947 Obituary ~J Peter Debye technic School in Aachen. Here he happened to take courses with Sommerfeld, who was then the young 24 March 1884 - 2 November 1966 Professor of Technical Mechanics. Sommerfeld did not fail to recognize this brilliant student and soon made By the death of Peter Debye on 2 November 1966 in him his assistant, and, in 1906, took him along to Ithaca, N.Y., the world has lost one of its great physi- Munich as assistant and doctorand. The subject of cists, whose grip on the physical situation, mastery of Debye's thesis was a new and much improved theory experimental and mathematical methods, realism and of the rainbow, based on the strict theory of diffraction down-to-earthness has thrust forward many branches of light by a small dielectric sphere. I cannot remember of pure and industrial research of such variety that he that this thesis was ever published in full, for it had was made successively the holder of famous chairs of outgrown the usual volume of a Ph. D. thesis, and Physics and of Chemistry. Teubner, the publishers, had offered to accept it as a Debye was born in Maastricht, The Netherlands, a book. But in these early years one important and topical town close to the German border. His father was subject after the other appeared more urgent to Debye foreman in a machine workshop, and much of his and was tackled by him in a masterful way, so that son's liveliness and quick perception was inherited the finishing of a book on a classical problem of lesser from him. After finishing school in Maastricht, Peter urgency was not so attractive. A mathematical method began the study of Engineering at the near-by Poly- developed for the thesis was published separately in 948 OBITUARY Math. Annalen (1910) and has since served in many factor in its present form. The interpretation of the places; this was the method of Steepest Descent (Sat- constant in the exponent was wrong by a factor of 2, telpunktsmethode) of evaluating complex integrals. as shown eleven years later by Ivar Waller - hence the In the years when Debye was in Munich, 1906-1911, name Debye-Waller factor. physics was in a very turbulent state. On the one hand Debye was given no chance in Utrecht to check his there was the upheaval of fundamentals in consequence theoretical work by measurements. He therefore ac- of Einstein's theory of relativity. On the other hand cepted a call to the University of G/Sttingen, where he the quantum of energy, hv, which Planck introduced remained from 1914 to 1920. Niels Bohr's first epochal in 1900 in order to arrive at the correct spectral distri- papers on quantized electron orbits in atoms had ap- bution of black-body radiation, remained at that time peared in 1913, and Debye argued that the truth of an isolated ad hoe assumption. Einstein, in one of his these atomic models could be established by observing four famous papers of 1905, was the first to detect the X-ray interference effects due to the spacing of the other manifestations of h: he assumed a solid body to electrons on their orbits. He and Paul Scherrer indeed contain mechanical resonators of frequency v and dis- obtained interference rings, using lithium fluoride as tributed energy on them according to the same function scatterer of monochromatic X-rays. But these rings that Planck had used for his radiation resonators. This were much too sharp to be caused by the small dis- gave the solid body a thermal-energy content, and its tances within the atom, and Debye and Scherrer quick- derivative, the heat capacity, which vanished at low ly realized the nature of crystal-powder diffraction and temperatures in a manner reconcilable with Nernst's developed the indexing and discussion of the rings. The early measurements. Further measurements showed powder diffraction method quickly became the favour- that Einstein's formula did not suffice; Nernst & Linde- ite method of crystal analysis and identification in mann increased the general bewilderment by suggesting most chemical, mineralogical and metallurgical labor- two kinds of resonators, of frequencies v and ½v. It atories. As one of the first applications, Scherrer elu- was at this stage that Debye saved the situation by cidated the nature of metallic colloidal solutions and applying Planck's distribution to the proper modes of showed how to determine the sizes and structures of vibration of an elastic solid - a very daring procedure particles far below the resolving power of the light seeing that these body-pervading states were substi- microscope. tuted for the atomic or molecular resonators Planck Debye was less interested in finding crystal structures and Einstein had been thinking of. Debye was an avid than in using diffraction for the exploration of the reader of classical mathematics and physics, and in distribution of electrons round the nucleus of atoms. particular of Lord Rayleigh's papers and his Theory of He confirmed the ionic charges of lithium and fluorine Sound. Proper modes of vibration had a high priority in LiF by extrapolating the atomic form factors to in his mind. zero scattering angle (1916), and showed, in 1919, that I will not dwell on the ingenious way by which Debye metals do not have a sodium-chloride-like arrangement obtained the distribution of proper frequencies by first of positive cores and electrons, as had been contended finding that of an elastic sphere according to the con- at the time. tinuum elastic theory, and then simply cutting off this The later work of Debye and his co-workers makes infinite spectrum when the number of frequencies cor- use of light, X-ray, and electron scattering for the study responding to the finite number of molecules is reached. of the structures of single molecules, fluids, and high It is a catch-as-catch-can method based, however, on polymers. The discussion is in most cases a combination deep physical insight. Its success was resounding, and of classical interference theory and classical statistics. its simplicity and appeal so great that for a long time A variety of physically significant quantities is derived Debye's method overshadowed the mathematically from the measurements: first- and second-order mo- sounder but less readily applicable work of Born and ments of the charge distribution, atomic distances and K~irmS.n. correlations, the end-to-end distance of chain poly- The combination of classical mechanics, electro- mers. Debye studied experimentally X-ray and electron dynamics and optics with Planck's quantum-governed diffraction of single molecules in gases by impact of energy distribution and other forms of statistics char- X-rays or cathode rays on a molecular beam, for in- acterizes most of Debye's further work. In 1913, when stance of carbon tetrachloride, in a common vacuum. the interpretation of Laue-Friedrich-Knipping's X-ray He was the first successfully to apply the rotating- diffraction photographs was still having teething trou- sector method for reducing the forward scattering in bles, Debye, then a newly married Professor of Theore- electron diffraction. He measured the anisotropy of tical Physics at Utrecht, tackled the problem of dif- light scattering in a flowing viscous liquid and in fraction of X-rays by a crystal made irregular by its strained high polymers and interpreted the results in temperature motion. This seemed an extremely hard terms of molecular or chain orientation. It seems fair problem at that time, and indeed, Debye solved it in to say that Debye invented more methods of obtaining stages. His third paper, in 1914, deals with a three- information from diffuse scattering than anybody else. dimensional crystal with atomic displacements coor- For that reason alone, whoever is interested in the dinated in elastic waves and introduces the temperature structure of matter owes him a great debt. OBITUARY 949 I am not aware of Debye having taken an active part (i) The explanation of the van der Waals forces be- in the development of wave mechanics or advanced tween molecules; quantum theory, except that, in a paper of 1923, he (ii) the theory of polar molecules, which opened a developed the theory of the Compton effect indepen- wide field of molecular structure studies on the basis dently of Compton, whose paper preceded Debye's by of dielectric measurements, and a few weeks. But he did make a brilliant contribution (iii) the general theory of strong electrolytes, the to the dynamics of solids at a Congress on the Kinetic Debye-Hfickel theory. Theory of Matter in G~Sttingen, Easter 1913. His own All three subjects were started between 1920 and papers on heat capacity and on the thermal scattering 1923, and they are all based on the interaction of di- of X-rays are based on the assumption of linear forces poles and quadrupoles in a statistical manner. Each between the atoms or linear elastic forces. It is impos- of the subjects has led to a great volume of further sible to explain with these the two general properties research of a physico-chemical nature. The latest inte- of solids, the thermal expansion and the slow conduc- rest of Debye centered on the determination of long- tion of heat. Debye showed in his contribution how range molecular forces from the observation of light to tackle the formidable problem posed by assuming scattering by liquid systems in their critical state. non-linear forces, using again a catch-as-catch-can Debye had a brilliant career in many renowned style. He showed how in a body with non-linear forces academic positions; he received many awards and heat waves build up local and temporal fluctuations honours for his outstanding work, including the Nobel of density and of elasticity, so that for the passage of Prize for Chemistry in 1936.