According to these arguments, if gravity’s strength has decreased with time, Earth would have become less tightly bound, gradually allowing it to spread outward. The generation of new matter in the universe, hypothesized in theories such as that of Hoyle and Narlikar, would augment this effect. The Hoyle-Narlikar theory predicted an expansion rate of about one-tenth of a millimeter (one-fortieth of an inch) per year. Other models predicted slightly lesser or greater rates, depending on whether or not they involved matter creation. Dicke believed that variable gravitation would produce only a minute expansion, not enough to explain continental drift. Hence, one critical test of theories of changing gravity has been whether geophysical evidence indicated expansion and, if so, at what rate the expansion took place.
In The Expanding Earth, published in 1966, Pascual Jordan explored possible connections between cosmology and geophysics. Jordan argued that terrestrial measurements offered an ideal test for particular models of gravitation. He realized this was a tricky business, given the contentious nature of the topic. “We cannot decide immediately,” he wrote, “whether the Dirac hypothesis really contradicts experience…. We are compelled to inspect the results of various empirical geophysical research and select those whose foundations appear sound, often from controversial statements.”
Physicist Engelbert Schucking, who worked with Jordan in Germany on some of his cosmological theories and is currently a professor at New York University, has grown doubtful about the notion that the gravitational constant is changing. “The idea of changing G,” asserts Schucking, “is very much refuted by observation—the laser ranging to the Moon. G could not have changed by more than 1 percent from the beginning.” He is also dubious about the validity of changing the fine-structure constant and varying the speed of light, two other recently proposed ideas.
With any luck, astrophysical and geophysical data will eventually converge on a consensus for whether or not the gravitational constant varies over time. Earth’s solidity, its relationship to the