Changing α with Time: Implications for Fifth-Force-Type Experiments and Quintessence

If the recent observations suggesting a time variation of the fine structure constant are correct, they imply the existence of an ultralight scalar particle. This particle inevitably couples to nucleons through the α dependence of their masses and thus mediates an isotope-dependent long-range force. The strength of the coupling is within a couple of orders of magnitude of the existing experimental bounds for such forces. The new force can be potentially measured in precision experimental tests of the equivalence principle. Because of a coincidence of the required time scales, the scalar field can at the same time play the role of a quintessence field.