**Chapter 5C**

**Planetary Science and Fundamental Physics**

In the case of planetary science, as Stephen Brush reminds us, the attempt to satisfy intellectual curiosity brought about many of the most significant advances of the past few centuries. In 1746, for example, d'Alambert won a prize for making one of the first consistent uses of partial differential equations. His was an essay on winds. The work of Laplace at the end of the 18th century resulted from the calculations in celestial mechanics of Clairut, Euler, and Lagrange. Legendre and Laplace originated the use of spherical harmonics in potential theory while trying to calculate the gravitational attraction of the Earth. This use was later of great value in electricity and in quantum mechanics. And many of Poincare's major works on mathematical analysis were inspired by problems in planetary mechanics.[1]

Similar remarks may be made about Gauss, one of the world's greatest mathematicians, who did much work in geodetic surveys and terrestrial magnetism. As Brush points out, "even an advocate of pure science might concede that Gauss' geophysical work provided the stimulus for some of his contributions to geometry and potential theory, just as his early work on the computation of orbits led to a major contribution to probability theory, the `Gaussian distribution', and could thereby be justified."[2] James Clerk Maxwell, clearly a giant of physics, won the Adams Prize with an essay on the stability of the rings of Saturn. This work was the basis for his kinetic theory of gas viscosity and eventually of his theory of transport processes. He later returned to the problem of the rings of Saturn and applied to it the methods he developed in his kinetic theory of gases.

A case of particular interest to Brush in establishing the connection between pure science and planetary science is the 19th century problem of the dependence of thermal radiation on the temperature of the source. This problem was of great significance to planetary science, of course, because the sun is the greatest source of radiation in the solar system. The outcome of the attempt to determine the surface temperature of the sun "was Stefan's suggestion (1879) that the data could best be represented by assuming that the rate of emission of energy is proportional to the 4th power of the absolute temperature."[3] Further experimental investigation, and Boltzman's theoretical derivation of such a formula, led to the subsequent work on the frequency distribution of black-body radiation. The search for the law that would govern such distribution can thus be seen as the genesis of Planck's quantum theory.

These remarks on the history of physics vindicate the claim that the study of the heavens at all levels has been a driving force in the development of fundamental science. This may come as a surprise to some. But it should not be a surprise if we consider the variety of interactions between cosmology and other areas of science, and between the different levels of cosmological research themselves. The study of the cosmos leads to the discovery of fundamental principles of physics, further development of physics in turn leads to new investigative tools of the cosmos, and so on. We may speak here of a dialectical relationship between areas and levels of science which results in the dynamic growth of science. In the case of planetary science we find no exception: fundamental research on physics and cosmology leads to changes in our ideas about the solar system and its planets. On the other hand, those ideas in turn give many hints as to useful lines of "pure" inquiry. This result is not merely part of the historical record: if anything it should receive greater prominence as space exploration multiplies our means of investigating the cosmos.

Indeed we will see presently that space science provides an excellent opportunity to enhance the relationship between astrophysics and the rest of fundamental physics.

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