Based on the principle of constant speed of light and the principle of minimum energy, this paper innovatively proposes the principle of relative change of light speed and constructs a theoretical system of vacuum dynamics. By revealing the dynamic mechanism of spontaneous force caused by light speed gradient, this theory provides an innovative dynamic interpretation framework for gravitational interaction and dark energy phenomena, making up for the deficiency in the discussion of dynamic mechanism in general relativity. The core research results are as follows: 1. Principle of spacetime-light speed covariance: It reveals the physical law that the ratio of spacetime change to relative change of light speed must be consistent. 2. Dynamic mechanism of vacuum light speed gradient: It is proposed that the light speed gradient can cause spontaneous motion of objects, and its acceleration is the negative gradient of the square of light speed: . 3. The gravitational acceleration derived from the vacuum dynamics theory is completely equivalent to that from general relativity. This thus supplements the dynamic mechanism for the geometric interpretation that "spacetime curvature is gravity". 4. A gradient distribution model of light speed in the universe with higher speed inside and lower speed outside is proposed: The essence of cosmic expansion is the spontaneous movement of celestial bodies towards regions with lower light speed (i.e., lower energy), and dark energy is essentially the internal energy released and converted into kinetic energy by celestial bodies in this process.
Li et al. (Mon,) studied this question.