Is Gravity the Weakest Force?

It has recently been suggested that "gravity is the weakest force" in any theory with a suitable UV completion within quantum gravity. One formulation of this statement is the scalar weak gravity conjecture, which states that gravity is weaker than the force originating from scalar fields. We study the scalar weak gravity conjecture in de Sitter space, and discuss its low-energy consequences in light of the experimental searches for fifth forces and violations of the equivalence principle. We point out that some versions of the scalar weak gravity conjecture forbid the existence of very light scalar particles, such as the quintessence and axion-like particles. The absence of the quintessence field means that these versions of the scalar weak gravity conjecture are in phenomenological tension with the recently-proposed de Sitter swampland conjecture and its refinements. Some other versions of the scalar weak gravity conjecture escape these constraints, and could have interesting phenomenological consequences.