Kurt Gottfried wrote, “For almost seven decades, [Hans Bethe’s] wife Rose was his constant companion and closest adviser” (Physics Today, October 2005, page 36). Sam Schweber (page 38) explained that Bethe’s 1928 thesis on electron diffraction in crystals built on previous work by Paul Ewald on the diffraction of x rays by crystals. The two statements are intimately connected.
In his 1981 “Reminiscences of the Early Days of Electron Diffraction,” Bethe wrote the following:
On the basis of my thesis, I was invited by P. P. Ewald to give a talk at a small conference on diffraction which he was arranging in Stuttgart in 1928. Apparently my talk pleased him, because a year later he asked me to become his assistant. I had a most enjoyable semester there, with a great deal of research, and close personal contact with Ewald and his family. Out of this I got a wife: Ewald’s daughter, then 12 years old, was already very attractive, but I did not dream of marrying her. Eight years later, I met her again, and in 1939 we got married. So I owe a great deal to electron diffraction. 1
Arnold Sommerfeld had proposed that Bethe make a detailed theory of electron diffraction in a crystal. He recommended as a model the theory by Ewald of the diffraction of x rays, written in 1917. Bethe found that electron diffraction was a great deal simpler. In the x-ray case one has to contend with a vector field. He retained only Ewald’s fundamental idea, the expansion of a spherical wave—that is, the wave scattered by an atom—in terms of plane waves. Thence Bethe developed the theory of electron diffraction in first-order perturbation theory.
