The Dynamics of the Constancy of Light Speed

Just as Einstein said, "Light is always propagated in empty space with a definite velocity \(c\) which is independent of the state of motion of the emitting body", which has been experimentally verified and is widely known. However, physics textbooks do not tell us the dynamic reason why the speed of light remains constant in a vacuum. By re-examining the simultaneity (measured by light and sound without Doppler effect) within inertial frames, the electrostatic field dragged by moving charges, the gravitational field dragged by the sun, and the Casimir effect of a vacuum (not empty), this paper presents that all non-zero-mass objects possess their respective static vacuum fields, and reintroduces the vacuum medium model in which the constancy of light speed in a vacuum obeys continuum mechanics. This result shows that relativity can be superseded by a simply modified classical theory, and that there is a Newtonian cosmic model which can exactly predict the cosmological redshift, the cosmic microwave background, and the relevant cosmological constants, thereby equipping the theoretical possibility to quantize the vacuum and unify quantum-level and macroscopic systems.