Theoretical Physics Conference, 1940
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THE SIXTH WASHINGTON CONFERENCE ON THEORETICAL PHYSICS
"The Interior of the Earth" was the subject of three days of intensive discussion at the Sixth Annual Conference on Theoretical Physics held in Washington March 21-23, 1940, under the joint auspices of the George Washington University and the Carnegie Institution of Washington. A group of fourteen investigators in geophysics and in theoretical physics from various universities in the United States joined a similar number of Washington investigators in a series of informal meetings, in large and small groups, for technical examination of some of the outstanding problems concerning matter in great bulk and under large pressures and temperatures, as found inside the Earth. The chief aim of the discussions was to formulate these problems more clearly for future joint efforts.
The first meeting was devoted to the pressure-volume relation at high pressures and associated questions concerning the probable composition and physical state of the Earth's deep interior. Professor Fermi introduced the topic. He showed that for pressures higher than about 10(8) atmospheres the pressure-volume relation can be estimated statistically, and that all materials must behave in a similar way. At these pressures the outer electronic shells of the atoms are crushed; the electrons continue to move in different orbits but can be treated as independent of each other. In this region the pressure increases with the 5/3-power of the density. However, the pressure in the interior of the Earth only reaches a value of about 3 x 10(6) atmospheres at the center. Professor Fermi reported the results of a paper by Jensen (Zs. Physik, 1938) who extended these calculations to somewhat lower pressures. Jensen's results are valid only down to about 10(7) atmospheres, but by interpolating between his results and the experiment data at 2 to 5 x 10(4) atmospheres one finds agreement with the densities and compressibilities which geophysicists have deduced for the iron in the core of the Earth. As one immediate result of these discussions calculations along similar lines are now in progress (by Dr. Beeman at Johns Hopkins) for the intermediate pressure region (10(5) to 10(6) atmospheres).
Professor Fermi also estimated the melting point of iron under a pressure of 3 x 10(6) atmospheres. He used a formula based on the assumption that a solid melts if the amplitude of the thermal vibration of its atoms reaches a certain fraction of the interatomic distances. On this basis the melting point of iron in the core of the Earth is approximately 10,000 degrees K, a value which is rather higher than previous estimates.
Dr. Gutenberg then discussed the evidence from seismology and earth-tides which indicates that the deep interior of the Earth is in a fluid rather than a solid state. Dr. Kracek presented various considerations bearing on the composition and probable stratification of the interior, and various points in this connection were discussed by Drs. Gutenberg, Griggs, Herzfeld, and Bardeen.
Related material of special interest was presented the following day by Dr. Goranson, who discussed new measurements of compressibility extending to a pressure above 200,000 atmospheres, made with a cascade-bomb equipment under development at the Geophysical Laboratory of the Institution for the past year or more.
The main topic of the discussions on the second day was the origin and maintenance of the great magnetic field of the Earth. Mr. McNish first gave a description of the magnetic moment, its representation by a minimum number of internal dipoles (fourteen dipoles at the surface of the core, 3,000 kilometers down, plus one eccentric dipole near the center), and discussed the secular variation. Dr. Vestine presented various data and calculations respecting the electrical conductivity of the Earth at different depths, as inferred from the diurnal and magnetic-storm variations.
Dr. Slsasser then discussed various theories of the Earth's magnetic field. In particular he presented his own recent ideas according to which the magnetic effects may be due to large thermoelectric currents maintained by mass-convection currents in the fluid core. The guiding of this convective flow by the Coriolis forces is used to explain the relation between the direction of the magnetic field and the Earth's rotational axis. Questions of energy, viscosity, eddy-currents, and differences in temperature and composition in various regions were taken up.
Calculations which throw some doubt on this theory were put forward by Drs. Inglis and Teller. The known heat-transport through the mantle limits the convection-currents and the Coriolis forces, so that the orientation and ordering of the currents remains unexplained. The discussions thus brought out new material on an obscure point; these calculations are now in press.
Dr. Gunn discussed the dynamo-theory in relation to the Earth's interior, and the possibility of a ferromagnetic core was discussed by Professor Slater, The Curie point is probably lowered by increase of pressure, but a review of the properties of metals in the iron group shows that none of these is at all likely to be ferromagnetic at any temperature which is reasonable for the Earth's core.
The remainder of the discussions on the second and third days were devoted to radioactivity in the Earth and to problems of viscosity .Dr. Urry presented the results of examinations of numerous measurements on the radioactivity of meteorites. If those are fragments of a former planet, the implication is that the interior of the Earth has perhaps a hundred times smaller concentration of radioactive material than the crustal rocks. However, even so small an amount of radioactivity distributed throughout the Earth produces more heat than is conducted away through the surface. It seems possible on this basis that the core may be several hundred degrees hotter than it was when the mantle first solidified. Dr. Adams summarized recent discussions of rocks which are considered most likely to be similar to the deep-lying parts of the mantle, and which show practically no radioactivity. This may be the real answer to the familiar dilemma of a cooling Earth which is overheating from within.
Professor Gutenberg pointed out that according to the analysis of earthquake waves the core of the Earth (the region below a depth of 3,000 kilometers) is liquid. From tidal deformations of the mantle, measured at the Earth's surface, one can conclude that the core must have a much lower viscosity than the mantle. Indeed, the low absorption of longitudinal earthquake waves in the core indicates that the core has a viscosity smaller than 10(10) CGS units, which is roughly the viscosity of ordinary pitch. The viscosity throughout the mantle is about 10(23) CGS units, while molten metals have viscosities smaller than one CGS unit. Viscous liquids as a rule contain complex chain-like molecules, while all monatomic liquids have low viscosities. It is probable that high pressure does not give rise to a high viscosity for liquid iron. The pressure raises the melting point, but when melting has taken place the viscosity of the metal should be quite low.
Dr. Griggs and Dr. Gutenberg also discussed general questions concerning the viscosity of solids. It seems that definitions and concepts regarding permanent and non-permanent deformations of solids are as yet not quite clear cut, partly because the behavior of matter in the solid form under continued stress is complicated.
This Conference had as its objectives: (a) To formulate the problems and data of geophysics which may be of interest to workers in theoretical physics and (b) to put at the service of workers in geophysics a growing theoretical knowledge concerning the behavior of matter under unusual conditions, especially at very high pressures. Experts in a variety of fields can thus be of mutual assistance if their attention is focused on certain problems of geophysics. That this meeting provided an immediate basis for further cooperative work has already been demonstrated, as extended calculations on several questions have already been undertaken by several theoretical physicists, and the Physics Seminar at one University will be devoted next year to Geophysics. A list of those attending the meetings is attached.
Edward Teller
George Washington University
M. A. Tuve
Department of Terrestrial Magnetism
Carnegie Institution of Washington
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Author or Source: Office of Public Relations; President's Papers/RG0002
Document Location: University Archives
Date Added to Encyclopedia: January 22, 2007
Prepared by: Lyle Slovick, Assistant University Archivist
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