Varying Gravity: Dirac’s Legacy in Cosmology and Geophysics

In 1937 the celebrated Nobel laureate and quantum theorist Paul Dirac suggested that the gravitational constant G slowly decreases in time, and on this non-Einsteinian basis he developed a cosmological model of the expanding universe. The audacious hypothesis was generally ignored but after World War II it attracted interest when it was applied to problems of paleoclimatology and global geophysics. According to Pascual Jordan, another quantum pioneer, Dirac’s G(t) hypothesis led to the consequence that the Earth expands. For a decade or so the expanding Earth hypothesis, whether based on varying gravity or not, was discussed as an alternative to the revived theory of continental drift and its development into mainstream plate tectonics. At about the same time several cosmological models based on versions of the G(t) hypothesis were proposed. Although, as seen from a modern point of view, the theories based on varying gravity were blind alleys, from a historical perspective the case is of considerable interest. For example, it illustrates the problems that may occur when scientists from two very different disciplinary traditions face the same subject matter, in this case the past of the Earth. Moreover, although Dirac’s version of the G(t) hypothesis is dead, the general idea of varying gravity is still entertained by some physicists. The talk discusses some of the main features of the debate as it evolved ca. 1950-1980. It also briefly alludes to other ideas concerning fundamental constants varying in time.