Asked by: Tim Farrow, Preston
Over the years, theorists have proposed modification to Einstein’s theory of gravity that allow this fundamental force to vary with time. In the 1930s, the eminent British physicist Paul Dirac suggested that gravity might get weaker as the Universe expanded, prompting astronomers to look for evidence. During the 1970s, studies of the Moon suggested it was moving away from the Earth. Most of the increase in distance could be explained using standard theories of how the gravity fields of the Moon and Earth interact. But some of the increase pointed to a weakening of the force of gravity itself, as Dirac had predicted. The claim attracted a lot of media interest, but by the early 1980s experiments involving precise timing of signals from planetary probes found no evidence for changes in the strength of gravity. The original claim is now thought to be the result of faulty analysis of the Moon’s orbital motion.