a government paper denying the value of an “objective” science, Ewald walked out of the assembly. Thenew rector, an ardent National Socialist, urged Ewald to resign and had him pensioned off three weeks later.

In 1937 Ewald left Germany, a step he had been considering since 1933. With the help of William Lawrence Bragg, he was able to continue research at Cambridge, supported by a grant. In 1939 he was appointed lecturer, and later professor, of mathematical physics at Queen’s University, Belfast. The financial circumstances of the family—there were four children—improved when Ewald accepted the latter postr and, from 1949, when he was professor of physics and head of the department at the Polytechnic Institute of Brooklyn. He retired in 1959.

Of outstanding importance was Ewald’s contribution to the formation and growth of crystallography. From 1924 to 1940 he was coeditor of Zeitschrift für Kristallogra[hie, founded by Paul von Groth. With his pupil Carl Hermann he published the first volume of Strukturbericht. 1913–1928 as a supplement to Zeitschrift für Kristallographie (1931). Six volumes covering the period to 1939 were to follow. After the war Ewald was instrumental in continuing this review work as Structure Reports. Through his initiative and with Bragg’s help, the International Union of Crystallography was founded in 1947 with Bragg as its president and Ewald as its vice president. From 1948 to 1958 Ewald served as one of the editors of its journal, Acta crystallographica

Ewald was a member or fellow of the American Academy of Arts and Sciences, the Royal Society, and the Deutsche Akademie der Naturforscher (Leopoldina). He received honorary doctorates from the University of Stuttgart in 1954, from the University of Paris in 1958, and from the University of Munich in 1968. In 1978 the Deutsche Physikalische Gesellschaft awarded him the Max Planck Medal, and the following year he received the first Gregori Aminoff Medal of the Royal Swedish Academy.

BIBLIOGRAPHY

I. Original Works. Ewald’s writings include “Dispersion und Doppelbrechung von Elektronengitten (Kristallen)”(Ph.D. dissertation. University of Munich. 1912; published at Göttingen. 1912): “Zur Theorie der interferenzen der Röntgenstrahlen in Kristallen,” in Physikalische Zeitschrift. 14 (1913). 465–472: “Zur Begründung der Kristalloptik. Teil I : Theorie der Dispersion,” in Annalen der Physick. 49 (1916). 1–38, an abbreviated version of his dissertation: “Zur Begründung der Kristalloptik. Teil II: Theorie der Reflexion und Brechung,” ibid.. 1171–43, translated as On the Foundation of Crystal Optics. Air Force Cambridge Research Laboratories. Translations no. 83 (Bedford, Mass., 1970); “Zur Begründung der Kristalloptik, Teil III: Die Kristalloptik der Röntgenstrahlen,” in Annalen der Physik, 54 (1917), 519–597, his Habilitationsschrift: Kristalle und Röntgenstrahlen (Berlin, 1923);The Physics of Solids und Fluids, With Recent Developments, written with T. Pöschl and L. Prandtl, J.Dougall and W. m. Deans, trans. (London and Glasgow, 1930;2nd enl.ed., 1936);“Die Erforschung des Aufbaues der Materie mit Röntgenstrahlen,” in Hans Geiger and Karl Scheel.eds., Hundbuch der Physik, 2nd ed., XXIII, ǀpt. 2 (Berlin. 1938);“some personal Experiences in the International Coordination of Crystal Diffractometry,” ǀin Physics Today, 6. no. 12 (1953). 12–17;as editor.Fifty Years of X-Ray Diffraction (Utrecht. 1962); “William Henry Bragg and the New Crystallography,” in Nature, ǀ195 (28 July 1962), 320–325;“The Myth of Myth; Comments on P. Forman’s paper on’ The Discovery of the Diffraction of X-Rays in Crystals, ’”in Archive for History of Exact sciences, 6 (1969/1970), 72–81; “Physicists I ǀHave Known,” in Physics Today, 27 (September 1974 ), ǀ42–47;and “Remmebring Peter Debye in Munich,” ibid. 38 (January 1985), 9. 122.

Letters from or to Ewald are in the following archives: American Institute of Physics: Olin Library. Cornell University; Landesbibliothek, Stuttgart; Bayerische Staatsbibliothek, Munich; Deutsches Museum. Munich; Perkins Library, Duke University: Bodleian Library, Oxford University; Library of Congress. Washington, D.C.; Bibliothek und Archiv, Max Planck Gesellschaft, Berlin; Staatsbibliothek Preussischer Kulturbesitz. Berlin; and Archive for History of Quantum Physics, Office for History of Science and Technology. University of California, Berkeley. The last also has interviews with Ewald, 29 March 1962, conducted by G. Uhlenbeck with T.S. Kuhn and Mrs. Ewald, and 8 May 1962, conducted by T.S.Kuhn. Personnel records are at the Rektoramt of the University of Stuttgart

II Secondary Literature. On Ewald or his work, see Michael Eckert, Willibald Pricha. Helmut Schubert, and Gisela Torkar, Geheimrat Sommerfeld—theoretischer Physiker (Munich, 1984): Paul Forman, “The Discovery of the Diffraction of X-Rays by Crystals; a Critique of the Myths,” in Archive for the History of Exact Sciences, 6 (1969/1970), 38–71; G. Hildebrandt. “Zum Tode von paul Peter Ewald,” in Physikalische Blätter, 41 (1985), 412–413; and David Phillips, “William Lawrence Bragg,” in Biographical Memoirs of Fellows of the Royal Society, 25 (1979), 75–143, which on pp. 115–116 provides details on the founding of the International Union of Crystallography.

Walter Kaiser