2 Putting new theories to the test
The results of tests of the inverse-square law are usually plotted in terms of two numbers: a length scale, λ, on the x-axis and a strength, |α|, on the y-axis, with |α| = 1 corresponding to a new effect with a strength equal to that of Newtonian gravity. New forces in the yellow region have been ruled out by experiments at the 95% confidence level (black lines). These experiments probe different length scales: the Stanford experiment, for example, explores the shortest lengths, while the Irvine experiment has set the lowest limits on |α| but at larger distances. The dashed red curve is a preliminary result from the Eöt-Wash group in Seattle. New interactions could still be present in the unexplored region, but more-sensitive instruments will be needed to detect them. The predictions of various theoretical scenarios are shown in blue: some of these have been ruled out (such as the two large extra dimensions in the Arkani-Hamed et al. model) but others have not.