"What would happen if you assumed very much shorter waves to travel in the crystal?" This was the question posited by Max von Laue, then an associate professor at the Institute of Theoretical Physics in Munich, to Paul Ewald who was a student of Arnold Sommerfeld, director of that institute, during a discussion on the propagation of light in crystals. As it turns out, that conversation laid the basis for modern X-ray crystallography. A few months later, in April 1912, the first demonstration of X-ray diffraction from a crystal lattice was achieved.

The 'shorter waves' mentioned by von Laue were the X-rays discovered by Röntgen 17 years before. At the time, the nature of these rays was the subject of intense debate. The photoelectric effect showing that gas molecules are ionized by an X-ray beam indicated a corpuscular nature, whereas the observations that X-rays are polarized and can be diffracted by fine slits supported a wave-like interpretation. Several researchers also estimated the wavelength of these rays to be around 0.5 Å, orders of magnitude smaller than light. When Ewald, who was developing a theoretical model to explain the double refraction of light passing through a crystal, described crystalline structures as a regular arrangement of resonators having a distance comparable to this short wavelength, von Laue resolved that the characteristic X-ray fluorescence emitted from these particles had to produce diffraction patterns.

At the beginning, this idea received some opposition; indeed, both Sommerfeld and Wilhelm Wien doubted that the emission coming from these atoms would be coherent and thought that the interference would be destroyed by thermal motion of the crystal. Nevertheless, in April 1912 von Laue was able to secure the help of two brilliant experimentalists, Walter Friedrich and Paul Knipping, to test his hypothesis. The two physicists used a powerful X-ray bulb and collimated a narrow primary beam on several crystals (copper sulphate pentahydrate and zinc sulphide, in particular) that, according to previous studies, contained metallic species showing strong X-ray fluorescence. Looking for interference from an isotropic radiation, they first positioned a collecting photographic plate parallel to the primary X-ray beam, but detected no signal. When they added a photographic plate behind the crystal, Friedrich and Knipping finally recorded traces of the diffracted beam, proving that the intuition of von Laue was true, though only in part (Milestone 3).

X-ray diffraction pattern from a zinc-blende (ZnS) crystal. Figure reprinted with permission from W. Friedrich et al. Annalen der Physik 346, 971–988 (1913). Credit: © WILEY-VCH

The results of the experiment and their theoretical interpretation were published in August 1912. Yet even before the papers were out, the success of the experiment spread around Europe: Max Planck recalled that scientists in Berlin "felt that a remarkable feat had been achieved" and Albert Einstein defined the experiment as "among the most glorious that physics has seen so far". The interference patterns supported the interpretation of X-rays as electromagnetic waves. Remarkably, these findings also had an exceptional resonance among crystallographers: those well-defined spots were seen as conclusive evidence that atoms arrange in a space-lattice configuration in crystals. As Alfred Tutton — an English crystallographer — stated in November 1912 "the space-lattice structure of crystals ... is now rendered visible to our eyes" (Milestone 1).