Essay on gravitation: Present-time variation of Newton's gravitational constant in superstring theories

Abstract

Superstring theories provide an appropriate framework for studying the time variation of fundamental coupling constants. The present time variation of coupling constants in Superstring theories with currently favorable internal backgrounds critically depends on the shape of the potential for the size of internal space. If the potential is almost flat, as in perturbation theory to all orders, the present value of ¦Ġ/G¦ for Newton's gravitational constant is calculable and estimated to be 1×10−11±1yr−1, which is just at the edge of the present observational bound forĠ/G. If the potential has a minimum with finite curvature due to unknown nonperturbative effects,Ġ/G will become unobservably small. The improvement of the measurement ofĠ/G of 1 or 2 orders of magnitude would discriminate between the two situations. Problems with the time variation of other coupling constants are also discussed.