Paul Peter Ewald

Paul Peter Ewald, FRS (January 23, 1888 – August 22, 1985) was a German crystallographer and physicist, a pioneer of X-ray diffraction methods.

Ewald received his early education in the classics at the Gymnasium in Berlin and Potsdam, where he learned to speak Greek, French, and English, in addition to his native German.

Ewald began his higher education in physics, chemistry, and mathematics at Gonville and Caius College in Cambridge, during the winter of 1905. In 1906 and 1907, he continued his formal education at the University of Göttingen, where his interests turned primarily to mathematics. At that time, Göttingen was a world-class center of mathematics under the three “Mandarins” of Göttingen: Felix Klein, David Hilbert, and Hermann Minkowski. While studying at Göttingen, Ewald was taken on by Hilbert as an Ausarbeiter, a paid position as a scribe, i.e. he would take notes in Hilbert’s classes, have the notes approved by Hilbert’s assistant — at that time, Ernst Hellinger — and then prepare a clean copy for the Lesezimmer — the mathematics reading room. In 1907, he continued his mathematical studies at the Ludwig Maximilian University of Munich (LMU), under Arnold Sommerfeld at his Institute for Theoretical Physics. He was granted his doctorate in 1912. His doctoral thesis developed the laws of propagation of X-rays in single crystals. After earning his doctorate, he was an assistant to Sommerfeld.

During the 1911 Christmas recess and in January 1912, Ewald was finishing the writing of his doctoral thesis. It was on a walk through Englischer Garten in Munich, in January, that Ewald was telling Max von Laue about his thesis topic. The wavelengths of concern to Ewald were in the visible region of the spectrum and hence much larger than the spacing between the resonators in Ewald’s crystal model. Laue seemed distracted and wanted to know what would be the effect if much smaller wavelengths were considered. It was not until June of that year that Ewald heard Sommerfeld report to the Physikalische Gesellschaft of Göttingen on the successful diffraction of X-rays by Max von Laue, Paul Knipping and Walter Friedrich at LMU, for which Laue would be awarded the Nobel Prize in Physics, in 1914.

With the rise of theoretical physics in the early part of the twentieth century and its foundation in mathematics, David Hilbert decided to lend an organizing hand to formalizing the sciences, starting with physics. In 1912, Hilbert asked his friend and colleague Arnold Sommerfeld to send him a special assistant for physics. Sommerfeld sent Ewald, who was dubbed as “Hilbert’s tutor for physics”, and he performed this function until 1913, when Sommerfeld sent another one of his students, Alfred Landé. The first problem assigned to Ewald was to review the controversy in the literature on the constants of elasticity in crystals and report back. A few years later, Max Born, at Göttingen, solved the problem.

During Ewald’s stay in Göttingen, he was often a visitor at El BoKaReBo, a boarding house run by Sister Annie at Dahlmannstrasse 17. The name was derived from the first letters of the last names of its boarders: “El” for Ella Philippson (a medical student), “Bo” for Max Born (a Privatdozent) and Hans Bolza (a physics student), “Ka” for Theodore von Kármán (a Privatdozent), and “Re” for Albrecht Renner (a medical student). Richard Courant, a mathematician and Privatdozent, called these people the “in group”. It was here that Ewald met Ella Philippson, who was to become his wife.

In the spring of 1913, Niels Bohr, of the Institute for Theoretical Physics at the University of Copenhagen, submitted his theory of the Bohr atomic model for publication. Later that year, Ewald attended the Birmingham meeting of the British Association where he heard accounts and discussions of James Jeans’ review on radiation theory and Bohr’s model. This ignited a major new area of research for Sommerfeld and his students — the study and interpretation of atomic spectra and molecular band spectroscopy and theoretical modeling of atomic and molecular structure.

During World War I, Ewald served in the German military as a medical technician. When he could, he continued to think about the physics of his doctoral thesis, and he developed the dynamical theory of X-ray diffraction, which he was later to use in his Habilitationsschrift. At the conclusion of the war, he returned to LMU as an assistant to Sommerfeld. He completed his Habilitation in 1917, and became a Privatdozent there, while remaining as an assistant to Sommerfeld.