Interferenz-Erscheinungen bei Röntgenstrahlen." - "Eine quantitative Prüfung der Theorie für den Interferenz-Erscheinungen bei Röntgenstrahlen." Offprint (containing both papers) from the Sitzungsberichte der Königlich Bayerischen Akademie der Wissenschaften Mathematisch-physikalische Klasse (1912)

Münich: F. Straub for the Verlag der Königlich Bayerischen Akademie der Wissenschaften, 1912. First edition. PMM 406 - 'One of the most beautiful discoveries in physics' (Einstein).

First edition, very rare offprint issue, of Laue's Nobel Prize-winning report of "one of the most beautiful discoveries in physics" (Einstein). X-rays had been in wide use since their discovery in 1895 but their exact nature as electromagnetic waves of short wavelength was first elucidated by Laue and his collaborators in the present papers. Laue (1879-1960) had moved in 1909 from Berlin (where he was Planck's assistant) to Ludwig Maximillians University in Münich, where he was Arnold Sommerfeld's Privatdozent. In the spring of 1912 he was asked by Sommerfeld's doctoral student Paul Ewald a question about the arrangement of atoms in a crystal. In attempting to answer this question "Laue had the crucial idea of sending X-rays through crystals. At this time scientists were very far from having proven the supposition that the radiation that Röntgen had discovered in 1895 actually consisted of very short electromagnetic waves. Similarly, the physical composition of crystals was in dispute, although it was frequently stated that a regular structure of atoms was the characteristic property of crystals. Laue argued that if these suppositions were correct, then the behavior of X-radiation upon penetrating a crystal should be approximately the same as that of light upon striking a diffraction grating" (DSB), an instrument used for measuring the wavelength of light, inapplicable to X-rays because their wavelength is too short. Sommerfeld was initially skeptical but Laue persisted, enlisting the help of Sommerfeld's experimental assistant Walter Friedrich (b. 1883) in his spare time as well as that of the doctoral student Paul Knipping. On April 12, 1912, Friedrich and Knipping succeeded in producing a regular pattern of dark spots on a photographic plate placed behind a copper sulphate crystal which had been bombarded with X-rays. Laue's second paper contains his complicated mathematical explanation of the phenomenon. "The awarding of the Nobel Prize in physics for 1914 to Laue indicated the significance of the discovery that Albert Einstein called 'one of the most beautiful in physics'. Subsequently it was possible to investigate X-radiation itself by means of wavelength determinations as well as to study the structure of the irradiated material. In the truest sense of the word scientists began to cast light on the structure of matter" (DSB). The following year the Prize was granted to the father and son team W. H. and W. L. Bragg for their exploration of crystal structure using X-rays. ABPC/RBH lists three other copies of this offprint (Christie's, 4 October 2002, lot 151, $5736; Sotheby's, 11 January 2001, lot 333, $10,200; Christie's 29 October 1998, lot 1161, $16,100).

In 1912, "the nature of the X rays discovered by Röntgen in 1895 was not known. Röntgen himself conjectured that they might be longitudinal ether waves as opposed to the transverse ones, the electromagnetic waves found by Hertz. Since in Röntgen's original experiment the X-rays originated from the point where cathode rays, i.e., electrons, hit matter, Wiechert and also Stokes suggested already in 1896 that X-rays were emitted by electrons while the latter were decelerated. In Maxwell's theory an electric charge with a velocity, which is not constant, emits electromagnetic waves. In the Hertzian dipole antenna the charges oscillate to and fro. In a Röntgen tube electrons lose their velocity hitting a piece of matter. The fact that interference effects, characteristic of all waves, in particular, light and Hertzian waves, were not observed for X-rays, did not preclude that they were electromagnetic waves. Their wavelength might be too small for the detection of interference ...

"The nature of X-rays, homogeneous or heterogeneous, remained a mystery. They could be understood as electromagnetic waves of short wavelengths or as new neutral particles. The former standpoint was taken, for instance, by Barkla, the latter by William Henry Bragg. One of Bragg's arguments ran like this: X-rays, produced by electrons falling on matter, fly more or less in the same direction as the incident electrons. That is easily understood if one assumes them to be particles. The production of X-rays can then be seen as a collision process, just as one billiard ball hitting another. For some time the two scientists fought out the Barkla-Bragg controversy in the columns of Nature. Sommerfeld showed that, contrary to the expectations of Bragg and others, electromagnetic radiation is emitted mostly in forward direction if a fast electron suffers a sudden deceleration. The German term bremsstrahlung [breaking radiation] is still used commonly in the literature.

"It was the work of Laue and the experiment done by Friedrich and Knipping on his suggestion that cleared up the nature of X-rays once and for all and that, moreover, beautifully demonstrated that crystals are composed of atoms arranged in a regular lattice. Laue had studied mathematics and physics in Strasbourg, Göttingen, Munich, and Berlin, where in 1903 he took his Ph.D. with a thesis under Planck. Feeling that he still had to continue his studies he went for another two years to Göttingen. In 1905 Planck offered him a position as his assistant. Laue worked with Planck on the latter's speciality, the entropy of radiation. In the autumn of 1905 Planck gave a talk in the Berlin Physics Colloquium on Einstein's first paper The Electrodynamics of Moving Bodies. Laue was deeply impressed. In 1906, when on a mountaineering trip in Switzerland, as one of the first (possibly the very first) visitor from abroad, he looked up Einstein in the patent office in Bern. In 1907 he published a paper in which he showed that classic experiment by Fizeau, who had measured the velocity of light in a moving liquid, was in accordance with Einstein's theory. Laue became a Privatdozent in Berlin and, also in that capacity, moved to Munich University in 1909. In 1910 he wrote the first book on the theory of relativity ...

"The towering figures in Munich were Röntgen (who, however, took only little part in research any more) and Sommerfeld. Early in 1912 Ewald, then a Ph.D. student of Sommerfeld, looked up Laue in his flat. Sommerfeld had asked him to study theoretically the behaviour of light waves in a spatial lattice of polarizable atoms. Laue could not help with the theory but it occurred to him that possibly a similar problem could be studied experimentally if one assumed that a crystal was a regular spatial lattice of atoms and if one passed X-rays through a crystal. (It had been conjectured in the nineteenth century, in particular by Bravais, that the regular shape of a crystal is due to the underlying regular lattice of atoms of which the crystal is composed. But that was a mere hypothesis and not widely discussed.) Assuming the wavelength of X-rays to be on the order of the distance between neighbouring atoms, one might be able to see interference effects. He mentioned this idea to Ewald and it soon got around in the closely-knit group of young physicists in Munich. Soon Friedrich, who had just obtained his Ph.D. with Röntgen and now was Sommerfeld's assistant, became interested. Sommerfeld had to be convinced that the experiment was important enough for Friedrich to start it right away in spite of other assignments. Knipping, a Ph.D. student of Röntgen, joined in the effort.

"Friedrich and Knipping sent a collimated beam of X-rays, 3 mm wide, onto a crystal of copper sulphate and placed a photographic plate at some distance behind the crystal. They observed a dark spot, where the undiffracted beam hit the plate. That spot was surrounded by a more or less regular pattern of further spots, which they attributed to diffraction by the crystal lattice. Only after Laue saw the plate did he start in earnest to analyse the problem of diffraction by a spatial lattice. Later he reminisced: 'In deep thought I went home by way of Leopoldstrasse after Friedrich had shown me the picture. And already near my flat, Bismarckstrasse 22, in front of the house Siegfriedstrasse 10, the thought came for the mathematical theory of the phenomenon.'

"Once the idea had come, the rest was simple. Laue advanced in three steps. First he recalled the laws of diffraction of light by a one-dimensional lattice or grid. If white light shines perpendicularly onto a grid made of fine wires, whose distance is on the order of the wavelength of this light, then on a screen behind the grid there will be a line of white light in the forward direction and a repetition of rainbow-like spectra to the left and to the right. If monochromatic light is used, there will be a series of regularly placed sharp lines. This is due to the fact that for a given wavelength only under certain angles light from equivalent points in the grid interferes constructively (i.e., has a phase difference corresponding to an integer number of wavelengths). He then turned to a two-dimensional lattice by replacing the grid by a mesh. Now two such conditions have to be met. Instead of a line pattern only a point pattern is observed for monochromatic light. For white light it is a pattern of patches each dis- playing side by side the colours of the rainbow. (The reader can easily do the experiment by looking at a street light through the cloth of her or his umbrella at night.) Up to here all was well known. But now Laue found that for a three- dimensional lattice a third condition had to be met. The spots are characterized by two angles (diffraction in horizontal and vertical direction). A third condition means that only a small number of spots is formed and only for certain wavelengths. Laue, of course, wrote his conditions in mathematical form, later called the Laue equations.

"Friedrich and Knipping improved their apparatus. They reduced the beam diameter, shielded their set-up by a lead box from stray radiation and, for the same reason, allowed the undiffracted beam to leave it through a lead tube. Instead of the original copper sulphate they used a carefully polished crystal of zinc blende. Also, they took great care to make the symmetry axis of the crystal coincide with the beam axis. As a result, they obtained photographs with beautifully symmetric sharp spots fitting Laue's theory. Such a photograph is now called a Laue diagram and the process leading to it is called Laue diffraction. A joint paper was written ['Interferenz-Erscheinungen bei Röntgenstrahlen'] with the theoretical part signed by Laue and the experimental part signed by Friedrich and Knipping and communicated by Sommerfeld on 8 June 1912 to the Bavarian Academy of Science. That same day Laue gave a talk on a session of the German Physical Society in Berlin 'at the same spot', as he proudly remembered, 'where in December 1900 Planck had first talked about his radiation law and the quantum theory'. Laue, Friedrich and Knipping assumed that the zinc blende crystal had a simple cubic lattice with zinc and sulphur atoms alternating on the corners of cubes of side length a. According to Laue's theory, the radiation responsible for one point in the diagram is monochromatic. All the points in the Laue diagram of zinc blende corresponded to just five different wavelengths. In July 1912 an extended numerical analysis of the diagram ['Eine quantitative Prüfung der Theorie für den Interferenz-Erscheinungen bei Röntgenstrahlen'] was published by Laue alone. He found that all the points in the diagram were explained by his theory but that not in every position in which his theory allowed for a point such a point was observed. He therefore assumed that the radiation emitted by the X-ray tube was a mixture of five monochromatic radiations, each with its own wave-length" (Brandt, pp. 80-83).

The two papers were first printed in 1912 in the Sitzungsberichte der Königlich Bayerischen Akademie der Wissenschaften; the offered offprint contains both papers. The two papers were reprinted the following year in Annalen der Physik (4 Folge, Bd. 41, pp. 971-988 & 989-1002). 

PMM 406a; Norman 1283. Brandt, The Harvest of a Century, 2009.

Offprint from the Sitzungsberichte der Königlich Bayerischen Akademie der Wissenschaften Mathematisch-physikalische Klasse (1912). Large 12mo (220 x 144 mm), pp. 303-322 & 363-373, with three line block diagrams in text and five collotype plates. Stapled in original printed green wrappers, a very fine copy. Preserved in a clamshell cloth box.