Physics in the 20th Century V. F. Weisskopf But to sacrifice particle physics certainly Professor Weisskopf spent several summer months for physicists; it was the task of the at CERN working in the Theory Division. During does not mean that the resources will go chemists to analyze and systematize them, his stay he has given three talks to the Summer into something equivalent. CERN and Vacation Students under the title 'Fundamental as was done so successfully a hundred related Laboratories are a successful ven­ Questions of Physics'. Some of the themes in years ago by Mendeleyev in his periodic ture in physics, and they themselves re­ his talks also appeared in the talk which he gave system of elements. The specific features at the Inaugural Conference of the European present just the tip of the iceberg in terms of the different species of atoms, their Physical Society in April. The Proceedings will of their total effect on physics. The ex­ be published as a special issue of 'La Rivista characteristics optical spectra, their che­ istence of their excellent experimental del Nuovo Cimento' and can be purchased from mical bounds, were known and catalogued facilities and the high calibre of scientist Messrs. Editrice Compositori, Viale XII Giugno 1, by the chemists, but they were not consi­ 40124 Bologna, at an estimated cost of $ 12. that they attract has an effect which gives dered a suitable subject for physicists. extra vitality, both directly and indirectly, The electron was already discovered to physics departments in Universities Reviewing the development of physics in before 1900 and it became obvious that throughout Europe. the 20th century is indeed a dazzling ex­ electrons must be essential parts of the Also CERN is a successful venture in perience. Relativity, quantum theory, ato­ atomic structure, but classical physics terms of the organization of large scale mic physics, molecular physics, the phy­ could not give any clue as to the kind of science. It is perhaps the most successful sics of the solid state, nuclear physics, structure one should expect within the of all the efforts at European collaboration. astrophysics, plasma physics, particle phy­ atoms. The discovery of a quantum of When there is much that has not suc­ sics, all these new insights into nature are electric charge, dominating all electric and ceeded, it would be a pity to jeopardize children of the 20th century. optical phenomena was the beginning of a that which has. There was a definite change in the cha­ long development, in which deep insights Despite the serious problems which in­ racter of physics at the turn of the century. into the essence of matter were gained. dividual countries seem to be confronting The older physics was under the spell of It behoves us to say, however, that the one after another, Europe still has colossal two fundamental forces of nature: gravity significance of this unit of charge is still resources both materially and intellec­ and electromagnetism. The development of a major riddle today, a hundred years after tually. Particle physics is a comparatively classical mechanics from GaHleo and New­ its discovery. small but significant field where, at ton to Lagrange and Hamilton had shown present, they are being successfully the validity of the same natural law, the In physics, the 20th century truly begins applied. law of gravity, on earth and in the uni­ in the year 1900. This date is not an acci­ verse. Electrodynamics, a child of the 19th dent, it is the year of publication of Max century, reared by Faraday, Maxwell and Planck's famous paper on the quantum of Hertz, was the first extensive application action, the birth year of quantum theory. of the field concept in physics; it revealed It is impressive to contemplate the rate of the importance of electric phenomena in progress in the first quarter of this cent­ matter. The discovery of the electromag­ ury: Planck's quantum of action in 1900, netic field as an independent entity in Einstein's special relativity theory in 1905, space, the spectrum of electromagnetic Rutherford's discovery of atomic structure waves, the electromagnetic nature of light, in 1911, Bohr's quantum orbits and expla­ are some of the greatest human insights nation of the hydrogen spectrum in 1913, into the natural world. But the properties Einstein's general relativity in 1916, Ruther­ of matter were not understood at that time, ford's first nuclear transformation in 1917, they were not deduced from more ele­ Bohr's explanation of the periodic table of mentary concepts, they were measured elements (Aufbauprinzip) in 1922, the dis­ and expressed in the form of specific covery of quantum mechanics by de Bro- constants of materials, such as elasticity, glie, Heisenberg, Schrôdinger and Bohr in compressibility, specific heat, viscosity, 1924-26, the exclusion principle by Pauli in conductivity of heat and electricity, dielec­ 1925, the electron spin by Uhlenbeck and tric and diamagnetic constants. Goudsmit in 1927, the relativistic quantum The physicists of the 19th century were mechanics by Dirac in 1928, Heitler-Lon- not unaware of the importance of inter­ don's theory of the chemical bond in 1927, atomic forces for the determination of the theory of metallic conductivity by material properties. But there was no way Bloch and Sommerfeld in 1930. Let us stop of telling what the origin of these inter­ there, although the rate of progress by no atomic forces was, and how to account means stopped in 1930; it went on for at for their strength or absence. The great least another ten years, before slowing variety among the properties of the differ­ down to the relatively slow pace of today. ent elements was not considered a topic Among the great systems of ideas which 295 were created in that period, relativity theo­ features which quantum mechanics has atom, as a direct consequence of quantum ry, special and general, has a place some­ brought to ojur view of the atomic world. mechanics. what different from the others. It was born First, it has introduced a characteristic A fundamental problem of natural philo­ in the 20th century as the brain child of length and energy which dominate the sophy was solved by the discovery of laws one towering personality. It is a new con­ atomic phenomena, endowing them with a which give rise to specific shapes and ceptual framework for the unification of scale and a measure. The combination of well-defined entities. Clearly, Nature is mechanics, electrodynamics and gravity, electrostatic attraction between the nu­ basically made of such entities, as our which brought with it a new perception of cleus and the electron on the one hand, experience tells us every day; materials space and time. This framework of ideas, the typical quantum kinetic energy of a have characteristic properties, iron re­ in some ways, is the crowning and syn­ confined electron on the other hand, mains the same iron after evaporation and thesis of 19th century physics, rather than define a length: the Bohr radius, and an recondensation. The specific properties of a break with the classic tradition. Quantum energy: the Rydberg unit. The size of the matter were the subject of chemistry be­ theory, however, was such a break; it was atoms is determined by the length which fore and not of physics. Quantum mecha­ a step into the unknown, into a world of is the combination h2/me2 of a few funda­ nics explains these properties and thus phenomena that did not fit into the web mental constants, the unit of charge e, the has eliminated chemistry as a separate of ideas of 19th century physics. New ways electron mass m, and the quantum of science. of formulation, new ways of thinking had to action h. The Rydberg unit is given by the be created in order to gain insight into the combination me4/h2. Thus atomic sizes and The infinitely varied, but well defined, world of atoms and molecules, with its energies are basically determined and ways in which atoms aggregate to larger discrete energy states and characteristic explained. units are now accessible to a rational patterns of spectra and bonds. Second, quantum mechanics introduces interpretation in quantum mechanical terms. A theory of the molecular bond These new ways of thinking were formu­ a 'morphic' trait, previously absent in came into being in which electron wave lated and codified in the midst of the physics. The electron wave functions re­ patterns keep atomic nuclei together in third decade of this century. The wave present special forms of patterns of simple the right arrangement. Since one again particle duality was proposed by de Bro- symmetry, characteristic of the symmetry deals here with the interaction of nuclear glie in 1924, the equation for particle- of the situation which the electron faces charges and electrons, the same sizes and waves was conceived by Schrôdinger in in the attractive field of the nucleus and energies must appear as in atoms, giving 1925. In these years the concepts of quan­ of the other electrons. These patterns are rise to interatomic distances of a few Bohr tum mechanics were expressed and criti­ the fundamental shapes of which all things radii and binding energies of the order of cally analyzed in Copenhagen under the in our environment are made. These electronvolts. Atomic aggregates consist of leadership of Niels Bohr, with the help of shapes are directly determined from the two kinds of particles, heavy nuclei and ideas of Heisenberg, Kramers, Pauli and fields of force which bind the electrons. light electrons, which are bound to each Born. The ink of these papers was hardly Here quantum mechanics has created the other by mutual attraction.