Do the constants of nature vary?

Full text: Speculation of 'varying constants' has a fascinating history. In 1937, Milne and Dirac independently explored the possibility that the gravitational constant G had changed with time. The debate was resurrected in 1961 by the Brans-Dicke theory, which appealed to some because it appeared to incorporate Mach's principle more convincingly than General Relativity. Suggestions that the fine-structure constant, alpha, may be time-dependent arose, with the quick recognition that this could be detected via shifts in the rest wavelengths of resonance transitions in QSO absorption systems. Current interest is high because in superstring theories which have additional spatial dimensions, compactified on tiny scales, any variation of the mean size of the extra dimensions results in changes of the 3-dimensional observed coupling constants. Also, we can now probe variations at the level predicted in inflationary models, where the amplitude and spectrum of spatial variations in alpha are linked to those of the gravitational wave and density fluctuations measured by COBE. Interestingly, our recent results hint that alpha may have actually been smaller in the past. Copyright (1999) Australian Optical Society