Modern physics requires a new gravity theory to explain relationship about gravity and quantum effects. However, quantum mechanics have an unsolved problem about the interpretation. Also, theory of relativity cannot explain dark energy. Currently, there is no theory that can comprehensively solve these problems. Here, as a result of reinterpreting the concept of reduced mass, this paper shows that quantum superposition can be explained as the intersection of spacetime trajectories of each quantum according to gravitational potential. Spacetime which is absolute in relativity theory is relative in this theory, and it can be explained why the speed of light is independent from the inertial system. Additionally, because the classical mechanical energy conservation law is relative, as in the Casimir effect, a new conservation law for a quantum version is proposed. Dark energy can be explained by the theory that rest mass changes according to gravitational potential. Dark energy is an action along the dimension of gravitational potential, and its size increases as that of rest energy decreases. Dark energy affects quantum effects and the asymptotic freedom of strong interaction can be explained similarly.