Changing α with time: Implications for fifth-force-type experiments and quintessence

The time variation of fine structure constant which implies existence of a very weakly coupled ultralight scalar particle is discussed. The α-ion couples to ordinary nucleons, protons, and neutrons 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 experimental bounds for such forces. The scalar field can play the role of a quintessence field due to a coincidence of the required time scales.